I observed that there is one msix vector for config and one shared vector
for all queues in below qemu cmdline, when the num-queues for virtio-blk
is more than the number of possible cpus:
qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6"
# cat /proc/interrupts
CPU0 CPU1 CPU2 CPU3
... ...
24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues
... ...
However, when num-queues is the same as number of possible cpus:
qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4"
# cat /proc/interrupts
CPU0 CPU1 CPU2 CPU3
... ...
24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0
26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1
27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2
28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3
... ...
In above case, there is one msix vector per queue.
This is because the max number of queues is not limited by the number of
possible cpus.
By default, nvme (regardless about write_queues and poll_queues) and
xen-blkfront limit the number of queues with num_possible_cpus().
Is this by design on purpose, or can we fix with below?
diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 4bc083b..df95ce3 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
if (err)
num_vqs = 1;
+ num_vqs = min(num_possible_cpus(), num_vqs);
+
vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
if (!vblk->vqs)
return -ENOMEM;
--
PS: The same issue is applicable to virtio-scsi as well.
Thank you very much!
Dongli Zhang
On Tue, 12 Mar 2019 10:22:46 -0700 (PDT)
Dongli Zhang <[email protected]> wrote:
> I observed that there is one msix vector for config and one shared vector
> for all queues in below qemu cmdline, when the num-queues for virtio-blk
> is more than the number of possible cpus:
>
> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6"
>
> # cat /proc/interrupts
> CPU0 CPU1 CPU2 CPU3
> ... ...
> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
> 25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues
> ... ...
>
>
> However, when num-queues is the same as number of possible cpus:
>
> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4"
>
> # cat /proc/interrupts
> CPU0 CPU1 CPU2 CPU3
> ... ...
> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
> 25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0
> 26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1
> 27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2
> 28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3
> ... ...
>
> In above case, there is one msix vector per queue.
Please note that this is pci-specific...
>
>
> This is because the max number of queues is not limited by the number of
> possible cpus.
>
> By default, nvme (regardless about write_queues and poll_queues) and
> xen-blkfront limit the number of queues with num_possible_cpus().
...and these are probably pci-specific as well.
>
>
> Is this by design on purpose, or can we fix with below?
>
>
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 4bc083b..df95ce3 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
> if (err)
> num_vqs = 1;
>
> + num_vqs = min(num_possible_cpus(), num_vqs);
> +
> vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
> if (!vblk->vqs)
> return -ENOMEM;
virtio-blk, however, is not pci-specific.
If we are using the ccw transport on s390, a completely different
interrupt mechanism is in use ('floating' interrupts, which are not
per-cpu). A check like that should therefore not go into the generic
driver.
On 3/13/19 1:33 AM, Cornelia Huck wrote:
> On Tue, 12 Mar 2019 10:22:46 -0700 (PDT)
> Dongli Zhang <[email protected]> wrote:
>
>> I observed that there is one msix vector for config and one shared vector
>> for all queues in below qemu cmdline, when the num-queues for virtio-blk
>> is more than the number of possible cpus:
>>
>> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6"
>>
>> # cat /proc/interrupts
>> CPU0 CPU1 CPU2 CPU3
>> ... ...
>> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
>> 25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues
>> ... ...
>>
>>
>> However, when num-queues is the same as number of possible cpus:
>>
>> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4"
>>
>> # cat /proc/interrupts
>> CPU0 CPU1 CPU2 CPU3
>> ... ...
>> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
>> 25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0
>> 26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1
>> 27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2
>> 28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3
>> ... ...
>>
>> In above case, there is one msix vector per queue.
>
> Please note that this is pci-specific...
>
>>
>>
>> This is because the max number of queues is not limited by the number of
>> possible cpus.
>>
>> By default, nvme (regardless about write_queues and poll_queues) and
>> xen-blkfront limit the number of queues with num_possible_cpus().
>
> ...and these are probably pci-specific as well.
Not pci-specific, but per-cpu as well.
>
>>
>>
>> Is this by design on purpose, or can we fix with below?
>>
>>
>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
>> index 4bc083b..df95ce3 100644
>> --- a/drivers/block/virtio_blk.c
>> +++ b/drivers/block/virtio_blk.c
>> @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
>> if (err)
>> num_vqs = 1;
>>
>> + num_vqs = min(num_possible_cpus(), num_vqs);
>> +
>> vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
>> if (!vblk->vqs)
>> return -ENOMEM;
>
> virtio-blk, however, is not pci-specific.
>
> If we are using the ccw transport on s390, a completely different
> interrupt mechanism is in use ('floating' interrupts, which are not
> per-cpu). A check like that should therefore not go into the generic
> driver.
>
So far there seems two options.
The 1st option is to ask the qemu user to always specify "-num-queues" with the
same number of vcpus when running x86 guest with pci for virtio-blk or
virtio-scsi, in order to assign a vector for each queue.
Or, is it fine for virtio folks to add a new hook to 'struct virtio_config_ops'
so that different platforms (e.g., pci or ccw) would use different ways to limit
the max number of queues in guest, with something like below?
So far both virtio-scsi and virtio-blk would benefit from the new hook.
---
drivers/block/virtio_blk.c | 2 ++
drivers/virtio/virtio_pci_common.c | 6 ++++++
drivers/virtio/virtio_pci_common.h | 2 ++
drivers/virtio/virtio_pci_legacy.c | 1 +
drivers/virtio/virtio_pci_modern.c | 2 ++
include/linux/virtio_config.h | 11 +++++++++++
6 files changed, 24 insertions(+)
diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 4bc083b..93cfeda 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
if (err)
num_vqs = 1;
+ num_vqs = virtio_calc_num_vqs(vdev, num_vqs);
+
vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
if (!vblk->vqs)
return -ENOMEM;
diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
index d0584c0..ce021d1 100644
--- a/drivers/virtio/virtio_pci_common.c
+++ b/drivers/virtio/virtio_pci_common.c
@@ -409,6 +409,12 @@ int vp_find_vqs(struct virtio_device *vdev, unsigned nvqs,
return vp_find_vqs_intx(vdev, nvqs, vqs, callbacks, names, ctx);
}
+/* the config->calc_num_vqs() implementation */
+unsigned short vp_calc_num_vqs(unsigned short num_vqs)
+{
+ return min_t(unsigned short, num_possible_cpus(), num_vqs);
+}
+
const char *vp_bus_name(struct virtio_device *vdev)
{
struct virtio_pci_device *vp_dev = to_vp_device(vdev);
diff --git a/drivers/virtio/virtio_pci_common.h b/drivers/virtio/virtio_pci_common.h
index 0227100..cc5ac80 100644
--- a/drivers/virtio/virtio_pci_common.h
+++ b/drivers/virtio/virtio_pci_common.h
@@ -134,6 +134,8 @@ int vp_find_vqs(struct virtio_device *vdev, unsigned nvqs,
struct virtqueue *vqs[], vq_callback_t *callbacks[],
const char * const names[], const bool *ctx,
struct irq_affinity *desc);
+/* the config->calc_num_vqs() implementation */
+unsigned short vp_calc_num_vqs(unsigned short num_vqs);
const char *vp_bus_name(struct virtio_device *vdev);
/* Setup the affinity for a virtqueue:
diff --git a/drivers/virtio/virtio_pci_legacy.c b/drivers/virtio/virtio_pci_legacy.c
index eff9ddc..69d1050 100644
--- a/drivers/virtio/virtio_pci_legacy.c
+++ b/drivers/virtio/virtio_pci_legacy.c
@@ -209,6 +209,7 @@ static const struct virtio_config_ops virtio_pci_config_ops = {
.bus_name = vp_bus_name,
.set_vq_affinity = vp_set_vq_affinity,
.get_vq_affinity = vp_get_vq_affinity,
+ .calc_num_vqs = vp_calc_num_vqs,
};
/* the PCI probing function */
diff --git a/drivers/virtio/virtio_pci_modern.c b/drivers/virtio/virtio_pci_modern.c
index 07571da..f04e44a 100644
--- a/drivers/virtio/virtio_pci_modern.c
+++ b/drivers/virtio/virtio_pci_modern.c
@@ -460,6 +460,7 @@ static const struct virtio_config_ops
virtio_pci_config_nodev_ops = {
.bus_name = vp_bus_name,
.set_vq_affinity = vp_set_vq_affinity,
.get_vq_affinity = vp_get_vq_affinity,
+ .calc_num_vqs = vp_calc_num_vqs,
};
static const struct virtio_config_ops virtio_pci_config_ops = {
@@ -476,6 +477,7 @@ static const struct virtio_config_ops virtio_pci_config_ops = {
.bus_name = vp_bus_name,
.set_vq_affinity = vp_set_vq_affinity,
.get_vq_affinity = vp_get_vq_affinity,
+ .calc_num_vqs = vp_calc_num_vqs,
};
/**
diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
index bb4cc49..f32368b 100644
--- a/include/linux/virtio_config.h
+++ b/include/linux/virtio_config.h
@@ -65,6 +65,7 @@ struct irq_affinity;
* the caller can then copy.
* @set_vq_affinity: set the affinity for a virtqueue (optional).
* @get_vq_affinity: get the affinity for a virtqueue (optional).
+ * @calc_num_vqs: calculate the number of virtqueue (optional)
*/
typedef void vq_callback_t(struct virtqueue *);
struct virtio_config_ops {
@@ -88,6 +89,7 @@ struct virtio_config_ops {
const struct cpumask *cpu_mask);
const struct cpumask *(*get_vq_affinity)(struct virtio_device *vdev,
int index);
+ unsigned short (*calc_num_vqs)(unsigned short num_vqs);
};
/* If driver didn't advertise the feature, it will never appear. */
@@ -207,6 +209,15 @@ int virtio_find_vqs_ctx(struct virtio_device *vdev,
unsigned nvqs,
desc);
}
+static inline
+unsigned short virtio_calc_num_vqs(struct virtio_device *vdev,
+ unsigned short num_vqs)
+{
+ if (vdev->config->calc_num_vqs)
+ return vdev->config->calc_num_vqs(num_vqs);
+ return num_vqs;
+}
+
/**
* virtio_device_ready - enable vq use in probe function
* @vdev: the device
--
2.7.4
Thank you very much!
Dongli Zhang
On Wed, 13 Mar 2019 11:26:04 +0800
Dongli Zhang <[email protected]> wrote:
> On 3/13/19 1:33 AM, Cornelia Huck wrote:
> > On Tue, 12 Mar 2019 10:22:46 -0700 (PDT)
> > Dongli Zhang <[email protected]> wrote:
> >
> >> I observed that there is one msix vector for config and one shared vector
> >> for all queues in below qemu cmdline, when the num-queues for virtio-blk
> >> is more than the number of possible cpus:
> >>
> >> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6"
> >>
> >> # cat /proc/interrupts
> >> CPU0 CPU1 CPU2 CPU3
> >> ... ...
> >> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
> >> 25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues
> >> ... ...
> >>
> >>
> >> However, when num-queues is the same as number of possible cpus:
> >>
> >> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4"
> >>
> >> # cat /proc/interrupts
> >> CPU0 CPU1 CPU2 CPU3
> >> ... ...
> >> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
> >> 25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0
> >> 26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1
> >> 27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2
> >> 28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3
> >> ... ...
> >>
> >> In above case, there is one msix vector per queue.
> >
> > Please note that this is pci-specific...
> >
> >>
> >>
> >> This is because the max number of queues is not limited by the number of
> >> possible cpus.
> >>
> >> By default, nvme (regardless about write_queues and poll_queues) and
> >> xen-blkfront limit the number of queues with num_possible_cpus().
> >
> > ...and these are probably pci-specific as well.
>
> Not pci-specific, but per-cpu as well.
Ah, I meant that those are pci devices.
>
> >
> >>
> >>
> >> Is this by design on purpose, or can we fix with below?
> >>
> >>
> >> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> >> index 4bc083b..df95ce3 100644
> >> --- a/drivers/block/virtio_blk.c
> >> +++ b/drivers/block/virtio_blk.c
> >> @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
> >> if (err)
> >> num_vqs = 1;
> >>
> >> + num_vqs = min(num_possible_cpus(), num_vqs);
> >> +
> >> vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
> >> if (!vblk->vqs)
> >> return -ENOMEM;
> >
> > virtio-blk, however, is not pci-specific.
> >
> > If we are using the ccw transport on s390, a completely different
> > interrupt mechanism is in use ('floating' interrupts, which are not
> > per-cpu). A check like that should therefore not go into the generic
> > driver.
> >
>
> So far there seems two options.
>
> The 1st option is to ask the qemu user to always specify "-num-queues" with the
> same number of vcpus when running x86 guest with pci for virtio-blk or
> virtio-scsi, in order to assign a vector for each queue.
That does seem like an extra burden for the user: IIUC, things work
even if you have too many queues, it's just not optimal. It sounds like
something that can be done by a management layer (e.g. libvirt), though.
> Or, is it fine for virtio folks to add a new hook to 'struct virtio_config_ops'
> so that different platforms (e.g., pci or ccw) would use different ways to limit
> the max number of queues in guest, with something like below?
That sounds better, as both transports and drivers can opt-in here.
However, maybe it would be even better to try to come up with a better
strategy of allocating msix vectors in virtio-pci. More vectors in the
num_queues > num_cpus case, even if they still need to be shared?
Individual vectors for n-1 cpus and then a shared one for the remaining
queues?
It might even be device-specific: Have some low-traffic status queues
share a vector, and provide an individual vector for high-traffic
queues. Would need some device<->transport interface, obviously.
On 3/13/19 5:39 PM, Cornelia Huck wrote:
> On Wed, 13 Mar 2019 11:26:04 +0800
> Dongli Zhang <[email protected]> wrote:
>
>> On 3/13/19 1:33 AM, Cornelia Huck wrote:
>>> On Tue, 12 Mar 2019 10:22:46 -0700 (PDT)
>>> Dongli Zhang <[email protected]> wrote:
>>>
>>>> I observed that there is one msix vector for config and one shared vector
>>>> for all queues in below qemu cmdline, when the num-queues for virtio-blk
>>>> is more than the number of possible cpus:
>>>>
>>>> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6"
>>>>
>>>> # cat /proc/interrupts
>>>> CPU0 CPU1 CPU2 CPU3
>>>> ... ...
>>>> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
>>>> 25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues
>>>> ... ...
>>>>
>>>>
>>>> However, when num-queues is the same as number of possible cpus:
>>>>
>>>> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4"
>>>>
>>>> # cat /proc/interrupts
>>>> CPU0 CPU1 CPU2 CPU3
>>>> ... ...
>>>> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
>>>> 25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0
>>>> 26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1
>>>> 27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2
>>>> 28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3
>>>> ... ...
>>>>
>>>> In above case, there is one msix vector per queue.
>>>
>>> Please note that this is pci-specific...
>>>
>>>>
>>>>
>>>> This is because the max number of queues is not limited by the number of
>>>> possible cpus.
>>>>
>>>> By default, nvme (regardless about write_queues and poll_queues) and
>>>> xen-blkfront limit the number of queues with num_possible_cpus().
>>>
>>> ...and these are probably pci-specific as well.
>>
>> Not pci-specific, but per-cpu as well.
>
> Ah, I meant that those are pci devices.
>
>>
>>>
>>>>
>>>>
>>>> Is this by design on purpose, or can we fix with below?
>>>>
>>>>
>>>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
>>>> index 4bc083b..df95ce3 100644
>>>> --- a/drivers/block/virtio_blk.c
>>>> +++ b/drivers/block/virtio_blk.c
>>>> @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
>>>> if (err)
>>>> num_vqs = 1;
>>>>
>>>> + num_vqs = min(num_possible_cpus(), num_vqs);
>>>> +
>>>> vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
>>>> if (!vblk->vqs)
>>>> return -ENOMEM;
>>>
>>> virtio-blk, however, is not pci-specific.
>>>
>>> If we are using the ccw transport on s390, a completely different
>>> interrupt mechanism is in use ('floating' interrupts, which are not
>>> per-cpu). A check like that should therefore not go into the generic
>>> driver.
>>>
>>
>> So far there seems two options.
>>
>> The 1st option is to ask the qemu user to always specify "-num-queues" with the
>> same number of vcpus when running x86 guest with pci for virtio-blk or
>> virtio-scsi, in order to assign a vector for each queue.
>
> That does seem like an extra burden for the user: IIUC, things work
> even if you have too many queues, it's just not optimal. It sounds like
> something that can be done by a management layer (e.g. libvirt), though.
>
>> Or, is it fine for virtio folks to add a new hook to 'struct virtio_config_ops'
>> so that different platforms (e.g., pci or ccw) would use different ways to limit
>> the max number of queues in guest, with something like below?
>
> That sounds better, as both transports and drivers can opt-in here.
>
> However, maybe it would be even better to try to come up with a better
> strategy of allocating msix vectors in virtio-pci. More vectors in the
> num_queues > num_cpus case, even if they still need to be shared?
> Individual vectors for n-1 cpus and then a shared one for the remaining
> queues?
>
> It might even be device-specific: Have some low-traffic status queues
> share a vector, and provide an individual vector for high-traffic
> queues. Would need some device<->transport interface, obviously.
>
This sounds a little bit similar to multiple hctx maps?
So far, as virtio-blk only supports set->nr_maps = 1, no matter how many hw
queues are assigned for virtio-blk, blk_mq_alloc_tag_set() would use at most
nr_cpu_ids hw queues.
2981 int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
... ...
3021 /*
3022 * There is no use for more h/w queues than cpus if we just have
3023 * a single map
3024 */
3025 if (set->nr_maps == 1 && set->nr_hw_queues > nr_cpu_ids)
3026 set->nr_hw_queues = nr_cpu_ids;
Even the block layer would limit the number of hw queues by nr_cpu_ids when
(set->nr_maps == 1).
That's why I think virtio-blk should use the similar solution as nvme
(regardless about write_queues and poll_queues) and xen-blkfront.
Added Jason again. I do not know why the mailing list of
[email protected] always filters out Jason's email...
Dongli Zhang
On Thu, 14 Mar 2019 14:12:32 +0800
Dongli Zhang <[email protected]> wrote:
> On 3/13/19 5:39 PM, Cornelia Huck wrote:
> > On Wed, 13 Mar 2019 11:26:04 +0800
> > Dongli Zhang <[email protected]> wrote:
> >
> >> On 3/13/19 1:33 AM, Cornelia Huck wrote:
> >>> On Tue, 12 Mar 2019 10:22:46 -0700 (PDT)
> >>> Dongli Zhang <[email protected]> wrote:
> >>>> Is this by design on purpose, or can we fix with below?
> >>>>
> >>>>
> >>>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> >>>> index 4bc083b..df95ce3 100644
> >>>> --- a/drivers/block/virtio_blk.c
> >>>> +++ b/drivers/block/virtio_blk.c
> >>>> @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
> >>>> if (err)
> >>>> num_vqs = 1;
> >>>>
> >>>> + num_vqs = min(num_possible_cpus(), num_vqs);
> >>>> +
> >>>> vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
> >>>> if (!vblk->vqs)
> >>>> return -ENOMEM;
> >>>
> >>> virtio-blk, however, is not pci-specific.
> >>>
> >>> If we are using the ccw transport on s390, a completely different
> >>> interrupt mechanism is in use ('floating' interrupts, which are not
> >>> per-cpu). A check like that should therefore not go into the generic
> >>> driver.
> >>>
> >>
> >> So far there seems two options.
> >>
> >> The 1st option is to ask the qemu user to always specify "-num-queues" with the
> >> same number of vcpus when running x86 guest with pci for virtio-blk or
> >> virtio-scsi, in order to assign a vector for each queue.
> >
> > That does seem like an extra burden for the user: IIUC, things work
> > even if you have too many queues, it's just not optimal. It sounds like
> > something that can be done by a management layer (e.g. libvirt), though.
> >
> >> Or, is it fine for virtio folks to add a new hook to 'struct virtio_config_ops'
> >> so that different platforms (e.g., pci or ccw) would use different ways to limit
> >> the max number of queues in guest, with something like below?
> >
> > That sounds better, as both transports and drivers can opt-in here.
> >
> > However, maybe it would be even better to try to come up with a better
> > strategy of allocating msix vectors in virtio-pci. More vectors in the
> > num_queues > num_cpus case, even if they still need to be shared?
> > Individual vectors for n-1 cpus and then a shared one for the remaining
> > queues?
> >
> > It might even be device-specific: Have some low-traffic status queues
> > share a vector, and provide an individual vector for high-traffic
> > queues. Would need some device<->transport interface, obviously.
> >
>
> This sounds a little bit similar to multiple hctx maps?
>
> So far, as virtio-blk only supports set->nr_maps = 1, no matter how many hw
> queues are assigned for virtio-blk, blk_mq_alloc_tag_set() would use at most
> nr_cpu_ids hw queues.
>
> 2981 int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
> ... ...
> 3021 /*
> 3022 * There is no use for more h/w queues than cpus if we just have
> 3023 * a single map
> 3024 */
> 3025 if (set->nr_maps == 1 && set->nr_hw_queues > nr_cpu_ids)
> 3026 set->nr_hw_queues = nr_cpu_ids;
>
> Even the block layer would limit the number of hw queues by nr_cpu_ids when
> (set->nr_maps == 1).
Correct me if I'm wrong, but there seem to be two kinds of limitations
involved here:
- Allocation of msix vectors by the virtio-pci transport. We end up
with shared vectors if we have more virtqueues than vcpus. Other
transports may or may not have similar issues, but essentially, this
is something that applies to all kind of virtio devices attached via
the virtio-pci transport.
- The block layer limits the number of hw queues to the number of
vcpus. This applies only to virtio devices that interact with the
block layer, but regardless of the virtio transport.
> That's why I think virtio-blk should use the similar solution as nvme
> (regardless about write_queues and poll_queues) and xen-blkfront.
Ok, the hw queues limit from above would be an argument to limit to
#vcpus in the virtio-blk driver, regardless of the transport used. (No
idea if there are better ways to deal with this, I'm not familiar with
the interface.)
For virtio devices that don't interact with the block layer and are
attached via the virtio-pci transport, it might still make sense to
revisit vector allocation.
On Tue, Mar 12, 2019 at 10:22:46AM -0700, Dongli Zhang wrote:
> I observed that there is one msix vector for config and one shared vector
> for all queues in below qemu cmdline, when the num-queues for virtio-blk
> is more than the number of possible cpus:
>
> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6"
So why do this?
> # cat /proc/interrupts
> CPU0 CPU1 CPU2 CPU3
> ... ...
> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
> 25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues
> ... ...
>
>
> However, when num-queues is the same as number of possible cpus:
>
> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4"
>
> # cat /proc/interrupts
> CPU0 CPU1 CPU2 CPU3
> ... ...
> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
> 25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0
> 26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1
> 27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2
> 28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3
> ... ...
>
> In above case, there is one msix vector per queue.
>
>
> This is because the max number of queues is not limited by the number of
> possible cpus.
>
> By default, nvme (regardless about write_queues and poll_queues) and
> xen-blkfront limit the number of queues with num_possible_cpus().
>
>
> Is this by design on purpose, or can we fix with below?
>
>
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 4bc083b..df95ce3 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
> if (err)
> num_vqs = 1;
>
> + num_vqs = min(num_possible_cpus(), num_vqs);
> +
> vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
> if (!vblk->vqs)
> return -ENOMEM;
> --
>
>
> PS: The same issue is applicable to virtio-scsi as well.
>
> Thank you very much!
>
> Dongli Zhang
I don't think this will address the issue if there's vcpu hotplug though.
Because it's not about num_possible_cpus it's about the # of active VCPUs,
right? Does block hangle CPU hotplug generally?
We could maybe address that by switching vq to msi vector mapping in
a cpu hotplug notifier...
--
MST
On 03/14/2019 08:32 PM, Michael S. Tsirkin wrote:
> On Tue, Mar 12, 2019 at 10:22:46AM -0700, Dongli Zhang wrote:
>> I observed that there is one msix vector for config and one shared vector
>> for all queues in below qemu cmdline, when the num-queues for virtio-blk
>> is more than the number of possible cpus:
>>
>> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6"
>
> So why do this?
I observed this when I was testing virtio-blk and block layer.
I just assign a very large 'num-queues' to virtio-blk and keep changing the
number of vcpu in order to study blk-mq.
The num-queues for nvme (qemu) is by default 64, while it is 1 for virtio-blk.
>
>> # cat /proc/interrupts
>> CPU0 CPU1 CPU2 CPU3
>> ... ...
>> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
>> 25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues
>> ... ...
>>
>>
>> However, when num-queues is the same as number of possible cpus:
>>
>> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4"
>>
>> # cat /proc/interrupts
>> CPU0 CPU1 CPU2 CPU3
>> ... ...
>> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
>> 25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0
>> 26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1
>> 27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2
>> 28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3
>> ... ...
>>
>> In above case, there is one msix vector per queue.
>>
>>
>> This is because the max number of queues is not limited by the number of
>> possible cpus.
>>
>> By default, nvme (regardless about write_queues and poll_queues) and
>> xen-blkfront limit the number of queues with num_possible_cpus().
>>
>>
>> Is this by design on purpose, or can we fix with below?
>>
>>
>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
>> index 4bc083b..df95ce3 100644
>> --- a/drivers/block/virtio_blk.c
>> +++ b/drivers/block/virtio_blk.c
>> @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
>> if (err)
>> num_vqs = 1;
>>
>> + num_vqs = min(num_possible_cpus(), num_vqs);
>> +
>> vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
>> if (!vblk->vqs)
>> return -ENOMEM;
>> --
>>
>>
>> PS: The same issue is applicable to virtio-scsi as well.
>>
>> Thank you very much!
>>
>> Dongli Zhang
>
> I don't think this will address the issue if there's vcpu hotplug though.
> Because it's not about num_possible_cpus it's about the # of active VCPUs,
> right? Does block hangle CPU hotplug generally?
> We could maybe address that by switching vq to msi vector mapping in
> a cpu hotplug notifier...
>
It looks it is about num_possible_cpus/nr_cpu_ids for cpu hotplug.
For instance, below is when only 2 out of 6 cpus are initialized while
virtio-blk has 6 queues.
"-smp 2,maxcpus=6" and "-device
virtio-blk-pci,drive=drive0,id=disk0,num-queues=6,iothread=io1"
# cat /sys/devices/system/cpu/present
0-1
# cat /sys/devices/system/cpu/possible
0-5
# cat /proc/interrupts | grep virtio
24: 0 0 PCI-MSI 65536-edge virtio0-config
25: 1864 0 PCI-MSI 65537-edge virtio0-req.0
26: 0 1069 PCI-MSI 65538-edge virtio0-req.1
27: 0 0 PCI-MSI 65539-edge virtio0-req.2
28: 0 0 PCI-MSI 65540-edge virtio0-req.3
29: 0 0 PCI-MSI 65541-edge virtio0-req.4
30: 0 0 PCI-MSI 65542-edge virtio0-req.5
6 + 1 irqs are assigned even 4 out of 6 cpus are still offline.
Below is about the nvme emulated by qemu. While 2 out of 6 cpus are initial
assigned, nvme has 64 queues by default.
"-smp 2,maxcpus=6" and "-device nvme,drive=drive1,serial=deadbeaf1"
# cat /sys/devices/system/cpu/present
0-1
# cat /sys/devices/system/cpu/possible
0-5
# cat /proc/interrupts | grep nvme
31: 0 16 PCI-MSI 81920-edge nvme0q0
32: 35 0 PCI-MSI 81921-edge nvme0q1
33: 0 42 PCI-MSI 81922-edge nvme0q2
34: 0 0 PCI-MSI 81923-edge nvme0q3
35: 0 0 PCI-MSI 81924-edge nvme0q4
36: 0 0 PCI-MSI 81925-edge nvme0q5
37: 0 0 PCI-MSI 81926-edge nvme0q6
6 io queues are assigned with irq, although only 2 cpus are online.
When only 2 out of 48 cpus are online, there are 48 hctx created by block layer.
"-smp 2,maxcpus=48" and "-device
virtio-blk-pci,drive=drive0,id=disk0,num-queues=48,iothread=io1"
# ls /sys/kernel/debug/block/vda/ | grep hctx | wc -l
48
The above indicates the number of hw queues/irq is related to
num_possible_cpus/nr_cpu_ids.
Dongli Zhang
On 03/14/2019 08:13 PM, Cornelia Huck wrote:
> On Thu, 14 Mar 2019 14:12:32 +0800
> Dongli Zhang <[email protected]> wrote:
>
>> On 3/13/19 5:39 PM, Cornelia Huck wrote:
>>> On Wed, 13 Mar 2019 11:26:04 +0800
>>> Dongli Zhang <[email protected]> wrote:
>>>
>>>> On 3/13/19 1:33 AM, Cornelia Huck wrote:
>>>>> On Tue, 12 Mar 2019 10:22:46 -0700 (PDT)
>>>>> Dongli Zhang <[email protected]> wrote:
>
>>>>>> Is this by design on purpose, or can we fix with below?
>>>>>>
>>>>>>
>>>>>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
>>>>>> index 4bc083b..df95ce3 100644
>>>>>> --- a/drivers/block/virtio_blk.c
>>>>>> +++ b/drivers/block/virtio_blk.c
>>>>>> @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
>>>>>> if (err)
>>>>>> num_vqs = 1;
>>>>>>
>>>>>> + num_vqs = min(num_possible_cpus(), num_vqs);
>>>>>> +
>>>>>> vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
>>>>>> if (!vblk->vqs)
>>>>>> return -ENOMEM;
>>>>>
>>>>> virtio-blk, however, is not pci-specific.
>>>>>
>>>>> If we are using the ccw transport on s390, a completely different
>>>>> interrupt mechanism is in use ('floating' interrupts, which are not
>>>>> per-cpu). A check like that should therefore not go into the generic
>>>>> driver.
>>>>>
>>>>
>>>> So far there seems two options.
>>>>
>>>> The 1st option is to ask the qemu user to always specify "-num-queues" with the
>>>> same number of vcpus when running x86 guest with pci for virtio-blk or
>>>> virtio-scsi, in order to assign a vector for each queue.
>>>
>>> That does seem like an extra burden for the user: IIUC, things work
>>> even if you have too many queues, it's just not optimal. It sounds like
>>> something that can be done by a management layer (e.g. libvirt), though.
>>>
>>>> Or, is it fine for virtio folks to add a new hook to 'struct virtio_config_ops'
>>>> so that different platforms (e.g., pci or ccw) would use different ways to limit
>>>> the max number of queues in guest, with something like below?
>>>
>>> That sounds better, as both transports and drivers can opt-in here.
>>>
>>> However, maybe it would be even better to try to come up with a better
>>> strategy of allocating msix vectors in virtio-pci. More vectors in the
>>> num_queues > num_cpus case, even if they still need to be shared?
>>> Individual vectors for n-1 cpus and then a shared one for the remaining
>>> queues?
>>>
>>> It might even be device-specific: Have some low-traffic status queues
>>> share a vector, and provide an individual vector for high-traffic
>>> queues. Would need some device<->transport interface, obviously.
>>>
>>
>> This sounds a little bit similar to multiple hctx maps?
>>
>> So far, as virtio-blk only supports set->nr_maps = 1, no matter how many hw
>> queues are assigned for virtio-blk, blk_mq_alloc_tag_set() would use at most
>> nr_cpu_ids hw queues.
>>
>> 2981 int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
>> ... ...
>> 3021 /*
>> 3022 * There is no use for more h/w queues than cpus if we just have
>> 3023 * a single map
>> 3024 */
>> 3025 if (set->nr_maps == 1 && set->nr_hw_queues > nr_cpu_ids)
>> 3026 set->nr_hw_queues = nr_cpu_ids;
>>
>> Even the block layer would limit the number of hw queues by nr_cpu_ids when
>> (set->nr_maps == 1).
>
> Correct me if I'm wrong, but there seem to be two kinds of limitations
> involved here:
> - Allocation of msix vectors by the virtio-pci transport. We end up
> with shared vectors if we have more virtqueues than vcpus. Other
> transports may or may not have similar issues, but essentially, this
> is something that applies to all kind of virtio devices attached via
> the virtio-pci transport.
It depends.
For virtio-net, we need to specify the number of available vectors on qemu side,
e.g.,:
-device virtio-net-pci,netdev=tapnet,mq=true,vectors=16
This parameter is specific for virtio-net.
Suppose if 'queues=8' while 'vectors=16', as 2*8+1 > 16, there will be lack of
vectors and guest would not be able to assign one vector for each queue.
I was tortured by this long time ago and it seems qemu would minimize the memory
allocation and the default 'vectors' is 3.
BTW, why cannot we have a more consistent configuration for most qemu devices,
e.g., so far:
virtio-blk use 'num-queues'
nvme use 'num_queues'
virtio-net use 'queues' for tap :)
> - The block layer limits the number of hw queues to the number of
> vcpus. This applies only to virtio devices that interact with the
> block layer, but regardless of the virtio transport.
Yes: virtio-blk and virtio-scsi.
>
>> That's why I think virtio-blk should use the similar solution as nvme
>> (regardless about write_queues and poll_queues) and xen-blkfront.
>
> Ok, the hw queues limit from above would be an argument to limit to
> #vcpus in the virtio-blk driver, regardless of the transport used. (No
> idea if there are better ways to deal with this, I'm not familiar with
> the interface.)
>
> For virtio devices that don't interact with the block layer and are
> attached via the virtio-pci transport, it might still make sense to
> revisit vector allocation.
>
As mentioned above, we need to specify 'vectors' for virtio-net as the default
value is only 3 (config + tx + rx). That would make a little difference?
Dongli Zhang
On 2019/3/14 下午2:12, Dongli Zhang wrote:
>
> On 3/13/19 5:39 PM, Cornelia Huck wrote:
>> On Wed, 13 Mar 2019 11:26:04 +0800
>> Dongli Zhang <[email protected]> wrote:
>>
>>> On 3/13/19 1:33 AM, Cornelia Huck wrote:
>>>> On Tue, 12 Mar 2019 10:22:46 -0700 (PDT)
>>>> Dongli Zhang <[email protected]> wrote:
>>>>
>>>>> I observed that there is one msix vector for config and one shared vector
>>>>> for all queues in below qemu cmdline, when the num-queues for virtio-blk
>>>>> is more than the number of possible cpus:
>>>>>
>>>>> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6"
>>>>>
>>>>> # cat /proc/interrupts
>>>>> CPU0 CPU1 CPU2 CPU3
>>>>> ... ...
>>>>> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
>>>>> 25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues
>>>>> ... ...
>>>>>
>>>>>
>>>>> However, when num-queues is the same as number of possible cpus:
>>>>>
>>>>> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4"
>>>>>
>>>>> # cat /proc/interrupts
>>>>> CPU0 CPU1 CPU2 CPU3
>>>>> ... ...
>>>>> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
>>>>> 25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0
>>>>> 26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1
>>>>> 27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2
>>>>> 28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3
>>>>> ... ...
>>>>>
>>>>> In above case, there is one msix vector per queue.
>>>> Please note that this is pci-specific...
>>>>
>>>>>
>>>>> This is because the max number of queues is not limited by the number of
>>>>> possible cpus.
>>>>>
>>>>> By default, nvme (regardless about write_queues and poll_queues) and
>>>>> xen-blkfront limit the number of queues with num_possible_cpus().
>>>> ...and these are probably pci-specific as well.
>>> Not pci-specific, but per-cpu as well.
>> Ah, I meant that those are pci devices.
>>
>>>>
>>>>>
>>>>> Is this by design on purpose, or can we fix with below?
>>>>>
>>>>>
>>>>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
>>>>> index 4bc083b..df95ce3 100644
>>>>> --- a/drivers/block/virtio_blk.c
>>>>> +++ b/drivers/block/virtio_blk.c
>>>>> @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
>>>>> if (err)
>>>>> num_vqs = 1;
>>>>>
>>>>> + num_vqs = min(num_possible_cpus(), num_vqs);
>>>>> +
>>>>> vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
>>>>> if (!vblk->vqs)
>>>>> return -ENOMEM;
>>>> virtio-blk, however, is not pci-specific.
>>>>
>>>> If we are using the ccw transport on s390, a completely different
>>>> interrupt mechanism is in use ('floating' interrupts, which are not
>>>> per-cpu). A check like that should therefore not go into the generic
>>>> driver.
>>>>
>>> So far there seems two options.
>>>
>>> The 1st option is to ask the qemu user to always specify "-num-queues" with the
>>> same number of vcpus when running x86 guest with pci for virtio-blk or
>>> virtio-scsi, in order to assign a vector for each queue.
>> That does seem like an extra burden for the user: IIUC, things work
>> even if you have too many queues, it's just not optimal. It sounds like
>> something that can be done by a management layer (e.g. libvirt), though.
>>
>>> Or, is it fine for virtio folks to add a new hook to 'struct virtio_config_ops'
>>> so that different platforms (e.g., pci or ccw) would use different ways to limit
>>> the max number of queues in guest, with something like below?
>> That sounds better, as both transports and drivers can opt-in here.
>>
>> However, maybe it would be even better to try to come up with a better
>> strategy of allocating msix vectors in virtio-pci. More vectors in the
>> num_queues > num_cpus case, even if they still need to be shared?
>> Individual vectors for n-1 cpus and then a shared one for the remaining
>> queues?
>>
>> It might even be device-specific: Have some low-traffic status queues
>> share a vector, and provide an individual vector for high-traffic
>> queues. Would need some device<->transport interface, obviously.
>>
> This sounds a little bit similar to multiple hctx maps?
>
> So far, as virtio-blk only supports set->nr_maps = 1, no matter how many hw
> queues are assigned for virtio-blk, blk_mq_alloc_tag_set() would use at most
> nr_cpu_ids hw queues.
>
> 2981 int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
> ... ...
> 3021 /*
> 3022 * There is no use for more h/w queues than cpus if we just have
> 3023 * a single map
> 3024 */
> 3025 if (set->nr_maps == 1 && set->nr_hw_queues > nr_cpu_ids)
> 3026 set->nr_hw_queues = nr_cpu_ids;
>
> Even the block layer would limit the number of hw queues by nr_cpu_ids when
> (set->nr_maps == 1).
>
> That's why I think virtio-blk should use the similar solution as nvme
> (regardless about write_queues and poll_queues) and xen-blkfront.
>
> Added Jason again. I do not know why the mailing list of
> [email protected] always filters out Jason's email...
>
>
> Dongli Zhang
Or something like I proposed several years ago?
https://do-db2.lkml.org/lkml/2014/12/25/169
Btw, for virtio-net, I think we actually want to go for having a maximum
number of supported queues like what hardware did. This would be useful
for e.g cpu hotplug or XDP (requires per cpu TX queue). But the current
vector allocation doesn't support this which will results all virtqueues
to share a single vector. We may indeed need more flexible policy here.
Thanks
On Fri, 15 Mar 2019 12:50:11 +0800
Jason Wang <[email protected]> wrote:
> Or something like I proposed several years ago?
> https://do-db2.lkml.org/lkml/2014/12/25/169
>
> Btw, for virtio-net, I think we actually want to go for having a maximum
> number of supported queues like what hardware did. This would be useful
> for e.g cpu hotplug or XDP (requires per cpu TX queue). But the current
> vector allocation doesn't support this which will results all virtqueues
> to share a single vector. We may indeed need more flexible policy here.
I think it should be possible for the driver to give the transport
hints how to set up their queues/interrupt structures. (The driver
probably knows best about its requirements.) Perhaps whether a queue is
high or low frequency, or whether it should be low latency, or even
whether two queues could share a notification mechanism without
drawbacks. It's up to the transport to make use of that information, if
possible.
On 2019/3/15 下午8:41, Cornelia Huck wrote:
> On Fri, 15 Mar 2019 12:50:11 +0800
> Jason Wang <[email protected]> wrote:
>
>> Or something like I proposed several years ago?
>> https://do-db2.lkml.org/lkml/2014/12/25/169
>>
>> Btw, for virtio-net, I think we actually want to go for having a maximum
>> number of supported queues like what hardware did. This would be useful
>> for e.g cpu hotplug or XDP (requires per cpu TX queue). But the current
>> vector allocation doesn't support this which will results all virtqueues
>> to share a single vector. We may indeed need more flexible policy here.
> I think it should be possible for the driver to give the transport
> hints how to set up their queues/interrupt structures. (The driver
> probably knows best about its requirements.) Perhaps whether a queue is
> high or low frequency, or whether it should be low latency, or even
> whether two queues could share a notification mechanism without
> drawbacks. It's up to the transport to make use of that information, if
> possible.
Exactly and it was what the above series tried to do by providing hints
of e.g which queues want to share a notification.
Thanks
Hi Jason,
On 3/18/19 3:47 PM, Jason Wang wrote:
>
> On 2019/3/15 下午8:41, Cornelia Huck wrote:
>> On Fri, 15 Mar 2019 12:50:11 +0800
>> Jason Wang <[email protected]> wrote:
>>
>>> Or something like I proposed several years ago?
>>> https://do-db2.lkml.org/lkml/2014/12/25/169
>>>
>>> Btw, for virtio-net, I think we actually want to go for having a maximum
>>> number of supported queues like what hardware did. This would be useful
>>> for e.g cpu hotplug or XDP (requires per cpu TX queue). But the current
>>> vector allocation doesn't support this which will results all virtqueues
>>> to share a single vector. We may indeed need more flexible policy here.
>> I think it should be possible for the driver to give the transport
>> hints how to set up their queues/interrupt structures. (The driver
>> probably knows best about its requirements.) Perhaps whether a queue is
>> high or low frequency, or whether it should be low latency, or even
>> whether two queues could share a notification mechanism without
>> drawbacks. It's up to the transport to make use of that information, if
>> possible.
>
>
> Exactly and it was what the above series tried to do by providing hints of e.g
> which queues want to share a notification.
>
I read about your patch set on providing more flexibility of queue-to-vector
mapping.
One use case of the patch set is we would be able to enable more queues when
there is limited number of vectors.
Another use case we may classify queues as hight priority or low priority as
mentioned by Cornelia.
For virtio-blk, we may extend virtio-blk based on this patch set to enable
something similar to write_queues/poll_queues in nvme, when (set->nr_maps != 1).
Yet, the question I am asking in this email thread is for a difference scenario.
The issue is not we are not having enough vectors (although this is why only 1
vector is allocated for all virtio-blk queues). As so far virtio-blk has
(set->nr_maps == 1), block layer would limit the number of hw queues by
nr_cpu_ids, we indeed do not need more than nr_cpu_ids hw queues in virtio-blk.
That's why I ask why not change the flow as below options when the number of
supported hw queues is more than nr_cpu_ids (and set->nr_maps == 1. virtio-blk
does not set nr_maps and block layer would set it to 1 when the driver does not
specify with a value):
option 1:
As what nvme and xen-netfront do, limit the hw queue number by nr_cpu_ids.
option 2:
If the vectors is not enough, use the max number vector (indeed nr_cpu_ids) as
number of hw queues.
option 3:
We should allow more vectors even the block layer would support at most
nr_cpu_ids queues.
I understand a new policy for queue-vector mapping is very helpful. I am just
asking the question from block layer's point of view.
Thank you very much!
Dongli Zhang
On 2019/3/19 上午10:22, Dongli Zhang wrote:
> Hi Jason,
>
> On 3/18/19 3:47 PM, Jason Wang wrote:
>> On 2019/3/15 下午8:41, Cornelia Huck wrote:
>>> On Fri, 15 Mar 2019 12:50:11 +0800
>>> Jason Wang <[email protected]> wrote:
>>>
>>>> Or something like I proposed several years ago?
>>>> https://do-db2.lkml.org/lkml/2014/12/25/169
>>>>
>>>> Btw, for virtio-net, I think we actually want to go for having a maximum
>>>> number of supported queues like what hardware did. This would be useful
>>>> for e.g cpu hotplug or XDP (requires per cpu TX queue). But the current
>>>> vector allocation doesn't support this which will results all virtqueues
>>>> to share a single vector. We may indeed need more flexible policy here.
>>> I think it should be possible for the driver to give the transport
>>> hints how to set up their queues/interrupt structures. (The driver
>>> probably knows best about its requirements.) Perhaps whether a queue is
>>> high or low frequency, or whether it should be low latency, or even
>>> whether two queues could share a notification mechanism without
>>> drawbacks. It's up to the transport to make use of that information, if
>>> possible.
>>
>> Exactly and it was what the above series tried to do by providing hints of e.g
>> which queues want to share a notification.
>>
> I read about your patch set on providing more flexibility of queue-to-vector
> mapping.
>
> One use case of the patch set is we would be able to enable more queues when
> there is limited number of vectors.
>
> Another use case we may classify queues as hight priority or low priority as
> mentioned by Cornelia.
>
> For virtio-blk, we may extend virtio-blk based on this patch set to enable
> something similar to write_queues/poll_queues in nvme, when (set->nr_maps != 1).
>
>
> Yet, the question I am asking in this email thread is for a difference scenario.
>
> The issue is not we are not having enough vectors (although this is why only 1
> vector is allocated for all virtio-blk queues). As so far virtio-blk has
> (set->nr_maps == 1), block layer would limit the number of hw queues by
> nr_cpu_ids, we indeed do not need more than nr_cpu_ids hw queues in virtio-blk.
>
> That's why I ask why not change the flow as below options when the number of
> supported hw queues is more than nr_cpu_ids (and set->nr_maps == 1. virtio-blk
> does not set nr_maps and block layer would set it to 1 when the driver does not
> specify with a value):
>
> option 1:
> As what nvme and xen-netfront do, limit the hw queue number by nr_cpu_ids.
How do they limit the hw queue number? A command?
>
> option 2:
> If the vectors is not enough, use the max number vector (indeed nr_cpu_ids) as
> number of hw queues.
We can share vectors in this case.
>
> option 3:
> We should allow more vectors even the block layer would support at most
> nr_cpu_ids queues.
>
>
> I understand a new policy for queue-vector mapping is very helpful. I am just
> asking the question from block layer's point of view.
>
> Thank you very much!
>
> Dongli Zhang
Don't know much for block, cc Stefan for more idea.
Thanks
On 3/20/19 8:53 PM, Jason Wang wrote:
>
> On 2019/3/19 上午10:22, Dongli Zhang wrote:
>> Hi Jason,
>>
>> On 3/18/19 3:47 PM, Jason Wang wrote:
>>> On 2019/3/15 下午8:41, Cornelia Huck wrote:
>>>> On Fri, 15 Mar 2019 12:50:11 +0800
>>>> Jason Wang <[email protected]> wrote:
>>>>
>>>>> Or something like I proposed several years ago?
>>>>> https://do-db2.lkml.org/lkml/2014/12/25/169
>>>>>
>>>>> Btw, for virtio-net, I think we actually want to go for having a maximum
>>>>> number of supported queues like what hardware did. This would be useful
>>>>> for e.g cpu hotplug or XDP (requires per cpu TX queue). But the current
>>>>> vector allocation doesn't support this which will results all virtqueues
>>>>> to share a single vector. We may indeed need more flexible policy here.
>>>> I think it should be possible for the driver to give the transport
>>>> hints how to set up their queues/interrupt structures. (The driver
>>>> probably knows best about its requirements.) Perhaps whether a queue is
>>>> high or low frequency, or whether it should be low latency, or even
>>>> whether two queues could share a notification mechanism without
>>>> drawbacks. It's up to the transport to make use of that information, if
>>>> possible.
>>>
>>> Exactly and it was what the above series tried to do by providing hints of e.g
>>> which queues want to share a notification.
>>>
>> I read about your patch set on providing more flexibility of queue-to-vector
>> mapping.
>>
>> One use case of the patch set is we would be able to enable more queues when
>> there is limited number of vectors.
>>
>> Another use case we may classify queues as hight priority or low priority as
>> mentioned by Cornelia.
>>
>> For virtio-blk, we may extend virtio-blk based on this patch set to enable
>> something similar to write_queues/poll_queues in nvme, when (set->nr_maps != 1).
>>
>>
>> Yet, the question I am asking in this email thread is for a difference scenario.
>>
>> The issue is not we are not having enough vectors (although this is why only 1
>> vector is allocated for all virtio-blk queues). As so far virtio-blk has
>> (set->nr_maps == 1), block layer would limit the number of hw queues by
>> nr_cpu_ids, we indeed do not need more than nr_cpu_ids hw queues in virtio-blk.
>>
>> That's why I ask why not change the flow as below options when the number of
>> supported hw queues is more than nr_cpu_ids (and set->nr_maps == 1. virtio-blk
>> does not set nr_maps and block layer would set it to 1 when the driver does not
>> specify with a value):
>>
>> option 1:
>> As what nvme and xen-netfront do, limit the hw queue number by nr_cpu_ids.
>
>
> How do they limit the hw queue number? A command?
The max #queue is also limited by other factors, e.g., kernel param
configuration, xen dom0 configuration or nvme hardware support.
Here we would ignore those factors for simplicity and only talk about the
relation between #queue and #cpu.
About nvme pci:
Regardless about new write_queues and poll_queues, the default queue type number
is limited by num_possible_cpus() as line 2120 and 252.
2113 static int nvme_setup_io_queues(struct nvme_dev *dev)
2114 {
2115 struct nvme_queue *adminq = &dev->queues[0];
2116 struct pci_dev *pdev = to_pci_dev(dev->dev);
2117 int result, nr_io_queues;
2118 unsigned long size;
2119
2120 nr_io_queues = max_io_queues();
2121 result = nvme_set_queue_count(&dev->ctrl, &nr_io_queues);
250 static unsigned int max_io_queues(void)
251 {
252 return num_possible_cpus() + write_queues + poll_queues;
253 }
The cons of this is there might be many unused hw queues and vectors when
num_possible_cpus() is very very large while only a small number of cpu are
online. I am looking if there is way to improve this.
About xen-blkfront:
Indeed the max #queue is limited by num_online_cpus() when xen-blkfront module
is loaded as line 2733 and 2736.
2707 static int __init xlblk_init(void)
... ...
2710 int nr_cpus = num_online_cpus();
... ...
2733 if (xen_blkif_max_queues > nr_cpus) {
2734 pr_info("Invalid max_queues (%d), will use default max: %d.\n",
2735 xen_blkif_max_queues, nr_cpus);
2736 xen_blkif_max_queues = nr_cpus;
2737 }
The cons of this is the number of hw-queue/hctx is limited and cannot increase
after cpu hotplug. I am looking if there is way to improve this.
While both have cons for cpu hotplug, they are trying to make #vector
proportional to the number of cpu.
For xen-blkfront and virtio-blk, as (set=nr_maps == 1), the number of hw queue
is limited by nr_cpu_ids again at block layer.
As virtio-blk is a PCI device, can we use the solution in nvme, that is, to use
num_possible_cpus to limited the max queues in virtio-blk?
Thank you very much!
Dongli Zhang
>
>
>>
>> option 2:
>> If the vectors is not enough, use the max number vector (indeed nr_cpu_ids) as
>> number of hw queues.
>
>
> We can share vectors in this case.
>
>
>>
>> option 3:
>> We should allow more vectors even the block layer would support at most
>> nr_cpu_ids queues.
>>
>>
>> I understand a new policy for queue-vector mapping is very helpful. I am just
>> asking the question from block layer's point of view.
>>
>> Thank you very much!
>>
>> Dongli Zhang
>
>
> Don't know much for block, cc Stefan for more idea.
>
> Thanks
>
On Wed, Mar 20, 2019 at 08:53:33PM +0800, Jason Wang wrote:
>
> On 2019/3/19 上午10:22, Dongli Zhang wrote:
> > Hi Jason,
> >
> > On 3/18/19 3:47 PM, Jason Wang wrote:
> > > On 2019/3/15 下午8:41, Cornelia Huck wrote:
> > > > On Fri, 15 Mar 2019 12:50:11 +0800
> > > > Jason Wang <[email protected]> wrote:
> > option 3:
> > We should allow more vectors even the block layer would support at most
> > nr_cpu_ids queues.
> >
> >
> > I understand a new policy for queue-vector mapping is very helpful. I am just
> > asking the question from block layer's point of view.
> >
> > Thank you very much!
> >
> > Dongli Zhang
>
>
> Don't know much for block, cc Stefan for more idea.
Thanks for CCing me. I don't have much input at this stage.
Stefan