On Thu, Sep 29, 2016 at 1:18 AM, Chris Wilson <[email protected]> wrote:
> On Thu, Sep 29, 2016 at 03:34:11PM +0800, Jisheng Zhang wrote:
>> On Marvell berlin arm64 platforms, I see the preemptoff tracer report
>> a max 26543 us latency at __purge_vmap_area_lazy, this latency is an
>> awfully bad for STB. And the ftrace log also shows __free_vmap_area
>> contributes most latency now. I noticed that Joel mentioned the same
>> issue[1] on x86 platform and gave two solutions, but it seems no patch
>> is sent out for this purpose.
>>
>> This patch adopts Joel's first solution, but I use 16MB per core
>> rather than 8MB per core for the number of lazy_max_pages. After this
>> patch, the preemptoff tracer reports a max 6455us latency, reduced to
>> 1/4 of original result.
>
> My understanding is that
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 91f44e78c516..3f7c6d6969ac 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -626,7 +626,6 @@ void set_iounmap_nonlazy(void)
> static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
> int sync, int force_flush)
> {
> - static DEFINE_SPINLOCK(purge_lock);
> struct llist_node *valist;
> struct vmap_area *va;
> struct vmap_area *n_va;
> @@ -637,12 +636,6 @@ static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
> * should not expect such behaviour. This just simplifies locking for
> * the case that isn't actually used at the moment anyway.
> */
> - if (!sync && !force_flush) {
> - if (!spin_trylock(&purge_lock))
> - return;
> - } else
> - spin_lock(&purge_lock);
> -
> if (sync)
> purge_fragmented_blocks_allcpus();
>
> @@ -667,7 +660,6 @@ static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
> __free_vmap_area(va);
> spin_unlock(&vmap_area_lock);
> }
> - spin_unlock(&purge_lock);
> }
>
[..]
> should now be safe. That should significantly reduce the preempt-disabled
> section, I think.
I believe that the purge_lock is supposed to prevent concurrent purges
from happening.
For the case where if you have another concurrent overflow happen in
alloc_vmap_area() between the spin_unlock and purge :
spin_unlock(&vmap_area_lock);
if (!purged)
purge_vmap_area_lazy();
Then the 2 purges would happen at the same time and could subtract
vmap_lazy_nr twice.
I had proposed to change it to mutex in [1]. How do you feel about
that? Let me know your suggestions, thanks. I am also Ok with reducing
the lazy_max_pages value.
[1] http://lkml.iu.edu/hypermail/linux/kernel/1603.2/04803.html
Regards,
Joel
On Sat, Oct 08, 2016 at 08:43:51PM -0700, Joel Fernandes wrote:
> On Thu, Sep 29, 2016 at 1:18 AM, Chris Wilson <[email protected]> wrote:
> > On Thu, Sep 29, 2016 at 03:34:11PM +0800, Jisheng Zhang wrote:
> >> On Marvell berlin arm64 platforms, I see the preemptoff tracer report
> >> a max 26543 us latency at __purge_vmap_area_lazy, this latency is an
> >> awfully bad for STB. And the ftrace log also shows __free_vmap_area
> >> contributes most latency now. I noticed that Joel mentioned the same
> >> issue[1] on x86 platform and gave two solutions, but it seems no patch
> >> is sent out for this purpose.
> >>
> >> This patch adopts Joel's first solution, but I use 16MB per core
> >> rather than 8MB per core for the number of lazy_max_pages. After this
> >> patch, the preemptoff tracer reports a max 6455us latency, reduced to
> >> 1/4 of original result.
> >
> > My understanding is that
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 91f44e78c516..3f7c6d6969ac 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -626,7 +626,6 @@ void set_iounmap_nonlazy(void)
> > static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
> > int sync, int force_flush)
> > {
> > - static DEFINE_SPINLOCK(purge_lock);
> > struct llist_node *valist;
> > struct vmap_area *va;
> > struct vmap_area *n_va;
> > @@ -637,12 +636,6 @@ static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
> > * should not expect such behaviour. This just simplifies locking for
> > * the case that isn't actually used at the moment anyway.
> > */
> > - if (!sync && !force_flush) {
> > - if (!spin_trylock(&purge_lock))
> > - return;
> > - } else
> > - spin_lock(&purge_lock);
> > -
> > if (sync)
> > purge_fragmented_blocks_allcpus();
> >
> > @@ -667,7 +660,6 @@ static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
> > __free_vmap_area(va);
> > spin_unlock(&vmap_area_lock);
> > }
> > - spin_unlock(&purge_lock);
> > }
> >
> [..]
> > should now be safe. That should significantly reduce the preempt-disabled
> > section, I think.
>
> I believe that the purge_lock is supposed to prevent concurrent purges
> from happening.
>
> For the case where if you have another concurrent overflow happen in
> alloc_vmap_area() between the spin_unlock and purge :
>
> spin_unlock(&vmap_area_lock);
> if (!purged)
> purge_vmap_area_lazy();
>
> Then the 2 purges would happen at the same time and could subtract
> vmap_lazy_nr twice.
That itself is not the problem, as each instance of
__purge_vmap_area_lazy() operates on its own freelist, and so there will
be no double accounting.
However, removing the lock removes the serialisation which does mean
that alloc_vmap_area() will not block on another thread conducting the
purge, and so it will try to reallocate before that is complete and the
free area made available. It also means that we are doing the
atomic_sub(vmap_lazy_nr) too early.
That supports making the outer lock a mutex as you suggested. But I think
cond_resched_lock() is better for the vmap_area_lock (just because it
turns out to be an expensive loop and we may want the reschedule).
-Chris
--
Chris Wilson, Intel Open Source Technology Centre
On Sun, Oct 9, 2016 at 5:42 AM, Chris Wilson <[email protected]> wrote:
[..]
>> > My understanding is that
>> >
>> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> > index 91f44e78c516..3f7c6d6969ac 100644
>> > --- a/mm/vmalloc.c
>> > +++ b/mm/vmalloc.c
>> > @@ -626,7 +626,6 @@ void set_iounmap_nonlazy(void)
>> > static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
>> > int sync, int force_flush)
>> > {
>> > - static DEFINE_SPINLOCK(purge_lock);
>> > struct llist_node *valist;
>> > struct vmap_area *va;
>> > struct vmap_area *n_va;
>> > @@ -637,12 +636,6 @@ static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
>> > * should not expect such behaviour. This just simplifies locking for
>> > * the case that isn't actually used at the moment anyway.
>> > */
>> > - if (!sync && !force_flush) {
>> > - if (!spin_trylock(&purge_lock))
>> > - return;
>> > - } else
>> > - spin_lock(&purge_lock);
>> > -
>> > if (sync)
>> > purge_fragmented_blocks_allcpus();
>> >
>> > @@ -667,7 +660,6 @@ static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
>> > __free_vmap_area(va);
>> > spin_unlock(&vmap_area_lock);
>> > }
>> > - spin_unlock(&purge_lock);
>> > }
>> >
>> [..]
>> > should now be safe. That should significantly reduce the preempt-disabled
>> > section, I think.
>>
>> I believe that the purge_lock is supposed to prevent concurrent purges
>> from happening.
>>
>> For the case where if you have another concurrent overflow happen in
>> alloc_vmap_area() between the spin_unlock and purge :
>>
>> spin_unlock(&vmap_area_lock);
>> if (!purged)
>> purge_vmap_area_lazy();
>>
>> Then the 2 purges would happen at the same time and could subtract
>> vmap_lazy_nr twice.
>
> That itself is not the problem, as each instance of
> __purge_vmap_area_lazy() operates on its own freelist, and so there will
> be no double accounting.
>
> However, removing the lock removes the serialisation which does mean
> that alloc_vmap_area() will not block on another thread conducting the
> purge, and so it will try to reallocate before that is complete and the
> free area made available. It also means that we are doing the
> atomic_sub(vmap_lazy_nr) too early.
>
> That supports making the outer lock a mutex as you suggested. But I think
> cond_resched_lock() is better for the vmap_area_lock (just because it
> turns out to be an expensive loop and we may want the reschedule).
> -Chris
Ok. So I'll submit a patch with mutex for purge_lock and use
cond_resched_lock for the vmap_area_lock as you suggested. I'll also
drop the lazy_max_pages to 8MB as Andi suggested to reduce the lock
hold time. Let me know if you have any objections.
Thanks,
Joel
On Sun, Oct 09, 2016 at 12:00:31PM -0700, Joel Fernandes wrote:
> Ok. So I'll submit a patch with mutex for purge_lock and use
> cond_resched_lock for the vmap_area_lock as you suggested. I'll also
> drop the lazy_max_pages to 8MB as Andi suggested to reduce the lock
> hold time. Let me know if you have any objections.
The downside of using a mutex here though, is that we may be called
from contexts that cannot sleep (alloc_vmap_area), or reschedule for
that matter! If we change the notion of purged, we can forgo the mutex
in favour of spinning on the direct reclaim path. That just leaves the
complication of whether to use cond_resched_lock() or a lock around
the individual __free_vmap_area().
-Chris
--
Chris Wilson, Intel Open Source Technology Centre
On Sun, Oct 9, 2016 at 12:26 PM, Chris Wilson <[email protected]> wrote:
> On Sun, Oct 09, 2016 at 12:00:31PM -0700, Joel Fernandes wrote:
>> Ok. So I'll submit a patch with mutex for purge_lock and use
>> cond_resched_lock for the vmap_area_lock as you suggested. I'll also
>> drop the lazy_max_pages to 8MB as Andi suggested to reduce the lock
>> hold time. Let me know if you have any objections.
>
> The downside of using a mutex here though, is that we may be called
> from contexts that cannot sleep (alloc_vmap_area), or reschedule for
> that matter! If we change the notion of purged, we can forgo the mutex
> in favour of spinning on the direct reclaim path. That just leaves the
> complication of whether to use cond_resched_lock() or a lock around
> the individual __free_vmap_area().
Good point. I agree with you. I think we still need to know if purging
is in progress to preserve previous trylock behavior. How about
something like the following diff? (diff is untested).
This drops the purge lock and uses a ref count to indicate if purging
is in progress, so that callers who don't want to purge if purging is
already in progress can be kept happy. Also I am reducing vmap_lazy_nr
as we go, and, not all at once, so that we don't reduce the counter
too soon as we're not holding purge lock anymore. Lastly, I added the
cond_resched as you suggested.
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index f2481cb..5616ca4 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -626,7 +626,7 @@ void set_iounmap_nonlazy(void)
static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
int sync, int force_flush)
{
- static DEFINE_SPINLOCK(purge_lock);
+ static atomic_t purging;
struct llist_node *valist;
struct vmap_area *va;
struct vmap_area *n_va;
@@ -638,10 +638,10 @@ static void __purge_vmap_area_lazy(unsigned long
*start, unsigned long *end,
* the case that isn't actually used at the moment anyway.
*/
if (!sync && !force_flush) {
- if (!spin_trylock(&purge_lock))
+ if (atomic_cmpxchg(&purging, 0, 1))
return;
} else
- spin_lock(&purge_lock);
+ atomic_inc(&purging);
if (sync)
purge_fragmented_blocks_allcpus();
@@ -655,9 +655,6 @@ static void __purge_vmap_area_lazy(unsigned long
*start, unsigned long *end,
nr += (va->va_end - va->va_start) >> PAGE_SHIFT;
}
- if (nr)
- atomic_sub(nr, &vmap_lazy_nr);
-
if (nr || force_flush)
flush_tlb_kernel_range(*start, *end);
@@ -665,9 +662,11 @@ static void __purge_vmap_area_lazy(unsigned long
*start, unsigned long *end,
spin_lock(&vmap_area_lock);
llist_for_each_entry_safe(va, n_va, valist, purge_list)
__free_vmap_area(va);
+ atomic_sub(1, &vmap_lazy_nr);
+ cond_resched_lock(&vmap_area_lock);
spin_unlock(&vmap_area_lock);
}
- spin_unlock(&purge_lock);
+ atomic_dec(&purging);
}
On Mon, Oct 10, 2016 at 10:06 PM, Joel Fernandes <[email protected]> wrote:
> On Sun, Oct 9, 2016 at 12:26 PM, Chris Wilson <[email protected]> wrote:
>> On Sun, Oct 09, 2016 at 12:00:31PM -0700, Joel Fernandes wrote:
>>> Ok. So I'll submit a patch with mutex for purge_lock and use
>>> cond_resched_lock for the vmap_area_lock as you suggested. I'll also
>>> drop the lazy_max_pages to 8MB as Andi suggested to reduce the lock
>>> hold time. Let me know if you have any objections.
>>
>> The downside of using a mutex here though, is that we may be called
>> from contexts that cannot sleep (alloc_vmap_area), or reschedule for
>> that matter! If we change the notion of purged, we can forgo the mutex
>> in favour of spinning on the direct reclaim path. That just leaves the
>> complication of whether to use cond_resched_lock() or a lock around
>> the individual __free_vmap_area().
>
> Good point. I agree with you. I think we still need to know if purging
> is in progress to preserve previous trylock behavior. How about
> something like the following diff? (diff is untested).
>
> This drops the purge lock and uses a ref count to indicate if purging
> is in progress, so that callers who don't want to purge if purging is
> already in progress can be kept happy. Also I am reducing vmap_lazy_nr
> as we go, and, not all at once, so that we don't reduce the counter
> too soon as we're not holding purge lock anymore. Lastly, I added the
> cond_resched as you suggested.
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index f2481cb..5616ca4 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -626,7 +626,7 @@ void set_iounmap_nonlazy(void)
> static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
> int sync, int force_flush)
> {
> - static DEFINE_SPINLOCK(purge_lock);
> + static atomic_t purging;
> struct llist_node *valist;
> struct vmap_area *va;
> struct vmap_area *n_va;
> @@ -638,10 +638,10 @@ static void __purge_vmap_area_lazy(unsigned long
> *start, unsigned long *end,
> * the case that isn't actually used at the moment anyway.
> */
> if (!sync && !force_flush) {
> - if (!spin_trylock(&purge_lock))
> + if (atomic_cmpxchg(&purging, 0, 1))
> return;
> } else
> - spin_lock(&purge_lock);
> + atomic_inc(&purging);
>
> if (sync)
> purge_fragmented_blocks_allcpus();
> @@ -655,9 +655,6 @@ static void __purge_vmap_area_lazy(unsigned long
> *start, unsigned long *end,
> nr += (va->va_end - va->va_start) >> PAGE_SHIFT;
> }
>
> - if (nr)
> - atomic_sub(nr, &vmap_lazy_nr);
> -
> if (nr || force_flush)
> flush_tlb_kernel_range(*start, *end);
>
> @@ -665,9 +662,11 @@ static void __purge_vmap_area_lazy(unsigned long
> *start, unsigned long *end,
> spin_lock(&vmap_area_lock);
> llist_for_each_entry_safe(va, n_va, valist, purge_list)
> __free_vmap_area(va);
> + atomic_sub(1, &vmap_lazy_nr);
> + cond_resched_lock(&vmap_area_lock);
> spin_unlock(&vmap_area_lock);
For this particular hunk, I forgot the braces. sorry, I meant to say:
@@ -665,9 +662,11 @@ static void __purge_vmap_area_lazy(unsigned long
*start, unsigned long *end,
spin_lock(&vmap_area_lock);
- llist_for_each_entry_safe(va, n_va, valist, purge_list)
+ llist_for_each_entry_safe(va, n_va, valist,
purge_list) {
__free_vmap_area(va);
+ atomic_sub(1, &vmap_lazy_nr);
+ cond_resched_lock(&vmap_area_lock);
+ }
spin_unlock(&vmap_area_lock);
Regards,
Joel