Commit b4c6f86ec2f64
("irq_work: Handle some irq_work in a per-CPU thread on PREEMPT_RT")
treat all irq_work without IRQ_WORK_HARD_IRQ flags to the lazy_list
in PREEMPT_RT. But this kind of irq_work still has some difference
with IRQ work set IRQ_WORK_LAZY. The difference as fellowing:
- With IRQ_WORK_LAZY: (type1)
This kind of work will be executed after the next time tick by wakeup
irq_workd, there will be more scheduling delays.
Let's mark this as type1
- Without IRQ_WORK_LAZY and IRQ_WORK_HARD_IRQ: (type2)
This kind of irq_work will have a faster response speed by wakeup
irq_workd from IPI interrupt.
Let's mark it as type2
I want to know if this difference is by design.
If this is by design, we have a problem that the irq_work of type2
will not execute as quickly as expected, it may be delayed by the
irq_work of type1.
Please consider the following scenarios:
If the CPU queued a type1 irq_work A, and then a type2 irq_work B.
But we won't make B executed quickly, because we won't issue the IPI
interrupt to wakeup irq_workd (the llist_add call will return false).
This PATCH will issue the IPI_IRQ_WORK to make B execute quickly.
One thing that needs to be optimized is that we now have
lazy_list.node.llist and lazy_work_raised which need to be granted
to be atomicity, disabled the local CPU IRQ to make this atomic.
There should be a better way to make these two variants to be atomically
and I can go in deep if this little problem is not by design, and need
to be fixed.
If these two types of irq_work should be the same with the priority.
maybe we should change.
if (!lazy_work || tick_nohz_tick_stopped()) {
arch_irq_work_raise();
}
to
if (!(lazy_work || rt_lazy_work) || tick_nohz_tick_stopped()) {
arch_irq_work_raise();
}
I'm uploading this patch just to explain the problem, hopefully
don't pay too much attention to the ugly changes below.
Signed-off-by: Schspa Shi <[email protected]>
---
kernel/irq_work.c | 40 +++++++++++++++++++++++++++++++++++++---
1 file changed, 37 insertions(+), 3 deletions(-)
diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index 7afa40fe5cc43..d5d0b720fac15 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -25,6 +25,7 @@
static DEFINE_PER_CPU(struct llist_head, raised_list);
static DEFINE_PER_CPU(struct llist_head, lazy_list);
static DEFINE_PER_CPU(struct task_struct *, irq_workd);
+static DEFINE_PER_CPU(bool, lazy_work_raised);
static void wake_irq_workd(void)
{
@@ -81,6 +82,7 @@ static void __irq_work_queue_local(struct irq_work *work)
bool rt_lazy_work = false;
bool lazy_work = false;
int work_flags;
+ unsigned long flags;
work_flags = atomic_read(&work->node.a_flags);
if (work_flags & IRQ_WORK_LAZY)
@@ -94,12 +96,35 @@ static void __irq_work_queue_local(struct irq_work *work)
else
list = this_cpu_ptr(&raised_list);
- if (!llist_add(&work->node.llist, list))
+ local_irq_save(flags);
+ if (!llist_add(&work->node.llist, list)) {
+ bool irq_raised;
+ /*
+ * In PREEMPT_RT, if we add a lazy work added to the list
+ * before, the work maybe not raised. We need a extra check
+ * for PREEMPT_RT.
+ */
+ irq_raised = !xchg(this_cpu_ptr(&lazy_work_raised), true);
+ local_irq_restore(flags);
+ if (unlikely(!irq_raised))
+ arch_irq_work_raise();
+
return;
+ }
/* If the work is "lazy", handle it from next tick if any */
- if (!lazy_work || tick_nohz_tick_stopped())
+ if (!lazy_work || tick_nohz_tick_stopped()) {
+ (void) xchg(this_cpu_ptr(&lazy_work_raised), true);
+ local_irq_restore(flags);
arch_irq_work_raise();
+ } else if (lazy_work || rt_lazy_work) {
+ /*
+ * The first added irq work not raise a irq work, we need to
+ * raise one for the next added irq work.
+ */
+ (void) xchg(this_cpu_ptr(&lazy_work_raised), false);
+ local_irq_restore(flags);
+ }
}
/* Enqueue the irq work @work on the current CPU */
@@ -151,9 +176,18 @@ bool irq_work_queue_on(struct irq_work *work, int cpu)
*/
if (IS_ENABLED(CONFIG_PREEMPT_RT) &&
!(atomic_read(&work->node.a_flags) & IRQ_WORK_HARD_IRQ)) {
+ unsigned long flags;
+
+ local_irq_save(flags);
+ if (!llist_add(&work->node.llist, &per_cpu(lazy_list, cpu))) {
+ if (!xchg(this_cpu_ptr(&lazy_work_raised), true))
+ arch_irq_work_raise();
- if (!llist_add(&work->node.llist, &per_cpu(lazy_list, cpu)))
+ local_irq_restore(flags);
goto out;
+ }
+ (void) xchg(this_cpu_ptr(&lazy_work_raised), true);
+ local_irq_restore(flags);
work = &per_cpu(irq_work_wakeup, cpu);
if (!irq_work_claim(work))
--
2.37.0
On 2022-07-12 01:23:15 [+0800], Schspa Shi wrote:
> I want to know if this difference is by design.
Yes. type1 (LAZY) does not need immediate action but can't be scheduled
regularly like a workqueue.
> If this is by design, we have a problem that the irq_work of type2
> will not execute as quickly as expected, it may be delayed by the
> irq_work of type1.
>
> Please consider the following scenarios:
>
> If the CPU queued a type1 irq_work A, and then a type2 irq_work B.
> But we won't make B executed quickly, because we won't issue the IPI
> interrupt to wakeup irq_workd (the llist_add call will return false).
But those two are different lists. So adding type1 to list1 does not
affect type2 with list2
> This PATCH will issue the IPI_IRQ_WORK to make B execute quickly.
>
> One thing that needs to be optimized is that we now have
> lazy_list.node.llist and lazy_work_raised which need to be granted
> to be atomicity, disabled the local CPU IRQ to make this atomic.
> There should be a better way to make these two variants to be atomically
> and I can go in deep if this little problem is not by design, and need
> to be fixed.
>
> If these two types of irq_work should be the same with the priority.
> maybe we should change.
>
> if (!lazy_work || tick_nohz_tick_stopped()) {
> arch_irq_work_raise();
> }
>
> to
>
> if (!(lazy_work || rt_lazy_work) || tick_nohz_tick_stopped()) {
> arch_irq_work_raise();
> }
but we wait for the timer for the lazy-work. RT has more LAZY items
compared to !RT. So if there is an error then it should be visible
there, too.
Is there a problem with this? Adding (as you call it) type1 item does
not affect type2 items. They will will processed asap.
Sebastian
Sebastian Andrzej Siewior <[email protected]> writes:
> On 2022-07-12 01:23:15 [+0800], Schspa Shi wrote:
>> I want to know if this difference is by design.
>
> Yes. type1 (LAZY) does not need immediate action but can't be scheduled
> regularly like a workqueue.
>
>> If this is by design, we have a problem that the irq_work of type2
>> will not execute as quickly as expected, it may be delayed by the
>> irq_work of type1.
>>
>> Please consider the following scenarios:
>>
>> If the CPU queued a type1 irq_work A, and then a type2 irq_work B.
>> But we won't make B executed quickly, because we won't issue the IPI
>> interrupt to wakeup irq_workd (the llist_add call will return false).
>
> But those two are different lists. So adding type1 to list1 does not
> affect type2 with list2
>
No, this will be added to same list (lazy_list).
All irq work without IRQ_WORK_HARD_IRQ flags will be added to lazy_list.
Maybe my description of type2 is not clear, type2 irq work means neither
the IRQ_WORK_LAZY flag nor the IRQ_WORK_HARD_IRQ flag is set.
>> This PATCH will issue the IPI_IRQ_WORK to make B execute quickly.
>>
>> One thing that needs to be optimized is that we now have
>> lazy_list.node.llist and lazy_work_raised which need to be granted
>> to be atomicity, disabled the local CPU IRQ to make this atomic.
>> There should be a better way to make these two variants to be atomically
>> and I can go in deep if this little problem is not by design, and need
>> to be fixed.
>>
>> If these two types of irq_work should be the same with the priority.
>> maybe we should change.
>>
>> if (!lazy_work || tick_nohz_tick_stopped()) {
>> arch_irq_work_raise();
>> }
>>
>> to
>>
>> if (!(lazy_work || rt_lazy_work) || tick_nohz_tick_stopped()) {
>> arch_irq_work_raise();
>> }
>
> but we wait for the timer for the lazy-work. RT has more LAZY items
> compared to !RT. So if there is an error then it should be visible
> there, too.
>
As type 2 work and type 1 work will be added to lazy_list, type 2 work
can be delayed and have same priority as type 1.
> Is there a problem with this? Adding (as you call it) type1 item does
> not affect type2 items. They will will processed asap.
>
I noticed this because there is a BUG before the patch
b4c6f86ec2f6 ("irq_work: Handle some irq_work in a per-CPU thread on PREEMPT_RT")
applied, which makes the task hang on when the CPU hotplug.
On some RT branches, lazy_work will be queued to ksoftirq via commit
https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/kernel/irq_work.c?h=linux-5.10.y-rt&id=c1ecdc62c514c2d541490026c312ec614ebd35aa
c1ecdc62c5 ("irqwork: push most work into softirq context")
Which makes the irq_work won't be executed due to we don't call arch_irq_work_raise();
and raise_softirq(TIMER_SOFTIRQ); won't be executed by this case too.
If there is no timer exists on the current CPU, it will hang forever.
Log as fellowing.
[32987.846092] INFO: task core_ctl:749 blocked for more than 120 seconds.
[32987.846106] Tainted: G O 5.10.59-rt52-g19228cd9c280-dirty #24
[32987.846117] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[32987.846125] task:core_ctl state:D stack: 0 pid: 749 ppid: 2 flags:0x00000028
[32987.846149] Call trace:
[32987.846155] __switch_to+0x164/0x17c
[32987.846175] __schedule+0x4cc/0x5c0
[32987.846190] schedule+0x7c/0xcc
[32987.846205] schedule_timeout+0x34/0xdc
[32987.846338] do_wait_for_common+0xa0/0x12c
[32987.846360] wait_for_common+0x44/0x68
[32987.846376] wait_for_completion+0x18/0x24
[32987.846391] __cpuhp_kick_ap+0x58/0x68
[32987.846408] cpuhp_kick_ap+0x38/0x94
[32987.846423] _cpu_down+0xbc/0x1f8
[32987.846443] cpu_down_maps_locked+0x20/0x34
[32987.846461] cpu_device_down+0x24/0x40
[32987.846477] cpu_subsys_offline+0x10/0x1c
[32987.846496] device_offline+0x6c/0xbc
[32987.846514] remove_cpu+0x24/0x40
[32987.846530] do_core_ctl+0x44/0x88 [cpuhp_qos]
[32987.846563] try_core_ctl+0x90/0xb0 [cpuhp_qos]
[32987.846587] kthread+0x114/0x124
[32987.846604] ret_from_fork+0x10/0x30
Please notice this patch is only used to explain the problem, don't
try to compile it.
> Sebastian
--
BRs
Schspa Shi
Schspa Shi <[email protected]> writes:
> Sebastian Andrzej Siewior <[email protected]> writes:
>
>> On 2022-07-12 01:23:15 [+0800], Schspa Shi wrote:
>>> I want to know if this difference is by design.
>>
>> Yes. type1 (LAZY) does not need immediate action but can't be scheduled
>> regularly like a workqueue.
>>
>>> If this is by design, we have a problem that the irq_work of type2
>>> will not execute as quickly as expected, it may be delayed by the
>>> irq_work of type1.
>>>
>>> Please consider the following scenarios:
>>>
>>> If the CPU queued a type1 irq_work A, and then a type2 irq_work B.
>>> But we won't make B executed quickly, because we won't issue the IPI
>>> interrupt to wakeup irq_workd (the llist_add call will return false).
>>
>> But those two are different lists. So adding type1 to list1 does not
>> affect type2 with list2
>>
>
> No, this will be added to same list (lazy_list).
> All irq work without IRQ_WORK_HARD_IRQ flags will be added to lazy_list.
> Maybe my description of type2 is not clear, type2 irq work means neither
> the IRQ_WORK_LAZY flag nor the IRQ_WORK_HARD_IRQ flag is set.
>
>>> This PATCH will issue the IPI_IRQ_WORK to make B execute quickly.
>>>
>>> One thing that needs to be optimized is that we now have
>>> lazy_list.node.llist and lazy_work_raised which need to be granted
>>> to be atomicity, disabled the local CPU IRQ to make this atomic.
>>> There should be a better way to make these two variants to be atomically
>>> and I can go in deep if this little problem is not by design, and need
>>> to be fixed.
>>>
>>> If these two types of irq_work should be the same with the priority.
>>> maybe we should change.
>>>
>>> if (!lazy_work || tick_nohz_tick_stopped()) {
>>> arch_irq_work_raise();
>>> }
>>>
>>> to
>>>
>>> if (!(lazy_work || rt_lazy_work) || tick_nohz_tick_stopped()) {
>>> arch_irq_work_raise();
>>> }
>>
>> but we wait for the timer for the lazy-work. RT has more LAZY items
>> compared to !RT. So if there is an error then it should be visible
>> there, too.
>>
>
> As type 2 work and type 1 work will be added to lazy_list, type 2 work
> can be delayed and have same priority as type 1.
>
>> Is there a problem with this? Adding (as you call it) type1 item does
>> not affect type2 items. They will will processed asap.
>>
>
> I noticed this because there is a BUG before the patch
> b4c6f86ec2f6 ("irq_work: Handle some irq_work in a per-CPU thread on PREEMPT_RT")
> applied, which makes the task hang on when the CPU hotplug.
>
> On some RT branches, lazy_work will be queued to ksoftirq via commit
> https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/kernel/irq_work.c?h=linux-5.10.y-rt&id=c1ecdc62c514c2d541490026c312ec614ebd35aa
> c1ecdc62c5 ("irqwork: push most work into softirq context")
>
> Which makes the irq_work won't be executed due to we don't call arch_irq_work_raise();
> and raise_softirq(TIMER_SOFTIRQ); won't be executed by this case too.
>
> If there is no timer exists on the current CPU, it will hang forever.
>
> Log as fellowing.
>
> [32987.846092] INFO: task core_ctl:749 blocked for more than 120 seconds.
> [32987.846106] Tainted: G O 5.10.59-rt52-g19228cd9c280-dirty #24
> [32987.846117] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [32987.846125] task:core_ctl state:D stack: 0 pid: 749 ppid: 2 flags:0x00000028
> [32987.846149] Call trace:
> [32987.846155] __switch_to+0x164/0x17c
> [32987.846175] __schedule+0x4cc/0x5c0
> [32987.846190] schedule+0x7c/0xcc
> [32987.846205] schedule_timeout+0x34/0xdc
> [32987.846338] do_wait_for_common+0xa0/0x12c
> [32987.846360] wait_for_common+0x44/0x68
> [32987.846376] wait_for_completion+0x18/0x24
> [32987.846391] __cpuhp_kick_ap+0x58/0x68
> [32987.846408] cpuhp_kick_ap+0x38/0x94
> [32987.846423] _cpu_down+0xbc/0x1f8
> [32987.846443] cpu_down_maps_locked+0x20/0x34
> [32987.846461] cpu_device_down+0x24/0x40
> [32987.846477] cpu_subsys_offline+0x10/0x1c
> [32987.846496] device_offline+0x6c/0xbc
> [32987.846514] remove_cpu+0x24/0x40
> [32987.846530] do_core_ctl+0x44/0x88 [cpuhp_qos]
> [32987.846563] try_core_ctl+0x90/0xb0 [cpuhp_qos]
> [32987.846587] kthread+0x114/0x124
> [32987.846604] ret_from_fork+0x10/0x30
>
Add missing logs to make this hang issues clear.
[32987.953814] NMI backtrace for cpu 3
[32987.953829] CPU: 3 PID: 30 Comm: cpuhp/3 Tainted: G O 5.10.59-rt52-g19228cd9c280-dirty #24
[32987.953849] Hardware name: Horizon Robotics Journey 5 DVB (DT)
[32987.953859] pstate: 60c00009 (nZCv daif +PAN +UAO -TCO BTYPE=--)
[32987.953876] pc : irq_work_sync+0x8/0x1c
[32987.953900] lr : cpufreq_dbs_governor_stop+0x50/0x7c
[32987.953923] sp : ffff80001280bd20
[32987.953930] pmr_save: 000000e0
[32987.953937] x29: ffff80001280bd20 x28: 0000000000000000
[32987.953960] x27: 0000000000000000 x26: 0000000000000000
[32987.953980] x25: 0000000000000000 x24: 0000000000000001
[32987.953999] x23: ffff800011403000 x22: ffff000183bf1800
[32987.954019] x21: ffff80001155e8e0 x20: 0000000000000080
[32987.954040] x19: ffff00018aafd400 x18: 0000000000000000
[32987.954059] x17: 0000000000000000 x16: 0000000000000000
[32987.954079] x15: 000000000000000a x14: 000000000000001e
[32987.954098] x13: ffff800012e2703c x12: ffffffffffffffff
[32987.954118] x11: ffffffffffffffff x10: 0000000000000a20
[32987.954138] x9 : ffff80001280bab0 x8 : ffff000180274480
[32987.954158] x7 : 00000000ffffffff x6 : ffff800012e255da
[32987.954177] x5 : ffff80001280bd00 x4 : 0000000000000000
[32987.954196] x3 : ffff80001280bd00 x2 : 0000000000000000
[32987.954216] x1 : 0000000000000023 x0 : ffff00018aafd448
[32987.954235] Call trace:
[32987.954243] irq_work_sync+0x8/0x1c
[32987.954263] cpufreq_stop_governor+0x6c/0x80
[32987.954282] cpufreq_offline+0xc4/0x1ec
[32987.954300] cpuhp_cpufreq_offline+0x10/0x20
[32987.954317] cpuhp_invoke_callback+0xc0/0x1b0
[32987.954334] cpuhp_thread_fun+0x124/0x170
[32987.954350] smpboot_thread_fn+0x1e8/0x1ec
[32987.954367] kthread+0x114/0x124
[32987.954384] ret_from_fork+0x10/0x30
>
> Please notice this patch is only used to explain the problem, don't
> try to compile it.
>
>> Sebastian
--
BRs
Schspa Shi