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Date: Tue, 26 Apr 2022 15:57:11 +0800
From: Hao Jia <jiahao.os@...edance.com>
To: Dietmar Eggemann <dietmar.eggemann@....com>, mingo@...hat.com,
peterz@...radead.org, juri.lelli@...hat.com,
vincent.guittot@...aro.org, rostedt@...dmis.org,
bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com
Cc: linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [PATCH v2 1/2] sched/core: Avoid obvious double
update_rq_clock warning
On 2022/4/25 Dietmar Eggemann wrote:
> On 22/04/2022 11:09, Hao Jia wrote:
>> When we use raw_spin_rq_lock to acquire the rq lock and have to
>> update the rq clock while holding the lock, the kernel may issue
>> a WARN_DOUBLE_CLOCK warning.
>>
>> Since we directly use raw_spin_rq_lock to acquire rq lock instead of
>> rq_lock, there is no corresponding change to rq->clock_update_flags.
>> In particular, we have obtained the rq lock of other cores,
>> the core rq->clock_update_flags may be RQCF_UPDATED at this time, and
>> then calling update_rq_clock will trigger the WARN_DOUBLE_CLOCK warning.
>>
>> So we need to clear RQCF_UPDATED of rq->clock_update_flags synchronously
>> to avoid the WARN_DOUBLE_CLOCK warning.
>>
>> Some call trace reports:
>> Call Trace 1:
>> <IRQ>
>> sched_rt_period_timer+0x10f/0x3a0
>> ? enqueue_top_rt_rq+0x110/0x110
>> __hrtimer_run_queues+0x1a9/0x490
>> hrtimer_interrupt+0x10b/0x240
>
> [...]
>
> For the sched_rt_period_timer() case you simply replace
> raw_spin_rq_lock()/raw_spin_rq_unlock() with rq_lock()/rq_unlock().
> I.e. you're not using the new double_rq_clock_clear_update() which
> is depicted in the text above.
>
Thanks for your review comments.
Yes, adding this description would make it clearer.
I will do it in patch v3.
Thanks.
>> Call Trace 2:
>> <TASK>
>> activate_task+0x8b/0x110
>> push_rt_task.part.108+0x241/0x2c0
>> push_rt_tasks+0x15/0x30
>> finish_task_switch+0xaa/0x2e0
>> ? __switch_to+0x134/0x420
>> __schedule+0x343/0x8e0
>
> [...]
>
>> Call Trace 3:
>> <TASK>
>> deactivate_task+0x93/0xe0
>> pull_rt_task+0x33e/0x400
>> balance_rt+0x7e/0x90
>> __schedule+0x62f/0x8e0
>> do_task_dead+0x3f/0x50
>
> [...]
>
>> Steps to reproduce:
>> 1. Enable CONFIG_SCHED_DEBUG when compiling the kernel
>> 2. echo 1 > /sys/kernel/debug/clear_warn_once
>> echo "WARN_DOUBLE_CLOCK" > /sys/kernel/debug/sched_features
>
> s/sched_features/sched/features
I will fix it in patch v3.
Thanks.
>
>> echo "NO_RT_PUSH_IPI" > /sys/kernel/debug/sched_features
>
> s/sched_features/sched/features
I will fix it in patch v3.
Thanks.
>
>> 3. Run some rt tasks that periodically change the priority and sleep
>
> Not sure about the `change the priority` part? I'm using rt-app with 2*n
> rt or dl (90% running) tasks (on a system with n CPUs) and can trigger
> all of these cases.
Yes, this problem is relatively easy to reproduce. I wrote a test case
for rt scheduler to trigger all cases. I create 150 rt threads (my test
environment has 96 cpus), change priority and sleep periodically.
Constantly changing priorities in order to trigger push/pull_rt_task.
void *ThreadFun(void *arg)
{
int cnt = *(int*)arg;
struct sched_param param;
while (1) {
sqrt(MAGIC_NUM);
cnt = cnt % 10 + 1;
param.sched_priority = cnt;
pthread_setschedparam(pthread_self(), SCHED_RR, ¶m);
sqrt(MAGIC_NUM);
sqrt(MAGIC_NUM);
sleep(cnt);
}
return NULL;
}
I will try to reproduce again using rt-app, and will change the
description of the reproduction steps in patch v3.
Thanks.
>
>> Signed-off-by: Hao Jia <jiahao.os@...edance.com>
>> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
>
> You can remove this Signed-off-by. I can provide a Reviewed-By for the
> next version.
OK, Thanks again for your review and suggestion, it really helped me.
>
> [...]
>
> When running PREEMPT_RT kernel there is another similar case in DL.
>
> [ 215.158100] ------------[ cut here ]------------
> [ 215.158105] rq->clock_update_flags & RQCF_UPDATED
> [ 215.158113] WARNING: CPU: 2 PID: 1942 at kernel/sched/core.c:690 update_rq_clock+0x128/0x1a0
> ...
> [ 215.158245] update_rq_clock+0x128/0x1a0
> [ 215.158253] migrate_task_rq_dl+0xec/0x310 <--- !!!
> [ 215.158259] set_task_cpu+0x84/0x1e4
> [ 215.158264] try_to_wake_up+0x1d8/0x5c0
> [ 215.158268] wake_up_process+0x1c/0x30
> [ 215.158272] hrtimer_wakeup+0x24/0x3c
> [ 215.158279] __hrtimer_run_queues+0x114/0x270
> [ 215.158285] hrtimer_interrupt+0xe8/0x244
> [ 215.158291] arch_timer_handler_phys+0x30/0x50
> [ 215.158301] handle_percpu_devid_irq+0x88/0x140
> [ 215.158306] generic_handle_domain_irq+0x40/0x60
> [ 215.158313] gic_handle_irq+0x48/0xe0
> [ 215.158320] call_on_irq_stack+0x2c/0x60
> [ 215.158327] do_interrupt_handler+0x80/0x84
>
> For a non_contending task, this is the same issue as in sched_rt_period_timer().
>
> -->8--
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index cfe7b40bc4ff..668a9910cd6d 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1804,6 +1804,7 @@ select_task_rq_dl(struct task_struct *p, int cpu, int flags)
>
> static void migrate_task_rq_dl(struct task_struct *p, int new_cpu __maybe_unused)
> {
> + struct rq_flags rf;
> struct rq *rq;
>
> if (READ_ONCE(p->__state) != TASK_WAKING)
> @@ -1815,7 +1816,7 @@ static void migrate_task_rq_dl(struct task_struct *p, int new_cpu __maybe_unused
> * from try_to_wake_up(). Hence, p->pi_lock is locked, but
> * rq->lock is not... So, lock it
> */
> - raw_spin_rq_lock(rq);
> + rq_lock(rq, &rf);
> if (p->dl.dl_non_contending) {
> update_rq_clock(rq);
> sub_running_bw(&p->dl, &rq->dl);
> @@ -1831,7 +1832,7 @@ static void migrate_task_rq_dl(struct task_struct *p, int new_cpu __maybe_unused
> put_task_struct(p);
> }
> sub_rq_bw(&p->dl, &rq->dl);
> - raw_spin_rq_unlock(rq);
> + rq_unlock(rq, &rf);
> }
>
I will fix it in patch v3.
Thanks.
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