linux-kernel - Re: timers: Move clearing of base::timer

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Date:   Mon, 7 Dec 2020 16:25:33 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Marco Elver <elver@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: Re: timers: Move clearing of base::timer_running under base::lock

On 2020-12-07 15:29:50 [+0100], Thomas Gleixner wrote:
> On Mon, Dec 07 2020 at 14:07, Sebastian Andrzej Siewior wrote:
> > One thing I noticed while testing it is that the "corner" case in
> > timer_sync_wait_running() is quite reliably hit by rcu_preempt
> > rcu_gp_fqs_loop() -> swait_event_idle_timeout_exclusive() invocation.
> 
> I assume it's something like this:
> 
>      timeout -> wakeup
> 
> ->preemption
>         del_timer_sync()
>                 .....

Yes, but it triggers frequently. Like `rcuc' is somehow is aligned with
the timeout.

|          <idle>-0       [007] dN.h4..    46.299705: sched_wakeup: comm=rcuc/7 pid=53 prio=98 target_cpu=007
|          <idle>-0       [007] d...2..    46.299728: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcuc/7 next_pid=53 next_prio=98
|          rcuc/7-53      [007] d...2..    46.299742: sched_switch: prev_comm=rcuc/7 prev_pid=53 prev_prio=98 prev_state=S ==> next_comm=ksoftirqd/7 next_pid=54 next_prio=120
|     ksoftirqd/7-54      [007] .....13    46.299750: timer_expire_entry: timer=000000003bd1e045 function=process_timeout now=4294903802 baseclk=4294903802
|     ksoftirqd/7-54      [007] d...213    46.299750: sched_waking: comm=rcu_preempt pid=11 prio=98 target_cpu=007
|     ksoftirqd/7-54      [007] dN..313    46.299754: sched_wakeup: comm=rcu_preempt pid=11 prio=98 target_cpu=007
|     ksoftirqd/7-54      [007] dN..213    46.299756: sched_stat_runtime: comm=ksoftirqd/7 pid=54 runtime=13265 [ns] vruntime=3012610540 [ns]
|     ksoftirqd/7-54      [007] d...213    46.299760: sched_switch: prev_comm=ksoftirqd/7 prev_pid=54 prev_prio=120 prev_state=R+ ==> next_comm=rcu_preempt next_pid=11 next_prio=98
|     rcu_preempt-11      [007] d...311    46.299766: sched_pi_setprio: comm=ksoftirqd/7 pid=54 oldprio=120 newprio=98
del_timer_sync()
|     rcu_preempt-11      [007] d...211    46.299773: sched_switch: prev_comm=rcu_preempt prev_pid=11 prev_prio=98 prev_state=R+ ==> next_comm=ksoftirqd/7 next_pid=54 next_prio=98
|     ksoftirqd/7-54      [007] .....13    46.299774: timer_expire_exit: timer=000000003bd1e045
|     ksoftirqd/7-54      [007] dN..311    46.299784: sched_pi_setprio: comm=ksoftirqd/7 pid=54 oldprio=98 newprio=120
|     ksoftirqd/7-54      [007] dN..311    46.299788: sched_waking: comm=rcu_preempt pid=11 prio=98 target_cpu=007
|     ksoftirqd/7-54      [007] dN..411    46.299790: sched_wakeup: comm=rcu_preempt pid=11 prio=98 target_cpu=007
|     ksoftirqd/7-54      [007] dN..311    46.299792: sched_stat_runtime: comm=ksoftirqd/7 pid=54 runtime=7404 [ns] vruntime=3012617944 [ns]
|     ksoftirqd/7-54      [007] d...2..    46.299797: sched_switch: prev_comm=ksoftirqd/7 prev_pid=54 prev_prio=120 prev_state=S ==> next_comm=rcu_preempt next_pid=11 next_prio=98


> Thanks,
> 
>         tglx

Sebastian