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

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Message-ID: <20201207160648.GF2657@paulmck-ThinkPad-P72>
Date:   Mon, 7 Dec 2020 08:06:48 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Marco Elver <elver@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: Re: timers: Move clearing of base::timer_running under base::lock

On Mon, Dec 07, 2020 at 04:25:33PM +0100, Sebastian Andrzej Siewior wrote:
> 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.

Given that a lot of RCU processing is event-driven based on timers,
and given that the scheduling-clock interrupts are synchronized for
energy-efficiency reasons on many configs, maybe this alignment is
expected behavior?

							Thanx, Paul

> |          <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