linux-kernel - Re: [PATCH v8 00/25] timer: Move from a push remote at enqueue to a pull at expiry model

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Message-ID: <87cyxa8zsj.fsf@somnus>
Date:   Thu, 19 Oct 2023 16:14:36 +0200
From:   Anna-Maria Behnsen <anna-maria@...utronix.de>
To:     K Prateek Nayak <kprateek.nayak@....com>,
        linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Arjan van de Ven <arjan@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        Lukasz Luba <lukasz.luba@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        Srinivas Pandruvada <srinivas.pandruvada@...el.com>
Subject: Re: [PATCH v8 00/25] timer: Move from a push remote at enqueue to a
 pull at expiry model

Hello Prateek,

I'm sorry for the late reply!

K Prateek Nayak <kprateek.nayak@....com> writes:

> Hello Anna-Maria,
>
> On 10/4/2023 6:04 PM, Anna-Maria Behnsen wrote:
>> [..snip..]
>> 
>> Ping Pong Oberservation
>> ^^^^^^^^^^^^^^^^^^^^^^^
>> 
>> During testing on a mostly idle machine a ping pong game could be observed:
>> a process_timeout timer is expired remotely on a non idle CPU. Then the CPU
>> where the schedule_timeout() was executed to enqueue the timer comes out of
>> idle and restarts the timer using schedule_timeout() and goes back to idle
>> again. This is due to the fair scheduler which tries to keep the task on
>> the CPU which it previously executed on.
>
> Regarding above, are you referring to "wake_up_process(timeout->task)" in
> "process_timeout()" ends up waking the task on an idle CPU instead of the
> CPU where process_timeout() ran?

Yes.

> In which case, have you tried using the "WF_CURRENT_CPU" flag for the
> wakeup? (landed upstream in v6.6-rc1) It is only used by wait queues in
> kernel/sched/wait.c currently but perhaps we can have a
> "wake_up_process_on_current_cpu()" that process_timeout() can call.
>
> Something along the lines of:
>
> 	int wake_up_process_on_current_cpu(struct task_struct *p)
> 	{
> 		return try_to_wake_up(p, TASK_NORMAL, WF_CURRENT_CPU);
> 	}
> 	EXPORT_SYMBOL(wake_up_process_on_current_cpu);
>
> Thoughts?

I didn't look into this again. Back than, I reported the observation to
scheduler people (others also already observed this behavior). I'm not
so familiar with scheduling, so I will ping scheduler people to give you
a feedback.

Thanks,

	Anna-Maria