linux-kernel - Re: [PATCH] RFC: sched: Rework task_sched_runtime to avoid calling update_rq

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Date: Fri, 14 Jun 2024 21:30:13 -0700
From: John Stultz <jstultz@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Qais Yousef <qyousef@...alina.io>, LKML <linux-kernel@...r.kernel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Frederic Weisbecker <frederic@...nel.org>, Ingo Molnar <mingo@...hat.com>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Daniel Bristot de Oliveira <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>, 
	Joel Fernandes <joel@...lfernandes.org>, kernel-team@...roid.com
Subject: Re: [PATCH] RFC: sched: Rework task_sched_runtime to avoid calling update_rq_clock

On Fri, Jun 14, 2024 at 2:48 AM Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Jun 13, 2024 at 12:51:42PM +0100, Qais Yousef wrote:
> > On 06/13/24 12:04, Peter Zijlstra wrote:
> > > @@ -5459,6 +5458,8 @@ void sched_tick(void)
> > >
> > >     sched_clock_tick();
> > >
> > > +   psi_account_irqtime(curr, &rq->psi_irq_time);
> > > +
> >
> > If wakeup preemption causes a context switch, wouldn't we lose this
> > information then? I *think* active migration might cause this information to be
> > lost too.
>
> I'm not sure what would be lost ?! the accounting is per cpu, not per
> task afaict. That said,...
>
> > pick_next_task() might be a better place to do the accounting?
>
> Additionally, when there has been an effective cgroup switch. Only on
> switch doesn't work for long running tasks, then the PSI information
> will be artitrarily long out of date.
>
> Which then gets me something like the (completely untested) below..
>
> Hmm?

I applied and booted with this. It still takes the accounting out of
the hotpath for the CLOCK_THREAD_CPUTIME_ID the microbenchmark
performance is back to 5.10 numbers.

I don't have any correctness tests for irqtime measurements, so I'll
have to try to work something up for that next week.

thanks
-john