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In-Reply-To: <20231217161927.vewygmvxvddhxs65@airbuntu>
From: John Stultz <jstultz@google.com>
Date: Mon, 18 Dec 2023 12:23:21 -0800
Message-ID: <CANDhNCraerdXXpxZ4nDB=aaNmmMnH+2M2yQ=NkFAWECOX6T5CA@mail.gmail.com>
Subject: Re: [PATCH v6 01/20] sched: Unify runtime accounting across classes
To: Qais Yousef <qyousef@layalina.io>
Cc: LKML <linux-kernel@vger.kernel.org>, Peter Zijlstra <peterz@infradead.org>, 
	Joel Fernandes <joelaf@google.com>, Qais Yousef <qyousef@google.com>, Ingo Molnar <mingo@redhat.com>, 
	Juri Lelli <juri.lelli@redhat.com>, Vincent Guittot <vincent.guittot@linaro.org>, 
	Dietmar Eggemann <dietmar.eggemann@arm.com>, Valentin Schneider <vschneid@redhat.com>, 
	Steven Rostedt <rostedt@goodmis.org>, Ben Segall <bsegall@google.com>, 
	Zimuzo Ezeozue <zezeozue@google.com>, Youssef Esmat <youssefesmat@google.com>, 
	Mel Gorman <mgorman@suse.de>, Daniel Bristot de Oliveira <bristot@redhat.com>, Will Deacon <will@kernel.org>, 
	Waiman Long <longman@redhat.com>, Boqun Feng <boqun.feng@gmail.com>, 
	"Paul E . McKenney" <paulmck@kernel.org>, kernel-team@android.com, 
	Mike Galbraith <efault@gmx.de>, Daniel Bristot de Oliveira <bristot@kernel.org>
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On Sun, Dec 17, 2023 at 8:19=E2=80=AFAM Qais Yousef <qyousef@layalina.io> w=
rote:
> On 11/06/23 19:34, John Stultz wrote:
> > From: Peter Zijlstra <peterz@infradead.org>
> >
> > All classes use sched_entity::exec_start to track runtime and have
> > copies of the exact same code around to compute runtime.
> >
> > Collapse all that.
> >
...
> Looks like this actually got merged into tip via the deadline server work=
 :-)

Oh! That's great to see! The patch has been floating around for a while.

> Though not sure if I caught a bug here
>
> > diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
> > index 85590599b4d6..7595494ceb6d 100644
> > --- a/kernel/sched/stop_task.c
> > +++ b/kernel/sched/stop_task.c
> > @@ -70,18 +70,7 @@ static void yield_task_stop(struct rq *rq)
> >
> >  static void put_prev_task_stop(struct rq *rq, struct task_struct *prev=
)
> >  {
> > -     struct task_struct *curr =3D rq->curr;
> > -     u64 now, delta_exec;
> > -
> > -     now =3D rq_clock_task(rq);
> > -     delta_exec =3D now - curr->se.exec_start;
> > -     if (unlikely((s64)delta_exec < 0))
> > -             delta_exec =3D 0;
>
> If negative instead of returning for stopper task; we set delta_exec to 0
>
> > -
> > -     schedstat_set(curr->stats.exec_max,
> > -                   max(curr->stats.exec_max, delta_exec));
> > -
> > -     update_current_exec_runtime(curr, now, delta_exec);
>
> And curry on to do time accounting
>
> > +     update_curr_common(rq);
>
> But the new function will return early without doing accounting. Wouldn't=
 this
> re-introrduce 8f6189684eb4 ("sched: Fix migration thread runtime bogosity=
")?

Hrm. So first, good eye for catching this!
Looking through the code, much of the accounting logic we end up
skipping doesn't have much effect when delta_exec =3D 0, so it seems
mostly harmless to return early without the accounting.

Though, there is one side-effect that does get skipped, which is the
removed update_current_exec_runtime() unconditionally sets:
  curr->se.exec_start =3D now;

Which basically resets the accounting window.

From the commit, It's unclear how intentional this side-effect is for
the edge case where the interval is negative.

I can't say I've really wrapped my head around the cases where the
se.exec_start would get ahead of the rq_clock_task(), so it's not
clear in which cases we would want to reset the accounting window vs
wait for the rq_clock_task() to catch up.  But as this is getting
called from put_prev_task_stop(), it seems we're closing the
accounting window here anyway, and later set_next_task_stop() would be
called (which sets se.exec_start, resetting the accounting) to start
the accounting window again.

So you are right that there is a practical change in behavior, but I
don't think I see it having an effect.

But I've added Mike and Daniel to the CC in case I'm missing something.

thanks
-john