linux-kernel - Re: [PATCH v3] sched: Don't try to catch up excess steal time.

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Message-ID: <CAJWu+opw2gohuwU2wie1eBghT0fL=fX60LdBu+_B2TjkHc4yyw@mail.gmail.com>
Date: Tue, 19 Nov 2024 09:22:20 +0900
From: Joel Fernandes <joelaf@...gle.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Suleiman Souhlal <suleiman@...gle.com>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, Paolo Bonzini <pbonzini@...hat.com>, seanjc@...gle.com, 
	Srikar Dronamraju <srikar@...ux.ibm.com>, David Woodhouse <dwmw2@...radead.org>, vineethrp@...gle.com, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, ssouhlal@...ebsd.org
Subject: Re: [PATCH v3] sched: Don't try to catch up excess steal time.

On Tue, Nov 19, 2024 at 9:17 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Tue, 19 Nov 2024 09:10:41 +0900
> Joel Fernandes <joelaf@...gle.com> wrote:
>
> > > > +++ b/kernel/sched/core.c
> > > > @@ -766,13 +766,15 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
> > > >  #endif
> > > >  #ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
> > > >       if (static_key_false((&paravirt_steal_rq_enabled))) {
> > > > -             steal = paravirt_steal_clock(cpu_of(rq));
> > > > +             u64 prev_steal;
> > > > +
> > > > +             steal = prev_steal = paravirt_steal_clock(cpu_of(rq));
> > > >               steal -= rq->prev_steal_time_rq;
> > > >
> > > >               if (unlikely(steal > delta))
> > > >                       steal = delta;
> > >
> > > So is the problem just the above if statement? That is, delta is already
> > > calculated, but if we get interrupted by the host before steal is
> > > calculated and the time then becomes greater than delta, the time
> > > difference between delta and steal gets pushed off to the next task, right?
> >
> > Pretty much.. the steal being capped to delta means the rest of the
> > steal is pushed off to the future. Instead he discards the remaining
> > steal after this patch.
>
> Thanks for confirming. I just wanted to make sure I understand as the
> initial change log went into a lot of detail where I sorta got lost ;-)

No problem!! Glad we're on the same page about the change.

thanks,

 - Joel