linux-kernel - Re: [PATCH v5 3/4] sched, psi: Don't account irq time if sched_clock

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Message-ID: <ghu7irmizgbyso5hjemwsgscfoigdtzufpfckxkvdqibeo63uo@kvzncpy2rlit>
Date: Tue, 3 Dec 2024 11:15:24 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Yafang Shao <laoar.shao@...il.com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, 
	vincent.guittot@...aro.org, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, hannes@...xchg.org, 
	surenb@...gle.com, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/4] sched, psi: Don't account irq time if
 sched_clock_irqtime is disabled

On Tue, Dec 03, 2024 at 11:01:41AM GMT, Michal Koutný <mkoutny@...e.com> wrote:
> On Fri, Nov 08, 2024 at 09:29:03PM GMT, Yafang Shao <laoar.shao@...il.com> wrote:
> > sched_clock_irqtime may be disabled due to the clock source. When disabled,
> > irq_time_read() won't change over time, so there is nothing to account. We
> > can save iterating the whole hierarchy on every tick and context switch.
> > 
> > Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> > Acked-by: Johannes Weiner <hannes@...xchg.org>
> > ---
> >  kernel/sched/psi.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Reviewed-by: Michal Koutný <mkoutny@...e.com>

On second thought, similar guard may be useful in psi_show() too. Since
there's a difference between zero pressure and unmeasured pressure (it'd
fail with EOPNOTSUPP).

(How common is it actually that tsc_init fails?)

Michal

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