linux-kernel - Re: [PATCH] sched: Drop outdated compile-optimization comment

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Message-ID: <YqzlL26IGdIHczaj@google.com>
Date:   Fri, 17 Jun 2022 13:33:51 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ben Segall <bsegall@...gle.com>, linux-kernel@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] sched: Drop outdated compile-optimization comment

On Fri, Jun 17, 2022 at 12:10:05PM +0100, Valentin Schneider wrote:
> I'd argue that comment is still somewhat relevant but it applies to that
> block:
> 
> #ifdef CONFIG_HAVE_SCHED_AVG_IRQ
> 	if ((irq_delta + steal) && sched_feat(NONTASK_CAPACITY))
> 		update_irq_load_avg(rq, irq_delta + steal);
> #endif

Eh, I suppose. The confusion still stands though ;)

> if !CONFIG_HAVE_SCHED_AVG_IRQ then yes you'd expect the compiler to not
> even add a call to update_irq_load_avg() in there, but compilers aren't the
> most trustworthy things :-) If you feel like it, you could play with
> GCC/clang and see what they emit if you remove those #ifdefs.

FWIW, update_irq_load_avg() is just "return 0" in that case. I think
that'd be considered excessive paranoia if you think a modern compiler
would still somehow produce a suboptimal result for that case :)

But anyway, I tried CONFIG_IRQ_TIME_ACCOUNTING=n +
CONFIG_PARAVIRT_TIME_ACCOUNTING=n with these compilers:

  x86 gcc 11.2.0 (Debian)
  x86 clang 13.0.1 (Debian)
  aarch64-linux-gnu-gcc 11.2.0 (Debian)

and they all dropped the update_irq_load_avg() even without the #ifdef.

I'll drop it in a v2, since that seems to be the consensus.

Brian