linux-kernel - Re: [PATCH] sched: Drop outdated compile-optimization comment

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Message-ID: <YqyCK+31YgOlBY9U@worktop.programming.kicks-ass.net>
Date:   Fri, 17 Jun 2022 15:31:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     Brian Norris <briannorris@...omium.org>,
        Ingo Molnar <mingo@...hat.com>,
        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Subject: Re: [PATCH] sched: Drop outdated compile-optimization comment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:
> On 15/06/22 15:27, Brian Norris wrote:
> > Looks like this exists from way back in 2011 (commit 095c0aa83e52
> > ("sched: adjust scheduler cpu power for stolen time")), when there was a
> > little more aggressive use of #if around these variables. That #if is
> > gone, and the comment just confuses the reader now. (For one, we don't
> > call sched_rt_avg_update() directly any more either.)
> >
> 
> So that sched_rt_avg_update() became update_irq_load_avg() with
> 
>   91c27493e78d ("sched/irq: Add IRQ utilization tracking")
> 
> and then the #ifdef configs were reorganized in
> 
>   11d4afd4ff66 ("sched/pelt: Fix warning and clean up IRQ PELT config")
> 
> 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
> 
> 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.

Mostly I think it was the jump_label stuff getting them confused. I
suspect that's fixed in todays compilers tho, so yeah, it might be good
to get rid of it.