linux-kernel - Re: [PATCH 5/5] sched/deadline: Use return value of SCHED_WARN

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Message-ID: <20190730082316.GK31381@hirez.programming.kicks-ass.net>
Date:   Tue, 30 Jul 2019 10:23:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     luca abeni <luca.abeni@...tannapisa.it>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <Valentin.Schneider@....com>,
        Qais Yousef <Qais.Yousef@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] sched/deadline: Use return value of SCHED_WARN_ON()
 in bw accounting

On Mon, Jul 29, 2019 at 05:59:04PM +0100, Dietmar Eggemann wrote:
> On 7/29/19 5:54 PM, Peter Zijlstra wrote:
> > On Fri, Jul 26, 2019 at 12:18:19PM +0200, luca abeni wrote:
> >> Hi Dietmar,
> >>
> >> On Fri, 26 Jul 2019 09:27:56 +0100
> >> Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >>
> >>> To make the decision whether to set rq or running bw to 0 in underflow
> >>> case use the return value of SCHED_WARN_ON() rather than an extra if
> >>> condition.
> >>
> >> I think I tried this at some point, but if I remember well this
> >> solution does not work correctly when SCHED_DEBUG is not enabled.
> > 
> > Well, it 'works' in so far that it compiles. But it might not be what
> > one expects. That is, for !SCHED_DEBUG the return value is an
> > unconditional false.
> > 
> > In this case I think that's fine, the WARN _should_ not be happending.
> 
> But there is commit 6d3aed3d ("sched/debug: Fix SCHED_WARN_ON() to
> return a value on !CONFIG_SCHED_DEBUG as well")?
> 
> But it doesn't work with !CONFIG_SCHED_DEBUG
> 
> What compiles and work is (at least on Arm64).
> 
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 4012f98b9d26..494a767a4091 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -78,7 +78,7 @@
>  #ifdef CONFIG_SCHED_DEBUG
>  # define SCHED_WARN_ON(x)      WARN_ONCE(x, #x)
>  #else
> -# define SCHED_WARN_ON(x)      ({ (void)(x), 0; })
> +# define SCHED_WARN_ON(x)      ({ (void)(x), x; })

Why doesn't the ,0 compile? That should work just fine. The thing is,
the two existing users:

kernel/sched/fair.c:            if (SCHED_WARN_ON(!se->on_rq))
kernel/sched/fair.c:            if (SCHED_WARN_ON(!se->on_rq))

seem to compile just fine with it.