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

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Date:   Tue, 30 Jul 2019 17:08:35 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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 7/30/19 9:23 AM, Peter Zijlstra wrote:
> 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.

You're right, compiling is not an issue. But it looks like the code does
different things depending on CONFIG_SCHED_DEBUG.

E.g. in set_last_buddy() we would return if '!se->on_rq' with
CONFIG_SCHED_DEBUG and continue the for_each_sched_entity() with
!CONFIG_SCHED_DEBUG.

IMHO, this forced Luca e.g. in __sub_running_bw() to code:

SCHED_WARN_ON(dl_rq->running_bw > old);
if (dl_rq->running_bw > old)
    dl_rq->running_bw = 0;

and not:

if (SCHED_WARN_ON(dl_rq->running_bw > old))
    dl_rq->running_bw = 0;

I experimented with '# define SCHED_WARN_ON(x) ({ (void)(x), x; })' and
in this case code inside an 'if (SCHED_WARN_ON(cond))' is also executed
with !CONFIG_SCHED_DEBUG.