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Subject: Re: [PATCH 5/5] sched/deadline: Use return value of SCHED_WARN_ON()
 in bw accounting
To:     Peter Zijlstra <peterz@infradead.org>,
        luca abeni <luca.abeni@santannapisa.it>
Cc:     Ingo Molnar <mingo@kernel.org>, Juri Lelli <juri.lelli@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <Valentin.Schneider@arm.com>,
        Qais Yousef <Qais.Yousef@arm.com>, linux-kernel@vger.kernel.org
References: <20190726082756.5525-1-dietmar.eggemann@arm.com>
 <20190726082756.5525-6-dietmar.eggemann@arm.com>
 <20190726121819.32be6fb1@sweethome>
 <20190729165434.GO31398@hirez.programming.kicks-ass.net>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Message-ID: <a12e2330-50d4-b31f-14b5-5b386252d418@arm.com>
Date:   Mon, 29 Jul 2019 17:59:04 +0100
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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@arm.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; })