linux-kernel - Re: [PATCH v4 2/2] sched/uclamp: Protect uclamp fast path code with static key

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Message-ID: <jhja70ph0qu.mognet@arm.com>
Date:   Sat, 27 Jun 2020 00:21:29 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Patrick Bellasi <patrick.bellasi@...bug.net>
Cc:     Qais Yousef <qais.yousef@....com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Chris Redpath <chris.redpath@....com>,
        Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] sched/uclamp: Protect uclamp fast path code with static key


On 26/06/20 13:38, Patrick Bellasi wrote:
> On Thu, Jun 25, 2020 at 17:43:52 +0200, Qais Yousef <qais.yousef@....com> wrote...
>> @@ -994,9 +1013,16 @@ static inline void uclamp_rq_dec_id(struct rq *rq, struct task_struct *p,
>>      lockdep_assert_held(&rq->lock);
>>
>>      bucket = &uc_rq->bucket[uc_se->bucket_id];
>> -	SCHED_WARN_ON(!bucket->tasks);
>> -	if (likely(bucket->tasks))
>> -		bucket->tasks--;
>> +
>> +	/*
>> +	 * This could happen if sched_uclamp_used was enabled while the
>> +	 * current task was running, hence we could end up with unbalanced call
>> +	 * to uclamp_rq_dec_id().
>> +	 */
>> +	if (unlikely(!bucket->tasks))
>> +		return;
>> +
>> +	bucket->tasks--;
>>      uc_se->active = false;
>
> In this chunk you are indeed changing the code.
>
> Are we sure there are not issues with patterns like:
>
>   enqueue(taskA)
>   // uclamp gets enabled
>   enqueue(taskB)
>   dequeue(taskA)
>   // bucket->tasks is now 0
>   dequeue(taskB)
>
> TaskB has been enqueued with with uclamp enabled, thus it
> has got uc_se->active=True and enforced its clamp value at RQ level.
>
> But with your change above we don't reset that anymore.
>

Harumph indeed...

> As per my previous proposal: why not just removing the SCHED_WARN_ON?
> That's the only real problem in the code above, since now we are not
> more granted to have balanced inc/dec.
>

The SCHED_WARN_ON is gone, were you thinking of something else?