linux-kernel - Re: [PATCH 04/14] sched: SCHED_DEADLINE SMP-related data structures & logic.

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Message-ID: <525D0C91.1090700@gmail.com>
Date:	Tue, 15 Oct 2013 11:36:17 +0200
From:	Juri Lelli <juri.lelli@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	tglx@...utronix.de, mingo@...hat.com, rostedt@...dmis.org,
	oleg@...hat.com, fweisbec@...il.com, darren@...art.com,
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	bruce.ashfield@...driver.com--no-chain-reply-to
Subject: Re: [PATCH 04/14] sched: SCHED_DEADLINE SMP-related data structures
 & logic.

On 10/14/2013 02:03 PM, Peter Zijlstra wrote:
> On Mon, Oct 14, 2013 at 12:43:36PM +0200, Juri Lelli wrote:
>> +static inline void dl_set_overload(struct rq *rq)
>> +{
>> +	if (!rq->online)
>> +		return;
>> +
>> +	cpumask_set_cpu(rq->cpu, rq->rd->dlo_mask);
>> +	/*
>> +	 * Must be visible before the overload count is
>> +	 * set (as in sched_rt.c).
>> +	 */
>> +	wmb();
>> +	atomic_inc(&rq->rd->dlo_count);
>> +}
> 
> Please, make that smp_wmb() and modify the comment to point to the
> matching barrier ; I couldn't find one! Which suggests something is
> amiss.
> 
> Ideally we'd have something like smp_wmb__after_set_bit() but alas.
> 

The only user of this function is pull_dl_task (that tries to pull only if at
least one runqueue of the root_domain is overloaded). Surely makes sense to
ensure that changes in the dlo_mask have to be visible before we check if we
should look at that mask. Am I right if I say that the matching barrier is
constituted by the spin_lock on this_rq acquired by schedule() before calling
pre_schedule()?

Same thing in rt_set_overload(), do we need to modify the comment also there?

Thanks,

- Juri
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