linux-kernel - Re: [PATCH 1/3] sched/fair: Disable runtime

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Message-ID: <xm2661jr1qmw.fsf@sword-of-the-dawn.mtv.corp.google.com>
Date:	Mon, 23 Jun 2014 10:29:11 -0700
From:	bsegall@...gle.com
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Kirill Tkhai <ktkhai@...allels.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>, tkhai@...dex.ru,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Konstantin Khorenko <khorenko@...allels.com>, pjt@...gle.com
Subject: Re: [PATCH 1/3] sched/fair: Disable runtime_enabled on dying rq

Peter Zijlstra <peterz@...radead.org> writes:

> On Tue, Jun 17, 2014 at 05:24:10PM +0400, Kirill Tkhai wrote:
>> @@ -3790,6 +3803,12 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
>>  		cfs_rq->runtime_remaining = 1;
>>  		if (cfs_rq_throttled(cfs_rq))
>>  			unthrottle_cfs_rq(cfs_rq);
>> +
>> +		/*
>> +		 * Offline rq is schedulable till cpu is completely disabled
>> +		 * in take_cpu_down(), so we prevent new cfs throttling here.
>> +		 */
>> +		cfs_rq->runtime_enabled = 0;
>
> Does it make sense to clear this before calling unthrottle_cfs_rq()?
> Just to make sure they're in the right order..

I believe that order is irrelevant here - I do not believe that
unthrottle_cfs_rq(a) can cause a throttle_cfs_rq(a). In fact, I don't
see any code that will check it at all from unthrottle, although I might
be missing something. It _can_ cause a throttle_cfs_rq(parent_cfs_rq(a)),
but that should be fine as long as for_each_leaf_cfs_rq is sorted
correctly.

That said, migrate_tasks drops rq->lock, and I /think/ another cpu could
wake another task onto this cpu, which could then enqueue_throttle its
cfs_rq (which previously had no tasks and thus wasn't on
leaf_cfs_rq_list). You certainly could have tg_set_bandwidth come in and
turn runtime_enabled on.

I think the general idea of turning runtime_enabled off during offline
is fine, ccing pjt to double check.
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