linux-kernel - Re: [patch 09/16] sched: unthrottle cfs_rq(s) who ran out of quota at period refresh

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Message-ID: <CAPM31RJMiK5Omhgx3nm219x8BBPdhrBwU8s7fi4q8NXroZgL9w@mail.gmail.com>
Date:	Tue, 28 Jun 2011 20:37:19 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Nikhil Rao <ncrao@...gle.com>
Subject: Re: [patch 09/16] sched: unthrottle cfs_rq(s) who ran out of quota at
 period refresh

On Tue, Jun 28, 2011 at 2:11 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Mon, 2011-06-27 at 21:40 -0700, Paul Turner wrote:
>> >> +       if (unlikely(runtime_expires != cfs_b->runtime_expires))
>> >> +               goto out_unlock;
>> >
>> > it might help to explain how, runtime_expires is taken from cfs_b after
>> > calling __refill_cfs_bandwidth_runtime, and we're in the replenishment
>> > timer, so nobody is going to be adding new runtime.
>> >
>>
>> Good idea -- thanks
>
> Aside from being a good idea, I'm genuinely puzzled by that part and
> would love having it explained :-)
>

It's all the users fault!

While we were busy doing this they might have set some new bandwidth
limit (since we drop cfs_b->lock to distribute) via cgroupfs, in which
case we need to make sure we:

a) stop since setting the new runtime will have unthrottled everyone anyway
b) don't put our runtime in cfs_b->runtime as at this point we'd be
over-writing the new-runtime just set by tg_set_cfs_bandwidth

We can catch this happening however, since tg_set_cfs_bandwidth sets a
new expiration' which is what the check above enforces.
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