linux-kernel - Re: [PATCH v3 2/7] sched: accumulate per-cfs

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Date:	Wed, 13 Oct 2010 07:41:46 -0700
From:	Nikhil Rao <ncrao@...gle.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval.giani@...il.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Avi Kivity <avi@...hat.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Paul Menage <menage@...gle.com>,
	Mike Waychison <mikew@...gle.com>, Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v3 2/7] sched: accumulate per-cfs_rq cpu usage

On Wed, Oct 13, 2010 at 6:59 AM, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> * Nikhil Rao <ncrao@...gle.com> [2010-10-13 06:46:18]:
>
>> > Quick question for cfs_b->quota == RUNTIME_INF, won't cfs_b->runtime
>> > be always > 0?
>>
>> Hi Balbir,
>>
>> cfs_b->runtime can be 0 if the task group exhausts its quota.
>> cfs_b->runtime is a counter that is periodically refreshed to
>> cfs_b->quota, and is decremented every time a cfs_rq requests a slice.
>>
>
> Thanks, Nikhil
>
> I saw several checks for quota == RUNTIME_INF and saw updates not
> happening. Do we track usage for cgroups with infinite quota? IOW, is
> runtime updated? If so, why?
>

We don't track usage or update runtime for cgroups with infinite
quota. Most of these RUNTIME_INF checks are usually at the top of the
functions and return immediately. The checks in the middle of
functions are mostly to prevent races when cfs_b->quota is updated.

And ... I think I might have misunderstood your original question. Let
me try answering that again. We check for quota == RUNTIME_INF (in
addition to cfs_b->runtime > 0) because it is possible that the global
pool is updated to RUNTIME_INF between the quota check in
account_cfs_rq_quota() and when the actual quota is distributed in
tg_request_cfs_quota(). I hope that answers your original question.

-Thanks,
Nikhil

> --
>        Three Cheers,
>        Balbir
>
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