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

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Date:	Thu, 14 Oct 2010 02:53:42 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	bharata@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	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>,
	Ingo Molnar <mingo@...e.hu>,
	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>,
	Nikhil Rao <ncrao@...gle.com>
Subject: Re: [PATCH v3 2/7] sched: accumulate per-cfs_rq cpu usage

On Thu, Oct 14, 2010 at 2:27 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, 2010-10-14 at 02:14 -0700, Paul Turner wrote:
>> On Thu, Oct 14, 2010 at 2:01 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Tue, 2010-10-12 at 13:21 +0530, Bharata B Rao wrote:
>> >> +static void account_cfs_rq_quota(struct cfs_rq *cfs_rq,
>> >> +               unsigned long delta_exec)
>> >> +{
>> >> +       if (cfs_rq->quota_assigned == RUNTIME_INF)
>> >> +               return;
>> >> +
>> >> +       cfs_rq->quota_used += delta_exec;
>> >> +
>> >> +       if (cfs_rq->quota_used < cfs_rq->quota_assigned)
>> >> +               return;
>> >> +
>> >> +       cfs_rq->quota_assigned += tg_request_cfs_quota(cfs_rq->tg);
>> >> +}
>> >
>> > That looks iffy, quota_assigned is only ever incremented and can wrap.
>>
>> This can't advance at a rate faster than ~vruntime and we can't handle
>> wrapping there anyway (fortunately it would take something like 35k
>> years?)
>
> You can't go faster than wall-time, vruntime can actually go a lot
> faster and can deal with wrapping.

Right -- I hadn't previously noticed the normalization w/ min_vruntime
in min_vruntime()

Fortunately the wall_time constraint still holds :)
>
>> > Why not subtract delta_exec and replenish when <0? That keeps the
>> > numbers small.
>> >
>>
>> Accounting in the opposite direction allows us to catch-up in
>> subsequent periods when a task exceeds its bandwidth across an
>> interval where we are not able to immediately throttle it (e.g. costly
>> syscall without config_prempt).  Since we'll continue to accrue the
>> execution time in this case it will be effectively pre-charged against
>> the next slice received.
>
> Humm, how so, that's a simply matter of the quota going negative, right?
>

Then quota is signed and representation of infinite quota is more ambiguous.
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