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

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Message-ID: <1287046900.29097.161.camel@twins>
Date:	Thu, 14 Oct 2010 11:01:40 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	bharata@...ux.vnet.ibm.com
Cc:	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>,
	Paul Turner <pjt@...gle.com>, Nikhil Rao <ncrao@...gle.com>
Subject: Re: [PATCH v3 2/7] sched: accumulate per-cfs_rq cpu usage

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.
Why not subtract delta_exec and replenish when <0? That keeps the
numbers small.
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