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

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Date:	Thu, 14 Oct 2010 10:57:30 +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 u64 tg_request_cfs_quota(struct task_group *tg)
> +{
> +       struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(tg);
> +       u64 delta = 0;
> +
> +       if (cfs_b->runtime > 0 || cfs_b->quota == RUNTIME_INF) {
> +               raw_spin_lock(&cfs_b->lock);
> +               /*
> +                * it's possible a bandwidth update has changed the global
> +                * pool.
> +                */
> +               if (cfs_b->quota == RUNTIME_INF)
> +                       delta = sched_cfs_bandwidth_slice();
> +               else {
> +                       delta = min(cfs_b->runtime,
> +                                       sched_cfs_bandwidth_slice());
> +                       cfs_b->runtime -= delta;
> +               }
> +               raw_spin_unlock(&cfs_b->lock);
> +       }
> +       return delta;
> +} 

Since you check cfs_b->quote outside of cfs_b->lock anyway, you might as
well avoid taking the lock in that case and directly return slice.

Also, you possibly evaluate sched_cfs_bandwidth_slice() twice.
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