linux-kernel - Re: [patch 07/16] sched: expire invalid runtime

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Message-ID: <CAPM31RKsOoKMBVFCHC=1D+XGgdNbYXLA2Cur1vOzJ9RX=pbs2g@mail.gmail.com>
Date:	Tue, 28 Jun 2011 19:29:03 -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>
Subject: Re: [patch 07/16] sched: expire invalid runtime

On Mon, Jun 27, 2011 at 9:42 PM, Paul Turner <pjt@...gle.com> wrote:
> On Wed, Jun 22, 2011 at 8:47 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>> On Tue, 2011-06-21 at 00:16 -0700, Paul Turner wrote:
>>
>>> +     now = sched_clock_cpu(smp_processor_id());
>>> +     cfs_b->runtime_expires = now + ktime_to_ns(cfs_b->period);
>>
>>> +     if ((s64)(rq->clock - cfs_rq->runtime_expires) < 0)
>>
>> Is there a good reason to mix these two (related) time sources?
>>
>
> It does make sense to remove the (current) aliasing dependency, will
> use rq->clock for setting expiration.
>

So looking more closely at this I think i prefer the "mix" after all.

Using rq->clock within __refill_cfs_bandwidth_runtime adds the requirement
of taking rq->lock on the current cpu within the period timer so that
we can update rq->clock (which then just gets set to sched_clock
anyway).

Expiration logic is already dependent on the fact that rq->clock
snapshots sched_clock (the 2ms bound on clock-to-clock drift).  Given
that this is an infrequent (once/period) operation I think it's better
to leave it as an explicit sched_clock_cpu call, with an explanatory
comment.
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