linux-kernel - Re: [patch 12/16] sched: prevent interactions with throttled entities

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Date:	Mon, 27 Jun 2011 21:38:13 -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 12/16] sched: prevent interactions with throttled entities

On Thu, Jun 23, 2011 at 4:49 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Tue, 2011-06-21 at 00:17 -0700, Paul Turner wrote:
>> @@ -2635,8 +2704,10 @@ static int update_shares_cpu(struct task
>>
>>         raw_spin_lock_irqsave(&rq->lock, flags);
>>
>> -       update_rq_clock(rq);
>> -       update_cfs_load(cfs_rq, 1);
>> +       if (!throttled_hierarchy(cfs_rq)) {
>> +               update_rq_clock(rq);
>> +               update_cfs_load(cfs_rq, 1);
>> +       }
>>
>>         /*
>
> OK, so we can't contribute to load since we're throttled, but
> tg->load_weight might have changed meanwhile?
>

What's why we continue to update their shares (also at
enqueue/dequeue) but not their load, so that the weight will be
correct when unthrottling*

> Also, update_cfs_shares()->reweight_entity() can dequeue/enqueue the
> entity, doesn't that require an up-to-date rq->clock?
>

It shouldn't, we're only doing an account_enqueue/dequeue, and there
shouldn't be a cfs_rq->curr to lead to updates (on a throttled
entity).  I suppose we might do something interesting in the case of a
race with alb forcing something throttled to run intersecting with
update_shares()**.

The original concern here was [*] above, keeping shares current for
the unthrottle.  *However*, with the hierarchal throttle accounting in
the current version, I think this can be improved.

Instead, we should skip update_shares/update_cfs_shares for all
throttled entities and simply do a final update shares when
throttle_count goes to 0 in tg_throttle_down (which also avoids **).
I thought of doing this at the end of preparing the last patchset but
by that time it was tested and I didn't want to change things around
here at the last minute.

Will fix for this week.

>
>
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