Re: [PATCH 2/2] sched: Rewrite per entity runnable load average tracking

From: Peter Zijlstra
Date: Wed Jul 09 2014 - 14:45:59 EST

On Wed, Jul 09, 2014 at 09:07:53AM +0800, Yuyang Du wrote:
> That is chalenging... Can someone (Peter) grant us a lock of the remote rq? :)

Nope :-).. we got rid of that lock for a good reason.

Also, this is one area where I feel performance really trumps
correctness, we can fudge the blocked load a little. So the
sched_clock_cpu() difference is a strict upper bound on the
rq_clock_task() difference (and under 'normal' circumstances shouldn't
be much off).

So we could simply use a timestamps from dequeue and one from enqueue,
and use that.

As to the remote subtraction, a RMW on another cacheline than the
rq->lock one should be good; esp since we don't actually observe the
per-rq total often (once per tick or so) I think, no?

The thing is, we do not want to disturb scheduling on whatever cpu the
task last ran on if we wake it to another cpu. Taking rq->lock wrecks
that for sure.

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