linux-kernel - Re: [PATCH RFC/TEST] sched: make sync affine wakeups work

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Date:	Mon, 05 May 2014 07:28:45 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	umgwanakikbuti@...il.com, Peter Zijlstra <peterz@...radead.org>
CC:	Preeti Murthy <preeti.lkml@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Ingo Molnar <mingo@...nel.org>, george.mccollister@...il.com,
	ktkhai@...allels.com
Subject: Re: [PATCH RFC/TEST] sched: make sync affine wakeups work

On 05/05/2014 12:50 AM, Preeti U Murthy wrote:

> Yeah now I see it. But I still feel wake_affine() and
> select_idle_sibling() are not at fault primarily because when they were
> introduced, I don't think it was foreseen that the cpu topology would
> grow to the extent it is now.

It's not about "fault", it is about the fact that on current
large NUMA systems they are broken, and could stand some
improvement :)

> select_idle_sibling() for instance scans the cpus within the purview of
> the last level cache of a cpu and this was a small set. Hence there was
> no overhead. Now with many cpus sharing the L3 cache, we see an
> overhead. wake_affine() probably did not expect the NUMA nodes to come
> under its governance as well and hence it sees no harm in waking up
> tasks close to the waker because it still believes that it will be
> within a node.

If two tasks truly are related to each other, I think we
will want to have the wake_affine logic pull them towards
each other, all the way across a giant NUMA system if
needs be.

The problem is that the current wake_affine logic starts
in the ON position, and only switches off in a few very
specific scenarios.

I suspect we would be better off with the reverse, starting
with wake_affine in the off position, and switching it on
when we detect it makes sense to do so.

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