linux-kernel - Re: [PATCH RFC] sched,idle: teach select_idle

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Date:	Thu, 9 Oct 2014 18:04:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH RFC] sched,idle: teach select_idle_sibling about idle
 states

On Fri, Oct 03, 2014 at 11:37:31AM -0400, Rik van Riel wrote:
> Some more brainstorming points...
> 
> 1) We should probably (lazily/batched?) propagate load information
>    up the sched_group tree.  This will be useful for wake_affine,
>    load_balancing, find_idlest_cpu, and select_idle_sibling
> 
> 2) With both find_idlest_cpu and select_idle_sibling walking down
>    the tree from the LLC level, they could probably share code
> 
> 3) Counting both blocked and runnable load may give better long
>    term stability of loads, resulting in a reduction in work
>    preserving behaviour, but an improvement in locality - this
>    could be worthwhile, but it is hard to say in advance
> 
> 4) We can be pretty sure that CPU makers are not going to stop
>    at a mere 18 cores. We need to subdivide things below the LLC
>    level, turning select_idle_sibling and find_idlest_cpu into
>    a tree walk.
> 
>    This means whatever selection criteria are used by these need
>    to be propagated up the sched_group tree. This, in turn, means
>    we probably need to restrict ourselves to things that do not get
>    changed/updated too often.
> 
> Am I overlooking anything?

Well, we can certainly try something like that; but your last point
seems like a contradition; seeing how _the_ important point for
select_idle_sibling() is the actual idle state, and that per definition
is something that can change/update often.

But yes, the only viable option is some artificial breakup of the
topology and we can indeed try and bridge the gap with some caching.
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