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

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Date:	Fri, 03 Oct 2014 15:05:31 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Rik van Riel <riel@...hat.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, 2014-10-03 at 09:50 +0200, Peter Zijlstra wrote: 
> On Fri, Oct 03, 2014 at 08:23:04AM +0200, Mike Galbraith wrote:

> > A generic boo hiss aimed in the general direction of all of this let's
> > go look at every possibility on every wakeup stuff.  Less is more.
> 
> I hear you, can you see actual slowdown with the patch? While the worst
> case doesn't change, it does make the average case equal to the worst
> case iteration -- where we previously would average out at inspecting
> half the CPUs before finding an idle one, we'd now always inspect all of
> them in order to compare all idle ones on their properties.
> 
> Also, with the latest generation of Haswell Xeons having 18 cores (36
> threads) this is one massively painful loop for sure.

Yeah, the things are getting too damn big.  I didn't try the patch and
measure anything, my gut instantly said "nope, not worth it".
  
> I'm just not sure what to do about it.. I suppose we can artificially
> split it into smaller groups, but I bet that'll hurt some, but if we can
> show it gains more we might still be able to do it. The only real
> problem is actual numbers/workloads (isn't it always) :/
> 
> One thing I suppose we could try is keeping a 'busy' flag at the
> llc domain which is set when all CPUs are busy (we'll clear it from
> new_idle) that way we can avoid the entire iteration if we know its
> pointless.

On one of those huge packages, heck, even on a 8 core that could save a
substantial number of busy box cycles.

-Mike

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