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Date: Mon, 2 May 2016 18:04:04 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Galbraith <mgalbraith@suse.de>, Chris Mason <clm@fb.com>,
        Matt Fleming <matt@codeblueprint.co.uk>, linux-kernel@vger.kernel.org
Subject: Re: sched: tweak select_idle_sibling to look for idle threads
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* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, May 02, 2016 at 04:50:04PM +0200, Mike Galbraith wrote:
> > On Mon, 2016-05-02 at 10:46 +0200, Peter Zijlstra wrote:
> > 5226         /*
> > 5227          * If there are idle cores to be had, go find one.
> > 5228          */
> > 5229         if (sched_feat(IDLE_CORE) && test_idle_cores(target)) {
> > 5230                 i = select_idle_core(p, target);
> > 5231                 if ((unsigned)i < nr_cpumask_bits)
> > 5232                         return i;
> > 5233  
> > 5234                 /*
> > 5235                  * Failed to find an idle core; stop looking for one.
> > 5236                  */
> > 5237                 clear_idle_cores(target);
> > 5238         }
> > 5239 #if 1
> > 5240         for_each_cpu(i, cpu_smt_mask(target)) {
> > 5241                 if (idle_cpu(i))
> > 5242                         return i;
> > 5243         }
> > 5244  
> > 5245         return target;
> > 5246 #endif
> 
> And yes, I have a variant of that, that does indeed work way better than
> scanning the whole LLC domain for idle threads.
> 
> If you want a laugh, modify select_idle_core() to remember the last idle
> thread it encounters and have it return that when it fails to find an
> idle core.. I'm still stumped to explain why it behaves the way it does.

Assuming by 'behaving the way it does' means it improves things, such a dynamic 
with history/memory could be disrupting escalating feedback loops. Only guessing 
though.

Thanks,

	Ingo