* Peter Zijlstra <pet...@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
