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    SubjectRe: sched: tweak select_idle_sibling to look for idle threads

    * 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

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