On Mon, 2018-02-12 at 18:29 +0100, Peter Zijlstra wrote: > On Mon, Feb 12, 2018 at 02:58:56PM +0000, Mel Gorman wrote: > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index c1091cb023c4..28c8d9c91955 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -5747,7 +5747,16 @@ wake_affine_weight(struct sched_domain *sd, struct > > task_struct *p, > > prev_eff_load *= 100 + (sd->imbalance_pct - 100) / 2; > > prev_eff_load *= capacity_of(this_cpu); > > > > - return this_eff_load <= prev_eff_load ? this_cpu : nr_cpumask_bits; > > + /* > > + * If sync, adjust the weight of prev_eff_load such that if > > + * prev_eff == this_eff that select_idle_sibling will consider > > + * stacking the wakee on top of the waker if no other CPU is > > + * idle. > > + */ > > + if (sync) > > + prev_eff_load += 1; > > So where we had <= and would consistently favour pulling the task to the > waking CPU when all else what equal, you now switch to <, such that when > things are equal we do not pull. > > That makes sense I suppose. > > Except for sync wakeups, where you say, if everything else is equal, > pull, which also makes sense, because sync says 'current' promises to go > away. > > OK.
Tasks tend to not honor that promise.. a lot. Even if the sync hint were a golden promise, it wouldn't be bullet proof: migrating a compute hog based on a single "phoned home" wakeup remains a bad idea whether schedule() is called immediately after the wakeup or not. It's a pain, only useful informational content is "this is a communication wakeup". -Mike