On Mon, Feb 12, 2018 at 08:14:49PM +0100, Mike Galbraith wrote: > 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". >
Agreed and I'm aware of the hazard of sync wakeups being no guarantee that the task will immediately sleep. If there is any delay at all then stacking incurs a wakeup latency penalty but in many cases, it'll be an idle sibling that is used. The sync hint does give a stronger hint that the tasks are closely related though which is why I special-cased it slightly and I feel it's justified. I think in all cases where it mattered, it was due to pref_eff_load and this_eff_load begin equal to 0 when waking a task via a socket. -- Mel Gorman SUSE Labs
