Re: [PATCH] sched, fair: Allow a per-cpu kthread waking a task to stack on the same CPU

[Peter Zijlstra]

On Tue, Jan 28, 2020 at 09:10:12AM +0000, Mel Gorman wrote:
> Peter, Ingo and Vincent -- I know the timing is bad due to the merge
> window but do you have any thoughts on allowing select_idle_sibling to
> stack a wakee task on the same CPU as a waker in this specific case?

I sort of see, but *groan*...

so if the kworker unlocks a contended mutex/rwsem/completion...

I suppose the fact that it limits it to tasks that were running on the
same CPU limits the impact if we do get it wrong.

Elsewhere you write:

> I would prefer the wakeup code did not have to signal that it's a
> synchronous wakeup. Sync wakeups so exist but callers got it wrong many
> times where stacking was allowed and then the waker did not go to sleep.
> While the chain of events are related, they are not related in a very
> obvious way. I think it's much safer to keep this as a scheduler
> heuristic instead of depending on callers to have sufficient knowledge
> of the scheduler implementation.

That is true; the existing WF_SYNC has caused many issues for maybe
being too strong.

But what if we create a new hint that combines both these ideas? Say
WF_COMPLETE and subject that to these same criteria. This way we can
eliminate wakeups from locks and such (they won't have this set).

Or am I just making things complicated again?