Re: [RFC V2 2/2] sched/fair: Fallback to sched-idle CPU if idle CPU isn't found

From: Peter Zijlstra
Date: Fri May 10 2019 - 03:22:52 EST

On Thu, Apr 25, 2019 at 03:07:40PM +0530, Viresh Kumar wrote:
> We target for an idle CPU in select_idle_sibling() to run the next task,
> but in case we don't find idle CPUs it is better to pick a CPU which
> will run the task the soonest, for performance reason. A CPU which isn't
> idle but has only SCHED_IDLE activity queued on it should be a good
> target based on this criteria as any normal fair task will most likely
> preempt the currently running SCHED_IDLE task immediately. In fact,
> choosing a SCHED_IDLE CPU shall give better results as it should be able
> to run the task sooner than an idle CPU (which requires to be woken up
> from an idle state).
> This patch updates the fast path to fallback to a sched-idle CPU if the
> idle CPU isn't found, the slow path can be updated separately later.
> Following is the order in which select_idle_sibling() picks up next CPU
> to run the task now:
> 1. idle_cpu(target) OR sched_idle_cpu(target)
> 2. idle_cpu(prev) OR sched_idle_cpu(prev)
> 3. idle_cpu(recent_used_cpu) OR sched_idle_cpu(recent_used_cpu)
> 4. idle core(sd)
> 5. idle_cpu(sd)
> 6. sched_idle_cpu(sd)
> 7. idle_cpu(p) - smt
> 8. sched_idle_cpu(p)- smt
> Though the policy can be tweaked a bit if we want to have different
> priorities.

I don't hate his per se; but the whole select_idle_sibling() thing is
something that needs looking at.

There was the task stealing thing from Steve that looked interesting and
that would render your apporach unfeasible.