Re: [PATCH RFC 1/2] sched: Minimize the idle cpu selection race window.

From: Josef Bacik
Date: Thu Nov 23 2017 - 11:00:13 EST

On Thu, Nov 23, 2017 at 02:13:01PM +0100, Mike Galbraith wrote:
> On Thu, 2017-11-23 at 11:52 +0100, Uladzislau Rezki wrote:
> > Hello, Atish, Peter, all.
> >
> > I have a question about if a task's nr_cpus_allowed is 1.
> > In that scenario we do not call select_task_rq. Therefore
> > even thought a task "p" is placed on idle CPU that CPU
> > will not be marked as claimed for wake-up.
> >
> > What do you think about adding per_cpu(claim_wakeup, cpu) = 1;
> > to select_task_rq() instead and possibly get rid of them from
> > other places (increases a race window a bit)?
> My thoughts on all of this is that we need less SIS, not more.  Rather
> than trying so hard for the absolute lowest wakeup latency, which
> induces throughput/efficiency robbing bouncing, I think we'd be better
> of considering leaving an already llc affine task where it is if the
> average cycle time is sufficiently low that it will likely hit the CPU
> RSN.  Completely ignoring low utilization kernel threads would go a
> long way to getting rid of bouncing userspace (which tends to have a
> meaningful footprint), all over hell and creation.
> You could also periodically send mobile kthreads down the slow path to
> try to keep them the hell away from partially busy CPUs, as well as
> anything else that hasn't run for a while, to keep background cruft
> from continually injecting itself into the middle of a cross core
> cyber-sex.

And on this thanksgiving I'm thankful for Mike, and his entertaining early
morning emails.