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

[Atish Patra]

On 2017/11/23 10:00 AM, Josef Bacik wrote:
> 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. ÂRather
> > 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. ÂCompletely 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.
:) :).
> Josef