Re: [PATCH v3 5/7] sched: SIS_CORE to disable idle core search

[Subhra Mazumdar]

On 6/28/19 12:01 PM, Parth Shah wrote:
> On 6/27/19 6:59 AM, subhra mazumdar wrote:
> > Use SIS_CORE to disable idle core search. For some workloads
> > select_idle_core becomes a scalability bottleneck, removing it improves
> > throughput. Also there are workloads where disabling it can hurt latency,
> > so need to have an option.
> >
> > Signed-off-by: subhra mazumdar <subhra.mazumdar@xxxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 8 +++++---
> >   1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index c1ca88e..6a74808 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6280,9 +6280,11 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> >   	if (!sd)
> >   		return target;
> >
> > -	i = select_idle_core(p, sd, target);
> > -	if ((unsigned)i < nr_cpumask_bits)
> > -		return i;
> > +	if (sched_feat(SIS_CORE)) {
> > +		i = select_idle_core(p, sd, target);
> > +		if ((unsigned)i < nr_cpumask_bits)
> > +			return i;
> > +	}
> This can have significant performance loss if disabled. The select_idle_core spreads
> workloads quickly across the cores, hence disabling this leaves much of the work to
> be offloaded to load balancer to move task across the cores. Latency sensitive
> and long running multi-threaded workload should see the regression under this conditions.
Yes in case of SPARC SMT8 I did notice that (see cover letter). That's why
it is a feature that is ON by default, but can be turned OFF for specific
workloads on x86 SMT2 that can benefit from it.
> Also, systems like POWER9 has sd_llc as a pair of core only. So it
> won't benefit from the limits and hence also hiding your code in select_idle_cpu
> behind static keys will be much preferred.
If it doesn't hurt then I don't see the point.

Thanks,
Subhra