Re: [RESEND PATCH] sched/fair: consider RT/IRQ pressure in select_idle_sibling

[Dietmar Eggemann]

On 02/09/2018 11:05 PM, Rohit Jain wrote:
> On 02/09/2018 07:46 AM, Dietmar Eggemann wrote:
> > On 02/09/2018 01:53 PM, Peter Zijlstra wrote:
> > > On Mon, Jan 29, 2018 at 03:27:09PM -0800, Rohit Jain wrote:
> >
> > [...]
> >
> > > > @@ -6173,8 +6183,15 @@ static int select_idle_cpu(struct task_struct 
> > > > *p, struct sched_domain *sd, int t
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ return -1;
> > > > ÂÂÂÂÂÂÂÂÂ if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ continue;
> > > > +ÂÂÂÂÂÂÂ if (idle_cpu(cpu)) {
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ if (full_capacity(cpu)) {
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ best_cpu = cpu;
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ break;
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ } else if (capacity_of(cpu) > best_cap) {
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ best_cap = capacity_of(cpu);
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ best_cpu = cpu;
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ }
> > > > +ÂÂÂÂÂÂÂ }
> > >
> > > No need for the else. And you'll note you're once again inconsistent
> > > with your previous self.
> > >
> > > But here I worry about big.little a wee bit. I think we're allowed big
> > > and little cores on the same L3 these days, and you can't directly
> > > compare capacity between them.
> > >
> > > Morten / Dietmar, any comments?
> >
> > Yes, for DynamIQ (big.little successor) systems, those cpus can have 
> > different capacity_orig_of() values already.
>
> OK, given that there are asymmetric capacities in L3 cores, we would
> probably have something like the below(?) in select_idle_cpu:
>
>  ÂÂÂÂÂÂÂÂÂ if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
>  ÂÂÂÂÂÂÂÂÂÂÂÂÂ continue;
> + ÂÂ ÂÂÂ if (idle_cpu(cpu) && !reduced_capacity(cpu))
> +ÂÂÂ ÂÂÂ ÂÂÂ break;

Only returning the first idle cpu w/o reduced capacity will definitely 
work better here on future DynamIQ systems than trying to find the 
best_cap cpu by taking capacity_of() into consideration.