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

[Rohit Jain]

On 01/30/2018 05:57 PM, Joel Fernandes wrote:

<snip>
> > > > Signed-off-by: Rohit Jain<rohit.k.jain@xxxxxxxxxx>
> > > > ---
> > > >    kernel/sched/fair.c | 38 ++++++++++++++++++++++++++++----------
> > > >    1 file changed, 28 insertions(+), 10 deletions(-)
> > > >
> > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > index 26a71eb..ce5ccf8 100644
> > > > --- a/kernel/sched/fair.c
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -5625,6 +5625,11 @@ static unsigned long capacity_orig_of(int cpu)
> > > >           return cpu_rq(cpu)->cpu_capacity_orig;
> > > >    }
> > > >
> > > > +static inline bool full_capacity(int cpu)
> > > > +{
> > > > +       return capacity_of(cpu) >= (capacity_orig_of(cpu)*3)/4;
> > > > +}
> > > > +
> > > >    static unsigned long cpu_avg_load_per_task(int cpu)
> > > >    {
> > > >           struct rq *rq = cpu_rq(cpu);
> > > > @@ -6081,7 +6086,7 @@ static int select_idle_core(struct task_struct *p,
> > > > struct sched_domain *sd, int
> > > >
> > > >                   for_each_cpu(cpu, cpu_smt_mask(core)) {
> > > >                           cpumask_clear_cpu(cpu, cpus);
> > > > -                       if (!idle_cpu(cpu))
> > > > +                       if (!idle_cpu(cpu) || !full_capacity(cpu))
> > > >                                   idle = false;
> > > >                   }
> > > There's some difference in logic between select_idle_core and
> > > select_idle_cpu as far as the full_capacity stuff you're adding goes.
> > > In select_idle_core, if all CPUs are !full_capacity, you're returning
> > > -1. But in select_idle_cpu you're returning the best idle CPU that's
> > > the most cap among the !full_capacity ones. Why there is this
> > > different in logic? Did I miss something?

<snip>

> Dude :) That is hardly an answer to the question I asked. Hint:
> *different in logic*.

Let me re-try :)

For select_idle_core, we are doing a search for a fully idle and full
capacity core, the fail-safe is select_idle_cpu because we will re-scan
the CPUs. The notion is to select an idle CPU no matter what, because
being on an idle CPU is better than waiting on a non-idle one. In
select_idle_core you can be slightly picky about the core because
select_idle_cpu is a fail safe. I measured the performance impact of
choosing the "best among low cap" vs the code changes I have (for
select_idle_core) and could not find a statistically significant impact,
hence went with the simpler code changes.

Hope I answered your question.

Thanks,
Rohit