linux-kernel - Re: [RFC PATCH v1 1/1] sched/fair: select idle cpu from idle cpumask in sched domain

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Date:   Tue, 15 Sep 2020 16:47:24 +0800
From:   Jiang Biao <benbjiang@...il.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Aubrey Li <aubrey.li@...el.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [RFC PATCH v1 1/1] sched/fair: select idle cpu from idle cpumask
 in sched domain

Hi, Vincent

On Mon, 14 Sep 2020 at 20:26, Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> On Sun, 13 Sep 2020 at 05:59, Jiang Biao <benbjiang@...il.com> wrote:
> >
> > Hi, Aubrey
> >
> > On Fri, 11 Sep 2020 at 23:48, Aubrey Li <aubrey.li@...el.com> wrote:
> > >
> > > Added idle cpumask to track idle cpus in sched domain. When a CPU
> > > enters idle, its corresponding bit in the idle cpumask will be set,
> > > and when the CPU exits idle, its bit will be cleared.
> > >
> > > When a task wakes up to select an idle cpu, scanning idle cpumask
> > > has low cost than scanning all the cpus in last level cache domain,
> > > especially when the system is heavily loaded.
> > >
> > > Signed-off-by: Aubrey Li <aubrey.li@...ux.intel.com>
> > > ---
> > >  include/linux/sched/topology.h | 13 +++++++++++++
> > >  kernel/sched/fair.c            |  4 +++-
> > >  kernel/sched/topology.c        |  2 +-
> > >  3 files changed, 17 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> > > index fb11091129b3..43a641d26154 100644
> > > --- a/include/linux/sched/topology.h
> > > +++ b/include/linux/sched/topology.h
> > > @@ -65,8 +65,21 @@ struct sched_domain_shared {
> > >         atomic_t        ref;
> > >         atomic_t        nr_busy_cpus;
> > >         int             has_idle_cores;
> > > +       /*
> > > +        * Span of all idle CPUs in this domain.
> > > +        *
> > > +        * NOTE: this field is variable length. (Allocated dynamically
> > > +        * by attaching extra space to the end of the structure,
> > > +        * depending on how many CPUs the kernel has booted up with)
> > > +        */
> > > +       unsigned long   idle_cpus_span[];
> > >  };
> > >
> > > +static inline struct cpumask *sds_idle_cpus(struct sched_domain_shared *sds)
> > > +{
> > > +       return to_cpumask(sds->idle_cpus_span);
> > > +}
> > > +
> > >  struct sched_domain {
> > >         /* These fields must be setup */
> > >         struct sched_domain __rcu *parent;      /* top domain must be null terminated */
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 6b3b59cc51d6..3b6f8a3589be 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -6136,7 +6136,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> > >
> > >         time = cpu_clock(this);
> > >
> > > -       cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> > > +       cpumask_and(cpus, sds_idle_cpus(sd->shared), p->cpus_ptr);
> > Is the sds_idle_cpus() always empty if nohz=off?
>
> Good point
>
> > Do we need to initialize the idle_cpus_span with sched_domain_span(sd)?
> >
> > >
> > >         for_each_cpu_wrap(cpu, cpus, target) {
> > >                 if (!--nr)
> > > @@ -10182,6 +10182,7 @@ static void set_cpu_sd_state_busy(int cpu)
> > >         sd->nohz_idle = 0;
> > >
> > >         atomic_inc(&sd->shared->nr_busy_cpus);
> > > +       cpumask_clear_cpu(cpu, sds_idle_cpus(sd->shared));
> > >  unlock:
> > >         rcu_read_unlock();
> > >  }
> > > @@ -10212,6 +10213,7 @@ static void set_cpu_sd_state_idle(int cpu)
> > >         sd->nohz_idle = 1;
> > >
> > >         atomic_dec(&sd->shared->nr_busy_cpus);
> > > +       cpumask_set_cpu(cpu, sds_idle_cpus(sd->shared));
> > This only works when entering/exiting tickless mode? :)
> > Why not update idle_cpus_span during tick_nohz_idle_enter()/exit()?
>
> set_cpu_sd_state_busy is only called during a tick in order to limit
> the rate of the update to once per tick per cpu at most and prevents
> any kind of storm of update if short running tasks wake/sleep all the
> time. We don't want to update a cpumask at each and every enter/leave
> idle.
>
Agree. But set_cpu_sd_state_busy seems not being reached when
nohz=off, which means it will not work for that case? :)

Thx.
Regards,
Jiang