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

[Vincent Guittot]

On Tue, 15 Sep 2020 at 10:47, Jiang Biao <benbjiang@xxxxxxxxx> wrote:
>
> Hi, Vincent
>
> On Mon, 14 Sep 2020 at 20:26, Vincent Guittot
> <vincent.guittot@xxxxxxxxxx> wrote:
> >
> > On Sun, 13 Sep 2020 at 05:59, Jiang Biao <benbjiang@xxxxxxxxx> wrote:
> > >
> > > Hi, Aubrey
> > >
> > > On Fri, 11 Sep 2020 at 23:48, Aubrey Li <aubrey.li@xxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > > > ---
> > > >  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? :)

Yes set_cpu_sd_state_idle/busy are nohz function
>
> Thx.
> Regards,
> Jiang