Re: [PATCH v4 5/7] sched/fair: skip SIS domain search if fully busy

[Abel Wu]

On 6/29/22 9:11 AM, Josh Don Wrote:
> On Mon, Jun 27, 2022 at 11:53 PM Abel Wu <wuyun.abel@xxxxxxxxxxxxx> wrote:
> > > > -static inline bool test_idle_cores(int cpu)
> > > > +static inline enum sd_state sd_get_state(int cpu)
> > > >    {
> > > >           struct sched_domain_shared *sds;
> > > >
> > > >           sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> > > >           if (sds)
> > > > -               return READ_ONCE(sds->has_idle_cores);
> > > > +               return READ_ONCE(sds->state);
> > > >
> > > > -       return false;
> > > > +       return sd_has_icpus;
> > > > +}
> > >
> > > Why is default not sd_is_busy?
> >
> > The state of sd_is_busy will prevent us from searching the LLC. By
> > design, both sd_has_icores and sd_is_busy indicate deterministic
> > status: has idle cores / no idle cpu exists. While sd_has_icpus is
> > not deterministic, it means there could be unoccupied cpus.
> >
> > The naming seems misleading, it would be nice to have other options.
>
> sd_has_icores isn't deterministic; when the last fully idle core gets
> an occupied sibling, it will take until the next select_idle_cpu() to
> mark the state as sd_has_icpus instead.

Yes, it's not deterministic in nature, but we treat it as deterministic.
As long as sd_has_icores, a full scan will be fired no matter there are
any idle cores or not.

> A comment here and also at the enum definitions would be helpful I think.

Agreed. I will add some comments here. State descriptions are already
above their definitions, please let me know if any modification needed.

> > > > +
> > > > +static inline void set_idle_cores(int cpu, int idle)
> > >
> > > nit: Slightly confusing to call the param 'idle', since in the case it
> > > is false we still mark icpus. Consider possibly 'core_idle'.
> >
> > What about changing the param 'cpu' to 'core'?
>
> I think keeping it as "cpu" is fine, since as "core" that would imply
> some per-core state (when we're still setting this per-cpu).

The function has already been there for a long time, and I haven't
changed its semantics, so maybe it isn't that confusing.. Does the
following naming make things clearer?

static inline void set_idle_cores(int cpu, int has_icores);
static inline void set_idle_cpus(int cpu, int has_icpus);

> > > >           for_each_cpu_and(i, sched_group_span(group), env->cpus) {
> > > >                   struct rq *rq = cpu_rq(i);
> > > > @@ -8692,6 +8740,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> > > >                   nr_running = rq->nr_running;
> > > >                   sgs->sum_nr_running += nr_running;
> > > >
> > > > +               if (update_core)
> > > > +                       sd_classify(sds, rq);
> > > > +
> > > >                   if (nr_running > 1)
> > > >                           *sg_status |= SG_OVERLOAD;
> > > >
> > > > @@ -9220,6 +9271,12 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
> > > >           return idlest;
> > > >    }
> > > >
> > > > +static void sd_update_state(struct lb_env *env, struct sd_lb_stats *sds)
> > > > +{
> > > > +       if (sds->sd_state == sd_has_icpus && !test_idle_cpus(env->dst_cpu))
> > > > +               set_idle_cpus(env->dst_cpu, true);
> > > > +}
> > >
> > > We're only setting state to has_icpus here in sd_update_state. That
> > > doesn't feel good enough, since we're only updating state for
> > > env->dst_cpu; all the other per-cpu state will remain stale (ie.
> > > falsely sd_is_busy).
> >
> > It's LLC-wide shared :)
>
> Oh wow, yea that's the thing I missed... Thanks.
>
> > > I think you also want a case in __update_idle_core() to call
> > > set_idle_cores(core, 0) in the case where we have a non-idle sibling,
> > > since we want to at least mark has_icpus even if the entire core isn't
> > > idle.
>
> More specifically, in the __update_idle_core() function, if the
> sibling is still busy and the sd_state is sd_is_busy, we should
> instead mark it as sd_has_icpus, since the current cpu is guaranteed
> to be going idle.

The sd_is_busy state will be cleared during newidle balance. And the
state should not set back to is_busy between the gap before the cpu
actually goes idle.

> Additionally, to be consistent with what we're calling "idle"
> elsewhere, I think you mean to have __update_idle_core() check either
> available_idle_cpu() or sched_idle_cpu()?

I think the condition should be aligned with SIS that an unoccupied
cpu satisfies "idle_cpu(cpu) || sched_idle_cpu(cpu)". The function
available_idle_cpu() is not used in load balancing due to its not
being used immediately.

Thanks & BR,
Abel