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

[Abel Wu]

On 6/28/22 8:28 AM, Josh Don Wrote:
> On Sun, Jun 19, 2022 at 5:05 AM Abel Wu <wuyun.abel@xxxxxxxxxxxxx> wrote:
> > If a full scan on SIS domain failed, then no unoccupied cpus available
> > and the LLC is fully busy. In this case we'd better spend the time on
> > something more useful, rather than wasting it trying to find an idle
> > cpu that probably not exist.
> >
> > The fully busy status will be re-evaluated when any core of this LLC
> > domain enters load balancing, and cleared once idle cpus found.
> >
> > Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>
> > ---
> >   include/linux/sched/topology.h | 35 ++++++++++++++-
> >   kernel/sched/fair.c            | 82 +++++++++++++++++++++++++++++-----
> >   2 files changed, 104 insertions(+), 13 deletions(-)
> >
> > diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> > index 56cffe42abbc..3e99ac98d766 100644
> > --- a/include/linux/sched/topology.h
> > +++ b/include/linux/sched/topology.h
> > @@ -77,10 +77,43 @@ extern int sched_domain_level_max;
> >
> >   struct sched_group;
> >
> > +/*
> > + * States of the sched-domain
> > + *
> > + * - sd_has_icores
> > + *     This state is only used in LLC domains to indicate worthy
> > + *     of a full scan in SIS due to idle cores available.
> > + *
> > + * - sd_has_icpus
> > + *     This state indicates that unoccupied (sched-idle/idle) cpus
> > + *     might exist in this domain. For the LLC domains it is the
> > + *     default state since these cpus are the main targets of SIS
> > + *     search, and is also used as a fallback state of the other
> > + *     states.
> > + *
> > + * - sd_is_busy
> > + *     This state indicates there are no unoccupied cpus in this
> > + *     domain. So for LLC domains, it gives the hint on whether
> > + *     we should put efforts on the SIS search or not.
> > + *
> > + * For LLC domains, sd_has_icores is set when the last non-idle cpu of
> > + * a core becomes idle. After a full SIS scan and if no idle cores found,
> > + * sd_has_icores must be cleared and the state will be set to sd_has_icpus
> > + * or sd_is_busy depending on whether there is any idle cpu. And during
> > + * load balancing on each SMT domain inside the LLC, the state will be
> > + * re-evaluated and switch from sd_is_busy to sd_has_icpus if idle cpus
> > + * exist.
> > + */
> > +enum sd_state {
> > +       sd_has_icores,
> > +       sd_has_icpus,
> > +       sd_is_busy
> > +};
> > +
> >   struct sched_domain_shared {
> >          atomic_t        ref;
> >          atomic_t        nr_busy_cpus;
> > -       int             has_idle_cores;
> > +       int             state;  /* see enum sd_state */
>
> nit: s/int/enum sd_state

Will fix in next version.

> >   };
> >
> >   struct sched_domain {
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 1cc86e76e38e..2ca37fdc6c4d 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5642,11 +5642,15 @@ static inline void update_overutilized_status(struct rq *rq)
> >   static inline void update_overutilized_status(struct rq *rq) { }
> >   #endif
> >
> > +static int unoccupied_rq(struct rq *rq)
> > +{
> > +       return rq->nr_running == rq->cfs.idle_h_nr_running;
> > +}
>
> nit: static inline int

Will fix.

> > +
> >   /* Runqueue only has SCHED_IDLE tasks enqueued */
> >   static int sched_idle_rq(struct rq *rq)
> >   {
> > -       return unlikely(rq->nr_running == rq->cfs.idle_h_nr_running &&
> > -                       rq->nr_running);
> > +       return unlikely(rq->nr_running && unoccupied_rq(rq));
> >   }
> >
> >   /*
> > @@ -6197,24 +6201,44 @@ static inline int __select_idle_cpu(int cpu, struct task_struct *p)
> >   DEFINE_STATIC_KEY_FALSE(sched_smt_present);
> >   EXPORT_SYMBOL_GPL(sched_smt_present);
> >
> > -static inline void set_idle_cores(int cpu, int val)
> > +static inline void sd_set_state(int cpu, enum sd_state state)
> >   {
> >          struct sched_domain_shared *sds;
> >
> >          sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> >          if (sds)
> > -               WRITE_ONCE(sds->has_idle_cores, val);
> > +               WRITE_ONCE(sds->state, state);
> >   }
> >
> > -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.

> > +
> > +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'?

> > +{
> > +       sd_set_state(cpu, idle ? sd_has_icores : sd_has_icpus);
> > +}
> > +
> > +static inline bool test_idle_cores(int cpu)
> > +{
> > +       return sd_get_state(cpu) == sd_has_icores;
> > +}
> > +
> > +static inline void set_idle_cpus(int cpu, int idle)
> > +{
> > +       sd_set_state(cpu, idle ? sd_has_icpus : sd_is_busy);
> > +}
> > +
> > +static inline bool test_idle_cpus(int cpu)
> > +{
> > +       return sd_get_state(cpu) != sd_is_busy;
> >   }
> >
> >   /*
> > @@ -6298,7 +6322,7 @@ static int select_idle_smt(struct task_struct *p, int target)
> >
> >   #else /* CONFIG_SCHED_SMT */
> >
> > -static inline void set_idle_cores(int cpu, int val)
> > +static inline void set_idle_cores(int cpu, int idle)
> >   {
> >   }
> >
> > @@ -6307,6 +6331,15 @@ static inline bool test_idle_cores(int cpu)
> >          return false;
> >   }
> >
> > +static inline void set_idle_cpus(int cpu, int idle)
> > +{
> > +}
> > +
> > +static inline bool test_idle_cpus(int cpu)
> > +{
> > +       return true;
> > +}
> > +
> >   static inline int select_idle_core(struct task_struct *p, int core, struct cpumask *cpus, int *idle_cpu)
> >   {
> >          return __select_idle_cpu(core, p);
> > @@ -6382,7 +6415,9 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >                  }
> >          }
> >
> > -       if (has_idle_core)
> > +       if (idle_cpu == -1)
> > +               set_idle_cpus(target, false);
> > +       else if (has_idle_core)
> >                  set_idle_cores(target, false);
> >
> >          if (sched_feat(SIS_PROP) && !has_idle_core) {
> > @@ -6538,6 +6573,9 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> >                          if ((unsigned int)i < nr_cpumask_bits)
> >                                  return i;
> >                  }
> > +
> > +               if (!has_idle_core && !test_idle_cpus(target))
> > +                       return target;
> >          }
> >
> >          i = select_idle_cpu(p, sd, has_idle_core, target);
> > @@ -8303,6 +8341,8 @@ struct sd_lb_stats {
> >          unsigned long avg_load; /* Average load across all groups in sd */
> >          unsigned int prefer_sibling; /* tasks should go to sibling first */
> >
> > +       int sd_state;
> > +
> >          struct sg_lb_stats busiest_stat;/* Statistics of the busiest group */
> >          struct sg_lb_stats local_stat;  /* Statistics of the local group */
> >   };
> > @@ -8321,6 +8361,7 @@ static inline void init_sd_lb_stats(struct sd_lb_stats *sds)
> >                  .local = NULL,
> >                  .total_load = 0UL,
> >                  .total_capacity = 0UL,
> > +               .sd_state = sd_is_busy,
> >                  .busiest_stat = {
> >                          .idle_cpus = UINT_MAX,
> >                          .group_type = group_has_spare,
> > @@ -8661,6 +8702,12 @@ sched_asym(struct lb_env *env, struct sd_lb_stats *sds,  struct sg_lb_stats *sgs
> >          return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu);
> >   }
> >
> > +static inline void sd_classify(struct sd_lb_stats *sds, struct rq *rq)
> > +{
> > +       if (sds->sd_state != sd_has_icpus && unoccupied_rq(rq))
> > +               sds->sd_state = sd_has_icpus;
> > +}
> > +
> >   /**
> >    * update_sg_lb_stats - Update sched_group's statistics for load balancing.
> >    * @env: The load balancing environment.
> > @@ -8675,11 +8722,12 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> >                                        struct sg_lb_stats *sgs,
> >                                        int *sg_status)
> >   {
> > -       int i, nr_running, local_group;
> > +       int i, nr_running, local_group, update_core;
> >
> >          memset(sgs, 0, sizeof(*sgs));
> >
> >          local_group = group == sds->local;
> > +       update_core = env->sd->flags & SD_SHARE_CPUCAPACITY;
>
> Nothing special about SD_SHARE_CPUCAPACITY here other than you want to
> do the update early on at the lowest domain level during balancing
> right?

I'm not sure what you are suggesting, the only interested domain
is the SMT domain. It contains all the information we need without
irrelevant data.

> >          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 :)

> 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.
>
> Still, that doesn't feel quite good enough, since we're only updating
> the per_cpu sd state for the given cpu. That seems like it will
> frequently leave us with idle cpus, and select_idle_sibling() skipping
> select_idle_cpu due to a false negative from test_idle_cpus(). Am I
> missing something there?

Until this patch, the sd_smt_shared->state is not used.

> > +
> >   /**
> >    * update_sd_lb_stats - Update sched_domain's statistics for load balancing.
> >    * @env: The load balancing environment.
> > @@ -9270,8 +9327,9 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> >          /* Tag domain that child domain prefers tasks go to siblings first */
> >          sds->prefer_sibling = child && child->flags & SD_PREFER_SIBLING;
> >
> > -
> > -       if (env->sd->flags & SD_NUMA)
> > +       if (env->sd->flags & SD_SHARE_CPUCAPACITY)
> > +               sd_update_state(env, sds);
> > +       else if (env->sd->flags & SD_NUMA)
> >                  env->fbq_type = fbq_classify_group(&sds->busiest_stat);
> >
> >          if (!env->sd->parent) {
> > --
> > 2.31.1