Re: [RFC PATCH v2] sched/fair: select idle cpu from idle cpumask in sched domain

[Vincent Guittot]

On Mon, 21 Sep 2020 at 17:14, Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> On Thu, 17 Sep 2020 at 11:21, Li, Aubrey <aubrey.li@xxxxxxxxxxxxxxx> wrote:
> >
> > On 2020/9/16 19:00, Mel Gorman wrote:
> > > On Wed, Sep 16, 2020 at 12:31:03PM +0800, Aubrey Li 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.
> > >>
> > >> The following benchmarks were tested on a x86 4 socket system with
> > >> 24 cores per socket and 2 hyperthreads per core, total 192 CPUs:
> > >>
> > >
> > > This still appears to be tied to turning the tick off. An idle CPU
> > > available for computation does not necessarily have the tick turned off
> > > if it's for short periods of time. When nohz is disabled or a machine is
> > > active enough that CPUs are not disabling the tick, select_idle_cpu may
> > > fail to select an idle CPU and instead stack tasks on the old CPU.
> > >
> > > The other subtlety is that select_idle_sibling() currently allows a
> > > SCHED_IDLE cpu to be used as a wakeup target. The CPU is not really
> > > idle as such, it's simply running a low priority task that is suitable
> > > for preemption. I suspect this patch breaks that.
> > >
> > Thanks!
> >
> > I shall post a v3 with performance data, I made a quick uperf testing and
> > found the benefit is still there. So I posted the patch here and looking
> > forward to your comments before I start the benchmarks.
> >
> > Thanks,
> > -Aubrey
> >
> > -----------------------------------------------------------------------
> > 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..9a3c82645472 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6023,6 +6023,26 @@ void __update_idle_core(struct rq *rq)
> >         rcu_read_unlock();
> >  }
> >
> > +/*
> > + * Update cpu idle state and record this information
> > + * in sd_llc_shared->idle_cpus_span.
> > + */
> > +void update_idle_cpumask(struct rq *rq)
> > +{
> > +       struct sched_domain *sd;
> > +       int cpu = cpu_of(rq);
> > +
> > +       rcu_read_lock();
> > +       sd = rcu_dereference(per_cpu(sd_llc, cpu));
> > +       if (!sd || !sd->shared)
> > +               goto unlock;
> > +       if (!available_idle_cpu(cpu) || !sched_idle_cpu(cpu))
> > +               goto unlock;
> > +       cpumask_set_cpu(cpu, sds_idle_cpus(sd->shared));
> > +unlock:
> > +       rcu_read_unlock();
> > +}
> > +
> >  /*
> >   * Scan the entire LLC domain for idle cores; this dynamically switches off if
> >   * there are no idle cores left in the system; tracked through
> > @@ -6136,7 +6156,12 @@ 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);
> > +       /*
> > +        * sched_domain_shared is set only at shared cache level,
> > +        * this works only because select_idle_cpu is called with
> > +        * sd_llc.
> > +        */
> > +       cpumask_and(cpus, sds_idle_cpus(sd->shared), p->cpus_ptr);
> >
> >         for_each_cpu_wrap(cpu, cpus, target) {
> >                 if (!--nr)
> > @@ -6712,6 +6737,10 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> >
> >                 if (want_affine)
> >                         current->recent_used_cpu = cpu;
> > +
> > +               sd = rcu_dereference(per_cpu(sd_llc, new_cpu));
> > +               if (sd && sd->shared)
> > +                       cpumask_clear_cpu(new_cpu, sds_idle_cpus(sd->shared));
>
> Why are you clearing the bit only for the fast path ? the slow path
> can also select an idle CPU
>
> Then, I'm afraid that updating a cpumask at each and every task wakeup
> will be far too expensive. That's why we are ot updating

That's why we are not updating

> nohz.idle_cpus_mask at each and every enter/exit idle but only once
> per tick.
>
> And a quick test with hackbench on my octo cores arm64 gives for 12
> iterations of: hackbench -l 2560 -g 1
> tip/sched/core :  1.324(+/- 1.26%)
> with this patch :  2.419(+/- 12.31%) -82% regression
>
> >         }
> >         rcu_read_unlock();
> >
> > @@ -10871,6 +10900,9 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
> >                 /* ensure bandwidth has been allocated on our new cfs_rq */
> >                 account_cfs_rq_runtime(cfs_rq, 0);
> >         }
> > +       /* Update idle cpumask if task has idle policy */
> > +       if (unlikely(task_has_idle_policy(p)))
> > +               update_idle_cpumask(rq);
>
> it's wrong because a sched_idle task will run for time to time even
> when some cfs tasks are runnable
>
> >  }
> >
> >  void init_cfs_rq(struct cfs_rq *cfs_rq)
> > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> > index 1ae95b9150d3..876dfdfe35bb 100644
> > --- a/kernel/sched/idle.c
> > +++ b/kernel/sched/idle.c
> > @@ -405,6 +405,7 @@ static void put_prev_task_idle(struct rq *rq, struct task_struct *prev)
> >  static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
> >  {
> >         update_idle_core(rq);
> > +       update_idle_cpumask(rq);
> >         schedstat_inc(rq->sched_goidle);
> >  }
> >
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index c82857e2e288..7a3355f61bcf 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1069,6 +1069,7 @@ static inline void update_idle_core(struct rq *rq)
> >  #else
> >  static inline void update_idle_core(struct rq *rq) { }
> >  #endif
> > +void update_idle_cpumask(struct rq *rq);
> >
> >  DECLARE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
> >
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index 9079d865a935..f14a6ef4de57 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -1407,6 +1407,7 @@ sd_init(struct sched_domain_topology_level *tl,
> >                 sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
> >                 atomic_inc(&sd->shared->ref);
> >                 atomic_set(&sd->shared->nr_busy_cpus, sd_weight);
> > +               cpumask_copy(sds_idle_cpus(sd->shared), sched_domain_span(sd));
> >         }
> >
> >         sd->private = sdd;
> > @@ -1769,7 +1770,7 @@ static int __sdt_alloc(const struct cpumask *cpu_map)
> >
> >                         *per_cpu_ptr(sdd->sd, j) = sd;
> >
> > -                       sds = kzalloc_node(sizeof(struct sched_domain_shared),
> > +                       sds = kzalloc_node(sizeof(struct sched_domain_shared) + cpumask_size(),
> >                                         GFP_KERNEL, cpu_to_node(j));
> >                         if (!sds)
> >                                 return -ENOMEM;
> >