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

[Vincent Guittot]

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
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;
>