Re: [PATCH v3 6/8] sched/idle: Move busy_cpu accounting to idle callback

[Vincent Guittot <vincent.guittot@linaro.org>]

On Thu, 13 May 2021 at 09:41, Srikar Dronamraju
<srikar@linux.vnet.ibm.com> wrote:
>
> Currently we account nr_busy_cpus in no_hz idle functions.
> There is no reason why nr_busy_cpus should updated be in NO_HZ_COMMON
> configs only. Also scheduler can mark a CPU as non-busy as soon as an
> idle class task starts to run. Scheduler can then mark a CPU as busy
> as soon as its woken up from idle or a new task is placed on it's
> runqueue.
>
> Cc: LKML <linux-kernel@vger.kernel.org>
> Cc: Gautham R Shenoy <ego@linux.vnet.ibm.com>
> Cc: Parth Shah <parth@linux.ibm.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Valentin Schneider <valentin.schneider@arm.com>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Rik van Riel <riel@surriel.com>
> Cc: Aubrey Li <aubrey.li@linux.intel.com>
> Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> ---
>  kernel/sched/fair.c     |  6 ++++--
>  kernel/sched/idle.c     | 29 +++++++++++++++++++++++++++--
>  kernel/sched/sched.h    |  1 +
>  kernel/sched/topology.c |  2 ++
>  4 files changed, 34 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0dfe01de22d6..8f86359efdbd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10410,7 +10410,10 @@ static void set_cpu_sd_state_busy(int cpu)
>                 goto unlock;
>         sd->nohz_idle = 0;
>
> -       atomic_inc(&sd->shared->nr_busy_cpus);
> +       if (sd && per_cpu(is_idle, cpu)) {
> +               atomic_add_unless(&sd->shared->nr_busy_cpus, 1, per_cpu(sd_llc_size, cpu));
> +               per_cpu(is_idle, cpu) = 0;
> +       }
>  unlock:
>         rcu_read_unlock();
>  }
> @@ -10440,7 +10443,6 @@ static void set_cpu_sd_state_idle(int cpu)
>                 goto unlock;
>         sd->nohz_idle = 1;
>
> -       atomic_dec(&sd->shared->nr_busy_cpus);
>  unlock:
>         rcu_read_unlock();
>  }
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index a9f5a8ace59e..c13105fe06b3 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -431,12 +431,25 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl
>
>  static void put_prev_task_idle(struct rq *rq, struct task_struct *prev)
>  {
> -#ifdef CONFIG_SCHED_SMT
> +#ifdef CONFIG_SMP
> +       struct sched_domain_shared *sds;
>         int cpu = rq->cpu;
>
> +#ifdef CONFIG_SCHED_SMT
>         if (static_branch_likely(&sched_smt_present))
>                 set_core_busy(cpu);
>  #endif
> +
> +       rcu_read_lock();
> +       sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> +       if (sds) {
> +               if (per_cpu(is_idle, cpu)) {
> +                       atomic_inc(&sds->nr_busy_cpus);
> +                       per_cpu(is_idle, cpu) = 0;
> +               }
> +       }
> +       rcu_read_unlock();
> +#endif
>  }
>
>  static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
> @@ -448,9 +461,21 @@ static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool fir
>  struct task_struct *pick_next_task_idle(struct rq *rq)
>  {
>         struct task_struct *next = rq->idle;
> +#ifdef CONFIG_SMP
> +       struct sched_domain_shared *sds;
> +       int cpu = rq->cpu;
>
> -       set_next_task_idle(rq, next, true);
> +       rcu_read_lock();
> +       sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> +       if (sds) {
> +               atomic_add_unless(&sds->nr_busy_cpus, -1, 0);
> +               per_cpu(is_idle, cpu) = 1;
> +       }

One reason to update nr_busy_cpus only during tick is and not at each
and every single sleep/wakeup to limit the number of atomic_inc/dec in
case of storm of short running tasks. Because at the end , you waste
more time trying to accurately follow the current state of the CPU
than doing work

>
> +       rcu_read_unlock();
> +#endif
> +
> +       set_next_task_idle(rq, next, true);
>         return next;
>  }
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 98c3cfbc5d26..b66c4dad5fd2 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1496,6 +1496,7 @@ DECLARE_PER_CPU(int, sd_llc_id);
>  #ifdef CONFIG_SCHED_SMT
>  DECLARE_PER_CPU(int, smt_id);
>  #endif
> +DECLARE_PER_CPU(int, is_idle);
>  DECLARE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
>  DECLARE_PER_CPU(struct sched_domain __rcu *, sd_numa);
>  DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 232fb261dfc2..730252937712 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -647,6 +647,7 @@ DEFINE_PER_CPU(int, sd_llc_id);
>  #ifdef CONFIG_SCHED_SMT
>  DEFINE_PER_CPU(int, smt_id);
>  #endif
> +DEFINE_PER_CPU(int, is_idle);
>  DEFINE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
>  DEFINE_PER_CPU(struct sched_domain __rcu *, sd_numa);
>  DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
> @@ -673,6 +674,7 @@ static void update_top_cache_domain(int cpu)
>  #ifdef CONFIG_SCHED_SMT
>         per_cpu(smt_id, cpu) = cpumask_first(cpu_smt_mask(cpu));
>  #endif
> +       per_cpu(is_idle, cpu) = 1;
>         rcu_assign_pointer(per_cpu(sd_llc_shared, cpu), sds);
>
>         sd = lowest_flag_domain(cpu, SD_NUMA);
> --
> 2.18.2
>