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Date:   Wed, 9 Dec 2020 09:15:41 +0100
From:   Vincent Guittot <vincent.guittot@linaro.org>
To:     Aubrey Li <aubrey.li@linux.intel.com>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        mgorman@techsingularity.net, valentin.schneider@arm.com,
        qais.yousef@arm.com, dietmar.eggemann@arm.com, rostedt@goodmis.org,
        bsegall@google.com, tim.c.chen@linux.intel.com,
        linux-kernel@vger.kernel.org, Mel Gorman <mgorman@suse.de>,
        Jiang Biao <benbjiang@gmail.com>
Subject: Re: [RFC PATCH v7] sched/fair: select idle cpu from idle cpumask for
 task wakeup
Message-ID: <20201209081541.GA5071@vingu-book>
References: <20201209062404.175565-1-aubrey.li@linux.intel.com>
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Le mercredi 09 d?c. 2020 ? 14:24:04 (+0800), Aubrey Li a ?crit :
> Add idle cpumask to track idle cpus in sched domain. Every time
> a CPU enters idle, the CPU is set in idle cpumask to be a wakeup
> target. And if the CPU is not in idle, the CPU is cleared in idle
> cpumask during scheduler tick to ratelimit idle cpumask update.
> 
> When a task wakes up to select an idle cpu, scanning idle cpumask
> has lower cost than scanning all the cpus in last level cache domain,
> especially when the system is heavily loaded.
> 
> Benchmarks including hackbench, schbench, uperf, sysbench mysql
> and kbuild were tested on a x86 4 socket system with 24 cores per
> socket and 2 hyperthreads per core, total 192 CPUs, no regression
> found.
> 
> v6->v7:
> - place the whole idle cpumask mechanism under CONFIG_SMP.
> 
> v5->v6:
> - decouple idle cpumask update from stop_tick signal, set idle CPU
>   in idle cpumask every time the CPU enters idle
> 
> v4->v5:
> - add update_idle_cpumask for s2idle case
> - keep the same ordering of tick_nohz_idle_stop_tick() and update_
>   idle_cpumask() everywhere
> 
> v3->v4:
> - change setting idle cpumask from every idle entry to tickless idle
>   if cpu driver is available.
> - move clearing idle cpumask to scheduler_tick to decouple nohz mode.
> 
> v2->v3:
> - change setting idle cpumask to every idle entry, otherwise schbench
>   has a regression of 99th percentile latency.
> - change clearing idle cpumask to nohz_balancer_kick(), so updating
>   idle cpumask is ratelimited in the idle exiting path.
> - set SCHED_IDLE cpu in idle cpumask to allow it as a wakeup target.
> 
> v1->v2:
> - idle cpumask is updated in the nohz routines, by initializing idle
>   cpumask with sched_domain_span(sd), nohz=off case remains the original
>   behavior.
> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Qais Yousef <qais.yousef@arm.com>
> Cc: Valentin Schneider <valentin.schneider@arm.com>
> Cc: Jiang Biao <benbjiang@gmail.com>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
> ---
>  include/linux/sched/topology.h | 13 +++++++++
>  kernel/sched/core.c            |  2 ++
>  kernel/sched/fair.c            | 51 +++++++++++++++++++++++++++++++++-
>  kernel/sched/idle.c            |  5 ++++
>  kernel/sched/sched.h           |  4 +++
>  kernel/sched/topology.c        |  3 +-
>  6 files changed, 76 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index 820511289857..b47b85163607 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/core.c b/kernel/sched/core.c
> index c4da7e17b906..c4c51ff3402a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4011,6 +4011,7 @@ void scheduler_tick(void)
>  
>  #ifdef CONFIG_SMP
>  	rq->idle_balance = idle_cpu(cpu);
> +	update_idle_cpumask(cpu, false);

Test rq->idle_balance here instead of adding the test in update_idle_cpumask which is only
relevant for this situation.

if (!rq->idle_balance)
    update_idle_cpumask(cpu, false);

>  	trigger_load_balance(rq);
>  #endif
>  }
> @@ -7186,6 +7187,7 @@ void __init sched_init(void)
>  		rq->idle_stamp = 0;
>  		rq->avg_idle = 2*sysctl_sched_migration_cost;
>  		rq->max_idle_balance_cost = sysctl_sched_migration_cost;
> +		rq->last_idle_state = 1;
>  
>  		INIT_LIST_HEAD(&rq->cfs_tasks);
>  
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c0c4d9ad7da8..7306f8886120 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6146,7 +6146,12 @@ 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)
> @@ -6806,6 +6811,50 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  
>  	return newidle_balance(rq, rf) != 0;
>  }
> +
> +/*
> + * Update cpu idle state and record this information
> + * in sd_llc_shared->idle_cpus_span.
> + */
> +void update_idle_cpumask(int cpu, bool set_idle)
> +{
> +	struct sched_domain *sd;
> +	struct rq *rq = cpu_rq(cpu);
> +	int idle_state;
> +
> +	/*
> +	 * If called from scheduler tick, only update
> +	 * idle cpumask if the CPU is busy, as idle
> +	 * cpumask is also updated on idle entry.
> +	 *
> +	 */
> +	if (!set_idle && rq->idle_balance)
> +		return;
> +	/*
> +	 * Also set SCHED_IDLE cpu in idle cpumask to
> +	 * allow SCHED_IDLE cpu as a wakeup target
> +	 */
> +	idle_state = set_idle || sched_idle_cpu(cpu);
> +	/*
> +	 * No need to update idle cpumask if the state
> +	 * does not change.
> +	 */
> +	if (rq->last_idle_state == idle_state)
> +		return;
> +	/*
> +	 * Called with irq disabled, rcu_read_lock() is not needed.
> +	 */
> +	sd = rcu_dereference(per_cpu(sd_llc, cpu));
> +	if (unlikely(!sd))
> +		return;
> +
> +	if (idle_state)
> +		cpumask_set_cpu(cpu, sds_idle_cpus(sd->shared));
> +	else
> +		cpumask_clear_cpu(cpu, sds_idle_cpus(sd->shared));
> +
> +	rq->last_idle_state = idle_state;
> +}
>  #endif /* CONFIG_SMP */
>  
>  static unsigned long wakeup_gran(struct sched_entity *se)
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index f324dc36fc43..2c517d6a061a 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -257,6 +257,11 @@ static void do_idle(void)
>  			cpuhp_report_idle_dead();
>  			arch_cpu_idle_dead();
>  		}
> +		/*
> +		 * The CPU is about to go idle, set it in idle cpumask
> +		 * to be a wake up target.
> +		 */
> +		update_idle_cpumask(cpu, true);
>  
>  		arch_cpu_idle_enter();
>  
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 8d1ca65db3b0..2167ca48f3aa 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -976,6 +976,7 @@ struct rq {
>  
>  	unsigned char		nohz_idle_balance;
>  	unsigned char		idle_balance;
> +	unsigned char		last_idle_state;
>  
>  	unsigned long		misfit_task_load;
>  
> @@ -1516,6 +1517,8 @@ static inline unsigned int group_first_cpu(struct sched_group *group)
>  
>  extern int group_balance_cpu(struct sched_group *sg);
>  
> +void update_idle_cpumask(int cpu, bool set_idle);
> +
>  #if defined(CONFIG_SCHED_DEBUG) && defined(CONFIG_SYSCTL)
>  void register_sched_domain_sysctl(void);
>  void dirty_sched_domain_sysctl(int cpu);
> @@ -1536,6 +1539,7 @@ extern void flush_smp_call_function_from_idle(void);
>  
>  #else /* !CONFIG_SMP: */
>  static inline void flush_smp_call_function_from_idle(void) { }
> +static inline void update_idle_cpumask(int cpu, bool set_idle) { }
>  #endif
>  
>  #include "stats.h"
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 1bd7e3af904f..541bd3a7de4d 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;
> -- 
> 2.25.1
>