Re: [RFC PATCH v4] sched/fair: select idle cpu from idle cpumask for task wakeup

Vincent Guittot
2020-11-18T13:37:11Z

On Wed, 18 Nov 2020 at 04:48, Aubrey Li <aubrey.li@linux.intel.com> wrote:
>
> From: Aubrey Li <aubrey.li@intel.com>
>
> Add idle cpumask to track idle cpus in sched domain. When a CPU
> enters idle, if the idle driver indicates to stop tick, this CPU
> is set in the 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 low cost than scanning all the cpus in last level cache domain,
> especially when the system is heavily loaded.
>
> Benchmarks were tested on a x86 4 socket system with 24 cores per
> socket and 2 hyperthreads per core, total 192 CPUs. Hackbench and
> schbench have no notable change, uperf has:
>
> uperf throughput: netperf workload, tcp_nodelay, r/w size = 90
>
> threads baseline-avg %std patch-avg %std
> 96 1 0.83 1.23 3.27
> 144 1 1.03 1.67 2.67
> 192 1 0.69 1.81 3.59
> 240 1 2.84 1.51 2.67
>
> 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 | 52 +++++++++++++++++++++++++++++++++-
> kernel/sched/idle.c | 7 +++--
> kernel/sched/sched.h | 2 ++
> kernel/sched/topology.c | 3 +-
> 6 files changed, 74 insertions(+), 5 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 b1e0da56abca..c86ae0495163 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3994,6 +3994,7 @@ void scheduler_tick(void)
> rq_lock(rq, &rf);
>
> update_rq_clock(rq);
> + update_idle_cpumask(rq, false);
> thermal_pressure = arch_scale_thermal_pressure(cpu_of(rq));
> update_thermal_load_avg(rq_clock_thermal(rq), rq, thermal_pressure);
> curr->sched_class->task_tick(rq, curr, 0);
> @@ -7192,6 +7193,7 @@ void __init sched_init(void)
> rq_csd_init(rq, &rq->nohz_csd, nohz_csd_func);
> #endif
> #endif /* CONFIG_SMP */
> + rq->last_idle_state = 1;
> hrtick_rq_init(rq);
> atomic_set(&rq->nr_iowait, 0);
> }
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 48a6d442b444..d67fba5e406b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6145,7 +6145,12 @@ 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)
> @@ -6807,6 +6812,51 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> }
> #endif /* CONFIG_SMP */
>
> +/*
> + * Update cpu idle state and record this information
> + * in sd_llc_shared->idle_cpus_span.
> + */
> +void update_idle_cpumask(struct rq *rq, bool set_idle)
> +{
> + struct sched_domain *sd;
> + int cpu = cpu_of(rq);
> + 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 && idle_cpu(cpu))
> + 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;
> +
> + rcu_read_lock();
> + sd = rcu_dereference(per_cpu(sd_llc, cpu));
> + if (!sd || !sd->shared)
> + goto unlock;
> +
> + 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;
> +unlock:
> + rcu_read_unlock();
> +}
> +
> static unsigned long wakeup_gran(struct sched_entity *se)
> {
> unsigned long gran = sysctl_sched_wakeup_granularity;
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index f324dc36fc43..0bd83c00c22a 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -164,7 +164,7 @@ static void cpuidle_idle_call(void)
>
> if (cpuidle_not_available(drv, dev)) {
> tick_nohz_idle_stop_tick();
> -
> + update_idle_cpumask(this_rq(), true);
> default_idle_call();
> goto exit_idle;
> }
> @@ -205,9 +205,10 @@ static void cpuidle_idle_call(void)
> */
> next_state = cpuidle_select(drv, dev, &stop_tick);
>
> - if (stop_tick || tick_nohz_tick_stopped())
> + if (stop_tick || tick_nohz_tick_stopped()) {
> + update_idle_cpumask(this_rq(), true);
> tick_nohz_idle_stop_tick();

please keep same ordering here and above
tick_nohz_idle_stop_tick();
update_idle_cpumask(this_rq(), true);

Also, it might be good to call update_idle_cpumask everywhere
tick_nohz_idle_stop_tick is called. For example S2Idle case should
also set the mask

> - else
> + } else

Also add { } in the else

> tick_nohz_idle_retain_tick();
>
> entered_state = call_cpuidle(drv, dev, next_state);
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 8d1ca65db3b0..db460b20217a 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1004,6 +1004,7 @@ struct rq {
> /* This is used to determine avg_idle's max value */
> u64 max_idle_balance_cost;
> #endif /* CONFIG_SMP */
> + unsigned char last_idle_state;
>
> #ifdef CONFIG_IRQ_TIME_ACCOUNTING
> u64 prev_irq_time;
> @@ -1088,6 +1089,7 @@ 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, bool set_idle);
>
> DECLARE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
>
> 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
>