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

Li, Aubrey
2020-11-19T01:34:12Z

Hi Vincent,

On 2020/11/18 21:36, Vincent Guittot wrote:
> 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);

okay, will do, thanks!

>
> 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

Hmm, I did think about this, but not sure what's the point here to
set the mask when the whole system suspend to idle and all user space
are forzen?

Also tick_nohz_idle_stop_tick is called when cpu is offline in cpuidle_idle_call,
I think adding an offlined cpu to idle cpumask may not be right.

>
>
>> - else
>> + } else
>
> Also add { } in the else
>

Thanks, will added it.

-Aubrey