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

[Li, Aubrey]

Hi Vincent,

On 2020/11/18 21:36, Vincent Guittot wrote:
> On Wed, 18 Nov 2020 at 04:48, Aubrey Li <aubrey.li@xxxxxxxxxxxxxxx> wrote:
>>
>> From: Aubrey Li <aubrey.li@xxxxxxxxx>
>>
>> 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@xxxxxxx>
>> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>> Cc: Qais Yousef <qais.yousef@xxxxxxx>
>> Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
>> Cc: Jiang Biao <benbjiang@xxxxxxxxx>
>> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
>> Signed-off-by: Aubrey Li <aubrey.li@xxxxxxxxxxxxxxx>
>> ---
>>  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