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

["Li, Aubrey" <aubrey.li@linux.intel.com>]

On 2020/11/19 16:19, Vincent Guittot wrote:
> On Thu, 19 Nov 2020 at 02:34, Li, Aubrey <aubrey.li@linux.intel.com> wrote:
>>
>> 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?
> 
> when system is in suspend to idle state, the cpu is idle. Would be
> good to set it in the mask so it can be used after the resume without
> waiting for an idle load balance to pull task if any
> 

Okay, make sense, thanks!