Re: [RFC PATCH v8] sched/fair: select idle cpu from idle cpumask for task wakeup

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

On 2020/12/11 23:07, Vincent Guittot wrote:
> On Thu, 10 Dec 2020 at 02:44, Aubrey Li <aubrey.li@linux.intel.com> wrote:
>>
>> 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 have been tested on a x86 4 socket system with 24 cores per
>> socket and 2 hyperthreads per core, total 192 CPUs, no regression
>> found.
>>
>> v7->v8:
>> - refine update_idle_cpumask, no functionality change
>> - fix a suspicious RCU usage warning with CONFIG_PROVE_RCU=y
>>
>> 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>
> 
> This version looks good to me. I don't see regressions of v5 anymore
> and see some improvements on heavy cases

v5 or v8?

> 
> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> 
>> ---
>>  include/linux/sched/topology.h | 13 ++++++++++
>>  kernel/sched/core.c            |  2 ++
>>  kernel/sched/fair.c            | 45 +++++++++++++++++++++++++++++++++-
>>  kernel/sched/idle.c            |  5 ++++
>>  kernel/sched/sched.h           |  4 +++
>>  kernel/sched/topology.c        |  3 ++-
>>  6 files changed, 70 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..b136e2440ea4 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, rq->idle_balance);
>>         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..25f36ecfee54 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 +6811,44 @@ 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.
>> + *
>> + * This function is called with interrupts disabled.
>> + */
>> +void update_idle_cpumask(int cpu, bool idle)
>> +{
>> +       struct sched_domain *sd;
>> +       struct rq *rq = cpu_rq(cpu);
>> +       int idle_state;
>> +
>> +       /*
>> +        * Also set SCHED_IDLE cpu in idle cpumask to
>> +        * allow SCHED_IDLE cpu as a wakeup target.
>> +        */
>> +       idle_state = 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 protection is not needed.
>> +        */
>> +       sd = 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..4041d5a10de5 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 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 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
>>