Re: [RFC PATCH v7] sched/fair: select idle cpu from idle cpumask for task wakeup

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

On 2020/12/9 21:09, Vincent Guittot wrote:
> On Wed, 9 Dec 2020 at 11:58, Li, Aubrey <aubrey.li@linux.intel.com> wrote:
>>
>> On 2020/12/9 16:15, Vincent Guittot wrote:
>>> Le mercredi 09 déc. 2020 à 14:24:04 (+0800), Aubrey Li a écrit :
>>>> 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 were tested on a x86 4 socket system with 24 cores per
>>>> socket and 2 hyperthreads per core, total 192 CPUs, no regression
>>>> found.
>>>>
>>>> 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>
>>>> ---
>>>>  include/linux/sched/topology.h | 13 +++++++++
>>>>  kernel/sched/core.c            |  2 ++
>>>>  kernel/sched/fair.c            | 51 +++++++++++++++++++++++++++++++++-
>>>>  kernel/sched/idle.c            |  5 ++++
>>>>  kernel/sched/sched.h           |  4 +++
>>>>  kernel/sched/topology.c        |  3 +-
>>>>  6 files changed, 76 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..c4c51ff3402a 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, false);
>>>
>>> Test rq->idle_balance here instead of adding the test in update_idle_cpumask which is only
>>> relevant for this situation.
>>
>> If called from idle path, because !set_idle is false, rq->idle_balance won't be tested actually.
>>
>>         if (!set_idle && rq->idle_balance)
>>                 return;
>>
>> So is it okay to leave it here to keep scheduler_tick a bit concise?
> 
> I don't like having a tick specific condition in a generic function.
> rq->idle_balance is only relevant in this case
> 
> calling update_idle_cpumask(rq->idle_balance) in scheduler_tick()
> should do the job and we can remove the check of rq->idle_balance in
> update_idle_cpumask()
> 
> In case of scheduler_tick() called when idle , we will only test if
> (rq->last_idle_state == idle_state) and return
> 

I see, will come up with a v8 soon.

Thanks,
-Aubrey