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 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?

Thanks,
-Aubrey