linux-kernel - Re: [PATCH V3 2/2] sched/fair: Fallback to sched-idle CPU if idle CPU isn't found

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Date:   Mon, 1 Jul 2019 15:08:41 -0700
From:   Subhra Mazumdar <subhra.mazumdar@...cle.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>, tkjos@...gle.com,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        quentin.perret@...aro.org, chris.redpath@....com,
        steven.sistare@...cle.com, songliubraving@...com
Subject: Re: [PATCH V3 2/2] sched/fair: Fallback to sched-idle CPU if idle CPU
 isn't found


On 7/1/19 1:03 AM, Viresh Kumar wrote:
> On 28-06-19, 18:16, Subhra Mazumdar wrote:
>> On 6/25/19 10:06 PM, Viresh Kumar wrote:
>>> We try to find an idle CPU to run the next task, but in case we don't
>>> find an idle CPU it is better to pick a CPU which will run the task the
>>> soonest, for performance reason.
>>>
>>> A CPU which isn't idle but has only SCHED_IDLE activity queued on it
>>> should be a good target based on this criteria as any normal fair task
>>> will most likely preempt the currently running SCHED_IDLE task
>>> immediately. In fact, choosing a SCHED_IDLE CPU over a fully idle one
>>> shall give better results as it should be able to run the task sooner
>>> than an idle CPU (which requires to be woken up from an idle state).
>>>
>>> This patch updates both fast and slow paths with this optimization.
>>>
>>> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
>>> ---
>>>    kernel/sched/fair.c | 43 +++++++++++++++++++++++++++++++++----------
>>>    1 file changed, 33 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index 1277adc3e7ed..2e0527fd468c 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -5376,6 +5376,15 @@ static struct {
>>>    #endif /* CONFIG_NO_HZ_COMMON */
>>> +/* CPU only has SCHED_IDLE tasks enqueued */
>>> +static int sched_idle_cpu(int cpu)
>>> +{
>>> +	struct rq *rq = cpu_rq(cpu);
>>> +
>>> +	return unlikely(rq->nr_running == rq->cfs.idle_h_nr_running &&
>>> +			rq->nr_running);
>>> +}
>>> +
>> Shouldn't this check if rq->curr is also sched idle?
> Why wouldn't the current set of checks be enough to guarantee that ?
I thought nr_running does not include the on-cpu thread.
>
>> And why not drop the rq->nr_running non zero check?
> Because CPU isn't sched-idle if nr_running and idle_h_nr_running are both 0,
> i.e. it is an IDLE cpu in that case. And so I thought it is important to have
> this check as well.
>
idle_cpu() not only checks nr_running is 0 but also rq->curr == rq->idle