linux-kernel - Re: [PATCH v5 1/3] sched: Stop nohz stats when decayed

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Date:   Fri, 16 Feb 2018 14:44:04 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Morten Rasmussen <morten.rasmussen@...s.arm.com>,
        Brendan Jackman <brendan.jackman@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v5 1/3] sched: Stop nohz stats when decayed

On 16 February 2018 at 13:13, Valentin Schneider
<valentin.schneider@....com> wrote:
> On 02/14/2018 03:26 PM, Vincent Guittot wrote:
>> Stopped the periodic update of blocked load when all idle CPUs have fully
>> decayed. We introduce a new nohz.has_blocked that reflect if some idle
>> CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked
>> is set everytime that a Idle CPU can have blocked load and it is then clear
>> when no more blocked load has been detected during an update. We don't need
>> atomic operation but only to make cure of the right ordering when updating
>> nohz.idle_cpus_mask and nohz.has_blocked.
>>
>> Suggested-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>> ---
>>  kernel/sched/fair.c  | 122 ++++++++++++++++++++++++++++++++++++++++++---------
>>  kernel/sched/sched.h |   1 +
>>  2 files changed, 102 insertions(+), 21 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 7af1fa9..5a6835e 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>>
>> [...]
>>> @@ -9374,6 +9427,22 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>>
>>       SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK);
>>
>> +     /*
>> +      * We assume there will be no idle load after this update and clear
>> +      * the has_blocked flag. If a cpu enters idle in the mean time, it will
>> +      * set the has_blocked flag and trig another update of idle load.
>> +      * Because a cpu that becomes idle, is added to idle_cpus_mask before
>> +      * setting the flag, we are sure to not clear the state and not
>> +      * check the load of an idle cpu.
>> +      */
>> +     WRITE_ONCE(nohz.has_blocked, 0);
>> +
>> +     /*
>> +      * Ensures that if we miss the CPU, we must see the has_blocked
>> +      * store from nohz_balance_enter_idle().
>> +      */
>> +     smp_mb();
>> +
>>       for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
>>               if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
>>                       continue;
>> @@ -9383,11 +9452,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>>                * work being done for other cpus. Next load
>>                * balancing owner will pick it up.
>>                */
>> -             if (need_resched())
>> -                     break;
>> +             if (need_resched()) {
>> +                     has_blocked_load = true;
>> +                     goto abort;
>> +             }
>>
>>               rq = cpu_rq(balance_cpu);
>>
>
> I'd say it's safe to do the following here. The flag is raised in
> nohz_balance_enter_idle() before the smp_mb(), so we won't skip a CPU
> that just got added to nohz.idle_cpus_mask.

rq->has_blocked_load will be set before the barrier only if
nohz_tick_stopped is not already set,
Otherwise, we skip cpumask update and the barrier in  nohz_balance_enter_idle

>
>                 /*
>                  * This cpu doesn't have any remaining blocked load, skip it.
>                  * It's sane to do this because this flag is raised in
>                  * nohz_balance_enter_idle()
>                  */
>                 if ((flags & NOHZ_KICK_MASK) == NOHZ_STATS_KICK &&
>                     !rq->has_blocked_load)
>                         continue;
>
>> +             update_blocked_averages(rq->cpu);
>> +             has_blocked_load |= rq->has_blocked_load;
>> +
>>               /*
>>                * If time for next balance is due,
>>                * do the balance.
>> @@ -9400,7 +9474,6 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>>                       cpu_load_update_idle(rq);
>>                       rq_unlock_irq(rq, &rf);
>>
>> -                     update_blocked_averages(rq->cpu);
>>                       if (flags & NOHZ_BALANCE_KICK)
>>                               rebalance_domains(rq, CPU_IDLE);
>>               }
>> @@ -9415,7 +9488,13 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>>       if (flags & NOHZ_BALANCE_KICK)
>>               rebalance_domains(this_rq, CPU_IDLE);
>>
>> -     nohz.next_stats = next_stats;
>> +     WRITE_ONCE(nohz.next_blocked,
>> +             now + msecs_to_jiffies(LOAD_AVG_PERIOD));
>> +
>> +abort:
>> +     /* There is still blocked load, enable periodic update */
>> +     if (has_blocked_load)
>> +             WRITE_ONCE(nohz.has_blocked, 1);
>>
>>       /*
>>        * next_balance will be updated only when there is a need.