linux-kernel - Re: [PATCH v2 1/3] sched: Stop nohz stats when decayed

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Date:   Thu, 8 Feb 2018 14:36:40 +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 v2 1/3] sched: Stop nohz stats when decayed

On 8 February 2018 at 13:46, Valentin Schneider
<valentin.schneider@....com> wrote:
> On 02/06/2018 07:23 PM, Vincent Guittot wrote:
>> [...]
>> @@ -7826,8 +7842,8 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>>       for_each_cpu_and(i, sched_group_span(group), env->cpus) {
>>               struct rq *rq = cpu_rq(i);
>>
>> -             if (env->flags & LBF_NOHZ_STATS)
>> -                     update_nohz_stats(rq);
>> +             if ((env->flags & LBF_NOHZ_STATS) && update_nohz_stats(rq))
>> +                     env->flags |= LBF_NOHZ_AGAIN;
>>
>>               /* Bias balancing toward cpus of our domain */
>>               if (local_group)
>> @@ -7979,18 +7995,15 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>>       struct sg_lb_stats *local = &sds->local_stat;
>>       struct sg_lb_stats tmp_sgs;
>>       int load_idx, prefer_sibling = 0;
>> +     int has_blocked = READ_ONCE(nohz.has_blocked);
>>       bool overload = false;
>>
>>       if (child && child->flags & SD_PREFER_SIBLING)
>>               prefer_sibling = 1;
>>
>>  #ifdef CONFIG_NO_HZ_COMMON
>> -     if (env->idle == CPU_NEWLY_IDLE) {
>> +     if (env->idle == CPU_NEWLY_IDLE && has_blocked)
>>               env->flags |= LBF_NOHZ_STATS;
>> -
>> -             if (cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd)))
>> -                     nohz.next_stats = jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD);
>> -     }
>>  #endif
>>
>>       load_idx = get_sd_load_idx(env->sd, env->idle);
>> @@ -8046,6 +8059,15 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>>               sg = sg->next;
>>       } while (sg != env->sd->groups);
>>
>> +#ifdef CONFIG_NO_HZ_COMMON
>> +     if ((env->flags & LBF_NOHZ_AGAIN) &&
>> +         cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) {
>> +
>> +             WRITE_ONCE(nohz.next_blocked,
>> +                             jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD));
>
> Here we push the stats update forward if we visited all the nohz CPUs but they
> still have blocked load. IMO we should also clear the nohz.has_blocked flag
> if we visited all the nohz CPUs and none had blocked load left.
>
> If we don't do that, we could very well have cleared all of the nohz blocked
> load in idle_balance and successfully pulled a task, but the flag isn't
> cleared so we'll end up doing a _nohz_idle_balance() later on for nothing.
>
>
> As I said in a previous comment, we also have this problem with periodic
> load balance: if a CPU goes nohz (nohz.has_blocked is raised) but wakes up,
> e.g. before the next nohz.next_blocked, we should stop kicking ILBs
>
> Now I'd need to test this, but I think it can actually get worse: if that
> CPU keeps generating blocked load after this short idle period, no matter
> how many _nohz_idle_balance() we go through we will never reach a point
> where nohz.has_blocked gets cleared, and we'll keep kicking those ILBs to
> update blocked load that already gets updated in the periodic balance.
>
> I think that's where a nohz blocked load cpumask can also help: on top of
> skipping nohz CPUs that don't need an update, we can stop the whole remote
> update machinery when the last nohz cpu with blocked load wakes up, or say
> when it goes through its first periodic balance.

The main question is : Do we want to remove all useless kicks to ilb
or useless calls to _nohz_idle_balance at the cost of adding more
latency in the idle/wakeup path because of the additional atomic
operations to keep track of which CPUs are idle, tickless with blocked
load or do we accept to kick ilb or call _nohz_idle_balance uselessly
from time to time for some use cases

I agree with you that there are some useless calls with the proposal
which can be removed with additional checks, variables and cpumask
manipulation. Which use case will benefits from these additional
checks and does it worth ?

Vincent

>
>> +     }
>> +#endif
>> +
>