linux-kernel - Re: [PATCH v6 03/11] sched/rt: add rt

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Date:   Fri, 15 Jun 2018 14:18:07 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        viresh kumar <viresh.kumar@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Quentin Perret <quentin.perret@....com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v6 03/11] sched/rt: add rt_rq utilization tracking

Hi Dietmar,

On 15 June 2018 at 13:52, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> On 06/08/2018 02:09 PM, Vincent Guittot wrote:
>> schedutil governor relies on cfs_rq's util_avg to choose the OPP when cfs
>> tasks are running. When the CPU is overloaded by cfs and rt tasks, cfs tasks
>> are preempted by rt tasks and in this case util_avg reflects the remaining
>> capacity but not what cfs want to use. In such case, schedutil can select a
>> lower OPP whereas the CPU is overloaded. In order to have a more accurate
>> view of the utilization of the CPU, we track the utilization of rt tasks.
>>
>> rt_rq uses rq_clock_task and cfs_rq uses cfs_rq_clock_task but they are
>> the same at the root group level, so the PELT windows of the util_sum are
>> aligned.
>>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>
> [...]
>
> ;
>> diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
>> index 4174582..81c0d7e 100644
>> --- a/kernel/sched/pelt.c
>> +++ b/kernel/sched/pelt.c
>> @@ -307,3 +307,25 @@ int __update_load_avg_cfs_rq(u64 now, int cpu, struct cfs_rq *cfs_rq)
>>
>>       return 0;
>>   }
>> +
>> +/*
>> + * rt_rq:
>> + *
>> + *   util_sum = \Sum se->avg.util_sum but se->avg.util_sum is not tracked
>> + *   util_sum = cpu_scale * load_sum
>> + *   runnable_load_sum = load_sum
>> + *
>> + */
>> +
>> +int update_rt_rq_load_avg(u64 now, struct rq *rq, int running)
>> +{
>> +     if (___update_load_sum(now, rq->cpu, &rq->avg_rt,
>> +                             running,
>> +                             running,
>> +                             running)) {
>
> The patch clearly says that this is about utilization but what happens
> to load and runnable load for the rt rq part when you call
> ___update_load_sum() with load=[0,1] and runnable=[0,1]?

I would say the same than what happens for se which has
___update_load_sum(now, cpu, &se->avg, !!se->on_rq, !!se->on_rq,
cfs_rq->curr == se))

>
> It looks like that the math would require 1024 instead of 1 for load and
> runnable so that we would see a load_avg or runnable_load_avg != 0.

why does it require 1024 ? the min  weight of a task is 15 and the min
share of a sched group is 2. AFAICT, there is no requirement mainly
because we are not using them as they will not give any additional
information compare to util_avg

>
> 1594.075128: bprint: update_rt_rq_load_avg: now=1593937228087 cpu=4 running=1
> 1594.075129: bprint: update_rt_rq_load_avg: delta=3068 cpu=4 load=1 runnable=1 running=1 scale_freq=1024 scale_cpu=1024 periods=2
> 1594.075130: bprint: update_rt_rq_load_avg: load_sum=23927 +2879 runnable_load_sum=23927 +2879 util_sum=24506165 +2948096
> 1594.075130: bprint: update_rt_rq_load_avg: load_avg=0 runnable_load_avg=0 util_avg=513
>
> IMHO, the patch should say whether load and runnable load are supported as well or not.

Although it is stated that we track only utilization , i can probably
mentioned clearly that load_avg and runnable_load_avg are useless

Vincent
>
> [...]