linux-kernel - Re: [RFC PATCH v2 1/2] sched/fair: Introduce UTIL_FITS

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Date:   Tue, 24 Oct 2023 17:03:25 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Chen Yu <yu.c.chen@...el.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Swapnil Sapkal <Swapnil.Sapkal@....com>,
        Aaron Lu <aaron.lu@...el.com>, Tim Chen <tim.c.chen@...el.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>, x86@...nel.org
Subject: Re: [RFC PATCH v2 1/2] sched/fair: Introduce UTIL_FITS_CAPACITY
 feature (v2)

On 24/10/2023 08:10, Chen Yu wrote:
> On 2023-10-23 at 11:04:49 -0400, Mathieu Desnoyers wrote:
>> On 2023-10-23 10:11, Dietmar Eggemann wrote:
>>> On 19/10/2023 18:05, Mathieu Desnoyers wrote:

[...]

>>> Or like find_energy_efficient_cpu() (feec(), used in
>>> Energy-Aware-Scheduling (EAS)) which uses cpu_util(cpu, p, cpu, 0) to get:
>>>
>>>    max(util_avg(CPU + p), util_est(CPU + p))
>>
>> I've tried using cpu_util(), but unfortunately anything that considers
>> blocked/sleeping tasks in its utilization total does not work for my
>> use-case.
>>
>> From cpu_util():
>>
>>  * CPU utilization is the sum of running time of runnable tasks plus the
>>  * recent utilization of currently non-runnable tasks on that CPU.
>>
> 
> I thought cpu_util() indicates the utilization decay sum of task that was once
> "running" on this CPU, but will not sum up the "util/load" of the blocked/sleeping
> task?

cpu_util() here refers to:

    cpu_util(int cpu, struct task_struct *p, int dst_cpu, int boost)

which when called with (cpu, p, cpu, 0) and task_cpu(p) != cpu returns:

    max(util_avg(CPU + p), util_est(CPU + p))

The term `CPU utilization` in cpu_util()'s header stands for
cfs_rq->avg.util_avg.

It does not sum up the utilization of blocked tasks but it can contain
it. They have to be a blocked tasks and not tasks which were running in
cfs_rq since we subtract utilization of tasks which are migrating away
from the cfs_rq (cfs_rq->removed.util_avg in remove_entity_load_avg()
and update_cfs_rq_load_avg()).
> accumulate_sum()
>     /* only the running task's util will be sum up */
>     if (running)
>        sa->util_sum += contrib << SCHED_CAPACITY_SHIFT;
> 
> WRITE_ONCE(sa->util_avg, sa->util_sum / divider);

__update_load_avg_cfs_rq()

  ___update_load_sum(..., cfs_rq->curr != NULL
                          ^^^^^^^^^^^^^^^^^^^^
                          running
    accumulate_sum()

      if (periods)
        /* decay _sum */
        sa->util_sum = decay_load(sa->util_sum, ...)

        if (load)
          /* decay and accrue _sum */
          contrib = __accumulate_pelt_segments(...)

When crossing periods we decay the old _sum and when additionally load
!= 0 we decay and accrue the new _sum as well.