linux-kernel - Re: [PATCH v3 4/5] sched/pelt: Add a new runnable average signal

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Date:   Thu, 20 Feb 2020 16:11:18 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Phil Auld <pauld@...hat.com>, Parth Shah <parth@...ux.ibm.com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH v3 4/5] sched/pelt: Add a new runnable average signal

On 20/02/2020 14:36, Vincent Guittot wrote:
> I agree that setting by default to SCHED_CAPACITY_SCALE is too much
> for little core.
> The problem for little core can be fixed by using the cpu capacity instead
> 

So that's indeed better for big.LITTLE & co. Any reason however for not
aligning with the initialization of util_avg ?

With the default MC imbalance_pct (117), it takes 875 utilization to make
a single CPU group (with 1024 capacity) overloaded (group_is_overloaded()).
For a completely idle CPU, that means forking at least 3 tasks (512 + 256 +
128 util_avg)

With your change, it only takes 2 tasks. I know I'm being nitpicky here, but
I feel like those should be aligned, unless we have a proper argument against
it - in which case this should also appear in the changelog with so far only
mentions issues with util_avg migration, not the fork time initialization.

> @@ -796,6 +794,8 @@ void post_init_entity_util_avg(struct task_struct *p)
>                 }
>         }
> 
> +       sa->runnable_avg = cpu_scale;
> +
>         if (p->sched_class != &fair_sched_class) {
>                 /*
>                  * For !fair tasks do:
>>
>>>               sa->load_avg = scale_load_down(se->load.weight);
>>> +     }
>>>
>>>       /* when this task enqueue'ed, it will contribute to its cfs_rq's load_avg */
>>>  }