linux-kernel - Re: [RFC PATCH 3/4] sched/fair: delay update_tg_load_avg() for cfs

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Message-ID: <96efae1a-37f6-c8da-5cdb-07b460307d6b@arm.com>
Date:   Thu, 20 Jul 2023 17:22:26 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Aaron Lu <aaron.lu@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Nitin Tekchandani <nitin.tekchandani@...el.com>,
        Yu Chen <yu.c.chen@...el.com>,
        Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/4] sched/fair: delay update_tg_load_avg() for
 cfs_rq's removed load

On 20/07/2023 17:02, Vincent Guittot wrote:
> On Thu, 20 Jul 2023 at 16:42, Aaron Lu <aaron.lu@...el.com> wrote:
>>
>> On Thu, Jul 20, 2023 at 03:10:30PM +0200, Vincent Guittot wrote:
>>> On Wed, 19 Jul 2023 at 15:29, Aaron Lu <aaron.lu@...el.com> wrote:
>>>>
>>>> On Wed, Jul 19, 2023 at 11:47:06AM +0200, Vincent Guittot wrote:
>>>>> On Wed, 19 Jul 2023 at 10:01, Aaron Lu <aaron.lu@...el.com> wrote:
>>>>>>
>>>>>> On Wed, Jul 19, 2023 at 01:18:26PM +0800, Aaron Lu wrote:

[...]

> What was wrong with your proposal to limit the update inside
> update_tg_load_avg()  ? except maybe s/1000000/NSEC_PER_MSEC/ and
> computing delta after testing the time since last update
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a80a73909dc2..e48fd0e6982d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3665,6 +3665,7 @@ static inline bool cfs_rq_is_decayed(struct
> cfs_rq *cfs_rq)
>  static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
>  {
>         long delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
> +       u64 now = cfs_rq_clock_pelt(cfs_rq);

Could this be `u64 now = sched_clock_cpu()` like in
migrate_se_pelt_lag() or newidle_balance() to avoid the time morphing
due to PELT's frequency and uArch invariance?
> 
>         /*
>          * No need to update load_avg for root_task_group as it is not used.

[...]