linux-kernel - Re: [RFC PATCH 1/7] sched/pelt: Add a new function to approximate the future util

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Date:   Wed, 13 Sep 2023 19:22:10 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qyousef@...alina.io>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Lukasz Luba <lukasz.luba@....com>
Subject: Re: [RFC PATCH 1/7] sched/pelt: Add a new function to approximate the
 future util_avg value

On 10/09/2023 21:58, Qais Yousef wrote:
> On 09/07/23 13:12, Dietmar Eggemann wrote:
>> On 06/09/2023 23:19, Qais Yousef wrote:
>>> On 09/06/23 14:56, Dietmar Eggemann wrote:
>>>> On 28/08/2023 01:31, Qais Yousef wrote:

[...]

>> Another thing ... I guess if you call accumulate_sum with delta the PELT
>> machinery assumes `delta = now - sa->last_update_time` which means you
>> would have to use `clock_pelt + TICK_USEC` as delta.
> 
> Right.
> 
> The way I understood it is that at TICK we should do update_load_avg() which
> would call __update_load_sum() which uses
> 
> 	delta = now - sa->last_update_time
> 
> which passes this delta to accumulate_sum()
> 
> I can see we are not very accurate since there will be a small additional time
> besides TICK_USEC that we are not accounting for. But I can't see how this can
> cause a big error.
> 
> 	predicted (assumed) tick time/delta
> 
> 		sa->last_update_time = now
> 		tick_time = TICK_USEC + now
> 
> 		delta = tick_time - sa->last_update_time
> 		delta = TICK_USEC + now - now
> 		delta = TICK_USEC
> 
> 	but actual tick time/delta
> 
> 		sa->last_update_time = now - x
> 		tick_time = TICK_USEC + now
> 
> 		delta = tick_time - sa->last_update_time
> 		delta = TICK_USEC + now - (now - x)
> 		delta = TICK_USEC + x
> 
> So the delta I am using might be slightly shorter than it should be.
> 
> IIUC, what you're saying that the `x` in my equation above is clock_pelt,
> right?

No, I was wrong here. Calling accumulate_sum with `delta = TICK_USEC` is
fine.

accumulate_sum() will accrue `sa->util.sum` and ___update_load_avg()
will then adjust `sa->util_avg` accordingly.

delta should be 4000 on Arm64 boards so you will cross period
boundaries. In case `delta < 1024` you might want to not call
___update_load_avg() to be in pair with __update_load_avg_cfs_rq().