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

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Date:   Thu, 7 Sep 2023 13:12:47 +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 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:

[...]

>>> +/*
>>> + * Approximate the new util_avg value assuming an entity has continued to run
>>> + * for @delta us.
>>> + */
>>> +unsigned long approximate_util_avg(unsigned long util, u64 delta)
>>> +{
>>> +	struct sched_avg sa = {
>>> +		.util_sum = util * PELT_MIN_DIVIDER,
>>> +		.util_avg = util,
>>> +	};
>>> +
>>> +	if (unlikely(!delta))
>>> +		return util;
>>> +
>>> +	accumulate_sum(delta, &sa, 0, 0, 1);
>>
>> IMHO, you miss the handling of `periods != 0`. load = 0 eclipses this
>> code in accumulate_sum().

You could call accumulate_sum(delta, &sa, 1, 0, 1);

> 
> Yes. For some reason I got blank registered when I saw if this codepath can
> impact util_avg..

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.
>>
>>> +	___update_load_avg(&sa, 0);
>>> +
>>> +	return sa.util_avg;
>>> +}
>>
>> We already discussed something similar like this in Nov 22, the so
>> called UTIL_EST_FASTER thing.
>>
>> https://lkml.kernel.org/r/Y2kLA8x40IiBEPYg@hirez.programming.kicks-ass.net
>>
>> +/*
>> + * Compute a pelt util_avg assuming no history and @delta runtime.
>> + */
>> +unsigned long faster_est_approx(u64 delta)
>> +{
>> +	unsigned long contrib = (unsigned long)delta; /* p == 0 -> delta < 1024 */
>> +	u64 periods = delta / 1024;
>> +
>> +	if (periods) {
>> +		delta %= 1024;
>> +		contrib = __accumulate_pelt_segments(periods, 1024, delta);
>> +	}
>> +
>> +	return (contrib << SCHED_CAPACITY_SHIFT) / PELT_MIN_DIVIDER;
>> +}
>> +
> 
> I could look at using this version instead. This misses the decay part though?

__accumulate_pelt_segments(periods, ...) decays the periods. But
obviously not the util you pass into approximate_util_avg().