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

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Date:   Thu, 19 Oct 2023 10:49:09 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Chen Yu <yu.c.chen@...el.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 1/2] sched/fair: Introduce UTIL_FITS_CAPACITY feature

On 2023-10-19 09:28, Mathieu Desnoyers wrote:
> On 2023-10-19 07:35, Chen Yu wrote:
[...]
>>> +/*
>>> + * Returns true if adding the task utilization to the estimated
>>> + * utilization of the runnable tasks on @cpu does not exceed the
>>> + * capacity of @cpu.
>>> + *
>>> + * This considers only the utilization of _runnable_ tasks on the @cpu
>>> + * runqueue, excluding blocked and sleeping tasks. This is achieved by
>>> + * using the runqueue util_est.enqueued, and by estimating the capacity
>>> + * of @cpu based on arch_scale_cpu_capacity and 
>>> arch_scale_thermal_pressure
>>> + * rather than capacity_of() because capacity_of() considers
>>> + * blocked/sleeping tasks in other scheduler classes.
>>> + *
>>> + * The utilization vs capacity comparison is done without the margin
>>> + * provided by fits_capacity(), because fits_capacity() is used to
>>> + * validate whether the utilization of a task fits within the overall
>>> + * capacity of a cpu, whereas this function validates whether the task
>>> + * utilization fits within the _remaining_ capacity of the cpu, 
>>> which is
>>> + * more precise.
>>> + */
>>> +static inline bool task_fits_remaining_cpu_capacity(unsigned long 
>>> task_util,
>>> +                            int cpu)
>>> +{
>>> +    unsigned long total_util, capacity;
>>> +
>>> +    if (!sched_util_fits_capacity_active())
>>> +        return false;
>>> +    total_util = READ_ONCE(cpu_rq(cpu)->cfs.avg.util_est.enqueued) + 
>>> task_util;
>>> +    capacity = arch_scale_cpu_capacity(cpu) - 
>>> arch_scale_thermal_pressure(cpu);
>>
>> scale_rt_capacity(cpu) could provide the remaining cpu capacity after 
>> substracted by
>> the side activity(rt tasks/thermal pressure/irq time), maybe it would 
>> be more accurate?
> 
> AFAIU, scale_rt_capacity(cpu) works similarly to capacity_of(cpu) and 
> considers blocked and sleeping tasks in the rq->avg_rt.util_avg and 
> rq->avg_dl.util_avg. I'm not sure sure about rq->avg_irq.util_avg and 
> thermal_load_avg().
> 
> This goes against what is needed here: we need a utilization that only 
> considers enqueued runnable tasks (exluding blocked and sleeping tasks). 
> Or am I missing something ?
> 

I was wrong. Looking more closely at dl and rt sched classes, unlike the 
fair sched class, they don't appear to take into account 
sleeping/blocked tasks in their util_avg. They just accumulate the rq 
util_sum and derive a rq util_avg from it. Likewise for thermal and irq.

So both capacity_of(cpu) and scale_rt_capacity(cpu) would appear to do 
what we need here, but AFAIU capacity_of(cpu) is based on a metric which 
is only updated once per jiffy or so.

Let me try using scale_rt_capacity(cpu) then.

Thanks!

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com