linux-kernel - Re: [PATCH] sched/fair: Replace CFS internal cpu_util() with cpu_util

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Date:   Thu, 18 Nov 2021 09:07:05 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.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 <Valentin.Schneider@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Replace CFS internal cpu_util() with cpu_util_cfs()

On Wed, 17 Nov 2021 at 18:26, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>
> On 12.11.21 17:20, Vincent Guittot wrote:
> > On Fri, 12 Nov 2021 at 15:14, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >>
> >> cpu_util_cfs() was created by commit d4edd662ac16 ("sched/cpufreq: Use
> >> the DEADLINE utilization signal") to enable the access to CPU
> >> utilization from the Schedutil CPUfreq governor.
> >>
> >> Commit a07630b8b2c1 ("sched/cpufreq/schedutil: Use util_est for OPP
> >> selection") added util_est support later.
> >>
> >> The only thing cpu_util() is doing on top of what cpu_util_cfs() already
> >> does is to clamp the return value to the [0..capacity_orig] capacity
> >> range of the CPU. Integrating this into cpu_util_cfs() is not harming
> >> the existing users (Schedutil and CPUfreq cooling (latter via
> >> sched_cpu_util() wrapper)).
> >
> > Could you to update cpu_util_cfs() to use cpu as a parameter instead of rq ?
>
> I could but I decided to use use `struct rq *rq` instead.
>
> (A) We already know the rq in the following functions where we call
>     cpu_util_cfs():

The only user of cpu_util_cfs() is sugov_get_util() and it does
cpu_util_cfs(cpu_rq(sg_cpu->cpu)) because rq is only used as a
parameter of cpu_util_cfs()

all other ones are using cpu_util() which already uses cpu as a
parameter so it's more straight forward to keep using cpu

>
>   update_sg_lb_stats()
>   find_busiest_queue()
>   update_numa_stats()
>   sugov_get_util() (existing cpu_util_cfs() call *)
>
> (B) For the following three functions we would call cpu_rq() outside
>     cpu_util_cfs():
>
>   cpu_overutilized()
>   cpu_util_without()
>   sched_cpu_util() (*)
>
> So for (A) we wouldn't call cpu_rq(cpu) twice, avoiding issues with the
> RELOC_HIDE() thing in per_cpu(runqueues, cpu).
>
>
> And cpu_util_cfs()'s PELT counterparts, cpu_load() and cpu_runnable()
> also use rq.
>
> >> Remove cpu_util().
> >>
> >> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
> >> ---
> >>
> >> I deliberately got rid of the comment on top of cpu_util(). It's from
> >> the early days of using PELT utilization, it describes CPU utilization
> >> behaviour before PELT time-scaling and talks about current capacity
> >> which we don't maintain.
> >
> > would be good to keep an updated version in this case. There are lot
> > of interesting informations in the comment
>
> Yes, can do.
>
> Something like this:
>
> /**
>  * cpu_util_cfs() - Estimates the amount of CPU capacity used by CFS tasks.
>  * @cpu: the CPU to get the utilization for.

cpu is clearly the right parameter ;-)

>  *
>  * The unit of the return value must be the same as the one of CPU capacity
>  * so that CPU utilization can be compared with CPU capacity.
>  *
>  * CPU utilization is the sum of running time of runnable tasks plus the
>  * recent utilization of currently non-runnable tasks on that CPU.
>  * It represents the amount of CPU capacity currently used by CFS tasks in
>  * the range [0..max CPU capacity] with max CPU capacity being the CPU
>  * capacity at f_max.
>  *
>  * The estimated CPU utilization is defined as the maximum between CPU
>  * utilization and sum of the estimated utilization of the currently
>  * runnable tasks on that CPU. It preserves a utilization "snapshot" of
>  * previously-executed tasks, which helps better deduce how busy a CPU will
>  * be when a long-sleeping task wake up. Such task's contribution to CPU
>  * utilization would be decayed significantly at this point of time.
>  *
>  * CPU utilization can be higher than the current CPU capacity
>  * (f_curr/f_max * max CPU capacity) or even the max CPU capacity because
>  * of rounding errors as well as task migrations or wakeups of new tasks.
>  * CPU utilization has to be capped to fit into the [0..max CPU capacity]
>  * range. Otherwise a group of CPUs (CPU0 util = 121% + CPU1 util = 80%)
>  * could be seen as over-utilized even though CPU1 has 20% of spare CPU
>  * capacity. CPU utilization is allowed to overshoot current CPU capacity
>  * though since this is useful for predicting the CPU capacity required
>  * after task migrations (scheduler-driven DVFS).
>  *
>  * Return: (Estimated) utilization for the specified CPU.
>  */
>
> [...]