linux-kernel - Re: [RFC] Documentation/scheduler/schedutil.txt

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Date:   Mon, 23 Nov 2020 10:26:13 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        patrick.bellasi@...bug.net, lenb@...nel.org,
        linux-kernel@...r.kernel.org, valentin.schneider@....com,
        ionela.voinescu@....com, qperret@...gle.com,
        viresh.kumar@...aro.org
Subject: Re: [RFC] Documentation/scheduler/schedutil.txt

On 20/11/2020 08:55, Peter Zijlstra wrote:

[...]

> PELT (Per Entity Load Tracking)
> -------------------------------

[...]

> Using this we track 2 key metrics: 'running' and 'runnable'. 'Running'
> reflects the time an entity spends on the CPU, while 'runnable' reflects the
> time an entity spends on the runqueue. When there is only a single task these
> two metrics are the same, but once there is contention for the CPU 'running'
> will decrease to reflect the fraction of time each task spends on the CPU
> while 'runnable' will increase to reflect the amount of contention.

People might find it confusing to map 'running and 'runnable' into the 3
PELT signals (load_avg, runnable_avg and util_avg) being used in the
scheduler ... with load_avg being 'runnable' and 'weight' based.

> For more detail see: kernel/sched/pelt.c
> 
> 
> Frequency- / Heterogeneous Invariance
> -------------------------------------

We call 'Heterogeneous Invariance' CPU invariance in chapter 2.3
Documentation/scheduler/sched-capacity.rst.

[...]

> For more detail see:
> 
>  - kernel/sched/pelt.h:update_rq_clock_pelt()
>  - arch/x86/kernel/smpboot.c:"APERF/MPERF frequency ratio computation."

drivers/base/arch_topology.c:"f_cur/f_max ratio computation".

> UTIL_EST / UTIL_EST_FASTUP
> --------------------------

[...]

>   util_est := \Sum_t max( t_running, t_util_est_ewma )
> 
> For more detail see: kernel/sched/fair.h:util_est_dequeue()

s/fair.h/fair.c

> UCLAMP
> ------
> 
> It is possible to set effective u_min and u_max clamps on each task; the

s/on each task/on each CFS or RT task

[...]