linux-kernel - Re: [PATCH v4 6/8] sched/fair: Add sched group latency support

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Date:   Mon, 19 Sep 2022 13:55:15 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org,
        parth@...ux.ibm.com
Cc:     qais.yousef@....com, chris.hyser@...cle.com,
        valentin.schneider@....com, patrick.bellasi@...bug.net,
        David.Laight@...lab.com, pjt@...gle.com, pavel@....cz,
        tj@...nel.org, qperret@...gle.com, tim.c.chen@...ux.intel.com,
        joshdon@...gle.com
Subject: Re: [PATCH v4 6/8] sched/fair: Add sched group latency support

s/valentin.schneider@....com//

On 16/09/2022 10:03, Vincent Guittot wrote:
> Task can set its latency priority, which is then used to decide to preempt
> the current running entity of the cfs, but sched group entities still have
> the default latency offset.
> 
> Add a latency field in task group to set the latency offset of the
> sched_eneities of the group, which will be used against other entities in

s/sched_eneities/sched_entity

> the parent cfs when deciding which entity to schedule first.

So latency for cgroups does not follow any (existing) Resource
Distribution Model/Scheme (Documentation/admin-guide/cgroup-v2.rst)?
Latency values are only used to compare sched entities at the same level.

[...]

> +static int cpu_latency_write_s64(struct cgroup_subsys_state *css,
> +				struct cftype *cft, s64 latency)
> +{

There is no [MIN, MAX] checking?

min_weight = sched_latency_to_weight[0]  = -1024
max_weight = sched_latency_to_weight[39] =   973

[MIN, MAX] = [sysctl_sched_latency * min_weight >> NICE_LATENCY_SHIFT,
              sysctl_sched_latency * max_weight >> NICE_LATENCY_SHIFT]


With the `cpu.latency` knob user would have to know for example that the
value is -24,000,000ns to get the same behaviour as for a task latency
nice = -20 (latency prio = 0) (w/ sysctl_sched_latency = 24ms)?

For `nice` we have `cpu.weight.nice` next to `cpu.weight` in cgroup v2 ?

[...]