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

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Date:   Mon, 19 Sep 2022 17:49:27 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     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,
        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

On Mon, 19 Sept 2022 at 13:55, Dietmar Eggemann
<dietmar.eggemann@....com> wrote:
>
> 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.

Just like share/cpu.weight value does for time sharing

>
> [...]
>
> > +static int cpu_latency_write_s64(struct cgroup_subsys_state *css,
> > +                             struct cftype *cft, s64 latency)
> > +{
>
> There is no [MIN, MAX] checking?

This is done is sched_group_set_latency() which checks that
abs(latency) < sysctl_sched_latency

>
> 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)?

Yes, Tejun raised some concerns about adding an interface like nice in
the task group in v2 so I have removed it.

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

If everybody is ok, I can add back the cpu.latency.nice interface in
the v5 in addition to the cpu.latency

>
> [...]