linux-kernel - Re: [RFC 6/6] sched/fair: Add sched group latency support

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Message-ID: <Yjn8DohJ8CZ6pEvf@slm.duckdns.org>
Date:   Tue, 22 Mar 2022 06:40:46 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, linux-kernel@...r.kernel.org, parth@...ux.ibm.com,
        qais.yousef@....com, chris.hyser@...cle.com,
        pkondeti@...eaurora.org, Valentin.Schneider@....com,
        patrick.bellasi@...bug.net, David.Laight@...lab.com,
        pjt@...gle.com, pavel@....cz, dhaval.giani@...cle.com,
        qperret@...gle.com, tim.c.chen@...ux.intel.com
Subject: Re: [RFC 6/6] sched/fair: Add sched group latency support

Hello,

On Tue, Mar 22, 2022 at 05:10:36PM +0100, Vincent Guittot wrote:
> latency_nice is quite similar to nice. The nice latency is used as an
> index to get a latency weight in the range [-1024:1024].  latency_nice
> is in the range [-20:19] and latency_prio shifts it in the range
> [0:40] . This index is then used to get the latency weight similar to
> how the nice prio is used to get a weight. That being said, the
> latency should probably reflect the latency_weight instead of the
> latency_prio in order to be aligned with the weight and weight.nice
> fields of cgroups.
> 
> As described in patch 5 commit message, the weight is then used to
> compute a relative offset to check whether the waking task can preempt
> the current running task.

So, what I'm trying to say is if it is actually a weight, just use weight
values instead of arbitrary mapped nice values. Nobody can tell how the
latency nice value of -2 compares against, say, 3. If you can define it
clearly in terms of weights (or something else clearly describable), it'd be
a lot better.

Thanks.

-- 
tejun