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

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Date:   Tue, 22 Mar 2022 09:41:47 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Tejun Heo <tj@...nel.org>,
        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
Subject: Re: [RFC 6/6] sched/fair: Add sched group latency support

On Mon, 2022-03-21 at 07:24 -1000, Tejun Heo wrote:
> Hello,
> 
> On Fri, Mar 11, 2022 at 05:14:06PM +0100, Vincent Guittot wrote:
> > Tasks 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 priority.
> > 
> > Add a latency field in task group to set the latency priority of the group
> > which will be used against other entity in the parent cfs.
> 
> One thing that bothers me about this interface is that the configuration
> values aren't well defined. We have the same problems with the nice levels
> but at least have them map to well defined weight values, which likely won't
> change in any foreseeable future. The fact that we have the
> not-too-well-defined nice levels as an interface shouldn't be a reason we
> add another one. Provided that this is something scheduler folks want, it'd
> be really great if the interface can be better thought through. What are the
> numbers actually encoding?
> 
> 

The way I was interpreting the latency_nice number is as follow:
Given runnable tasks that have not exceeded their time slice,
the task with the lowest latency nice number run first.

The current patchset takes care of the
case of tasks within a run queue. I think we also need to
consider which cpu should be selected for a waking
task, if we cannot find an idle CPU.

It seems reasonable that we wake a task on a CPU that is running a task with
a higher latency nice value and lower load than previous CPU the waking task
has run on. That way we can wake latency sensitive tasks faster in a busy system.

Tim