linux-kernel - Re: [RFC PATCH 1/9] sched,cgroup: Add interface for latency-nice

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Date:   Thu, 5 Sep 2019 13:47:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Qais Yousef <qais.yousef@....com>,
        Subhra Mazumdar <subhra.mazumdar@...cle.com>,
        linux-kernel@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
        steven.sistare@...cle.com, dhaval.giani@...cle.com,
        daniel.lezcano@...aro.org, vincent.guittot@...aro.org,
        viresh.kumar@...aro.org, tim.c.chen@...ux.intel.com,
        mgorman@...hsingularity.net, parth@...ux.ibm.com
Subject: Re: [RFC PATCH 1/9] sched,cgroup: Add interface for latency-nice

On Thu, Sep 05, 2019 at 12:30:52PM +0100, Patrick Bellasi wrote:

> I see this concept possibly evolving into something more then just a
> binary switch. Not yet convinced if it make sense and/or it's possible
> but, in principle, I was thinking about these possible usages for CFS
> tasks:
> 
>  - dynamically tune the policy of a task among SCHED_{OTHER,BATCH,IDLE}
>    depending on crossing certain pre-configured threshold of latency
>    niceness.

A big part of BATCH is wakeup preemption (batch doesn't preempt itself),
and wakeup preemption is a task-task propery and can thus be completely
relative.

>  - dynamically bias the vruntime updates we do in place_entity()
>    depending on the actual latency niceness of a task.

That is dangerous; theory says we should keep track of the 0-lag point
and place it back where we found it. BFQ does this correctly IIRC, but
for CFS I've never done that because 'expensive'.

But yes, we could (carefully) fumble a bit there.

>  - bias the decisions we take in check_preempt_tick() still depending
>    on a relative comparison of the current and wakeup task latency
>    niceness values.

Ack.

Placing relative and absolute behaviour on the same scale is going to be
'fun' :-)