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

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Message-ID: <20190905113002.GK2349@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Sep 2019 13:30:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Patrick Bellasi <patrick.bellasi@....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 12:13:47PM +0100, Qais Yousef wrote:
> On 09/05/19 12:46, Peter Zijlstra wrote:

> > This is important because we want to be able to bias towards less
> > importance to (tail) latency as well as more importantance to (tail)
> > latency.
> > 
> > Specifically, Oracle wants to sacrifice (some) latency for throughput.
> > Facebook OTOH seems to want to sacrifice (some) throughput for latency.
> 
> Another use case I'm considering is using latency-nice to prefer an idle CPU if
> latency-nice is set otherwise go for the most energy efficient CPU.
> 
> Ie: sacrifice (some) energy for latency.
> 
> The way I see interpreting latency-nice here as a binary switch. But
> maybe we can use the range to select what (some) energy to sacrifice
> mean here. Hmmm.

It cannot be binary, per definition is must be ternary, that is, <0, ==0
and >0 (or middle value if you're of that persuasion).

In your case, I'm thinking you mean >0, we want to lower the latency.

Anyway; there were a number of things mentioned at OSPM that we could
tie into this thing and finding sensible mappings is going to be a bit
of trial and error I suppose.

But as patrick said; we're very much exporting a BIAS knob, not a set of
behaviours.