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

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Message-ID: <87ef0v2cem.fsf@arm.com>
Date:   Thu, 05 Sep 2019 12:46:57 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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 12:40:30 +0100, Peter Zijlstra wrote...

> On Thu, Sep 05, 2019 at 12:18:55PM +0100, Patrick Bellasi wrote:
>
>> Right, we have this dualism to deal with and current mainline behaviour
>> is somehow in the middle.
>> 
>> BTW, the FB requirement is the same we have in Android.
>> We want some CFS tasks to have very small latency and a low chance
>> to be preempted by the wake-up of less-important "background" tasks.
>> 
>> I'm not totally against the usage of a signed range, but I'm thinking
>> that since we are introducing a new (non POSIX) concept we can get the
>> chance to make it more human friendly.
>
> I'm arguing that signed _is_ more human friendly ;-)

... but you are not human. :)

>> Give the two extremes above, would not be much simpler and intuitive to
>> have 0 implementing the FB/Android (no latency) case and 1024 the
>> (max latency) Oracle case?
>
> See, I find the signed thing more natural, negative is a bias away from
> latency sensitive, positive is a bias towards latency sensitive.
>
> Also; 0 is a good default value ;-)

Yes, that's appealing indeed.

>> Moreover, we will never match completely the nice semantic, give that
>> a 1 nice unit has a proper math meaning, isn't something like 10% CPU
>> usage change for each step?
>
> Only because we were nice when implementing it. Posix leaves it
> unspecified and we could change it at any time. The only real semantics
> is a relative 'weight' (opengroup uses the term 'favourable').

Good to know, I was considering it a POXIS requirement.

>> Could changing the name to "latency-tolerance" break the tie by marking
>> its difference wrt prior/nice levels? AFAIR, that was also the original
>> proposal [1] by PaulT during the OSPM discussion.
>
> latency torrerance could still be a signed entity, positive would
> signify we're more tolerant of latency (ie. less sensitive) while
> negative would be less tolerant (ie. more sensitive).

Right.

>> For latency-nice instead we will likely base our biasing strategies on
>> some predefined (maybe system-wide configurable) const thresholds.
>
> I'm not quite sure; yes, for some of these things, like the idle search
> on wakeup, certainly. But say for wakeup-preemption, we could definitely
> make it a task relative attribute.

-- 
#include <best/regards.h>

Patrick Bellasi