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

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Message-ID: <87h85r2d5f.fsf@arm.com>
Date:   Thu, 05 Sep 2019 12:30:52 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 12:13:47 +0100, Qais Yousef wrote...

> On 09/05/19 12:46, Peter Zijlstra wrote:
>> On Thu, Sep 05, 2019 at 10:45:27AM +0100, Patrick Bellasi wrote:
>> 
>> > > From just reading the above, I would expect it to have the range
>> > > [-20,19] just like normal nice. Apparently this is not so.
>> > 
>> > Regarding the range for the latency-nice values, I guess we have two
>> > options:
>> > 
>> >   - [-20..19], which makes it similar to priorities
>> >   downside: we quite likely end up with a kernel space representation
>> >   which does not match the user-space one, e.g. look at
>> >   task_struct::prio.
>> > 
>> >   - [0..1024], which makes it more similar to a "percentage"
>> > 
>> > Being latency-nice a new concept, we are not constrained by POSIX and
>> > IMHO the [0..1024] scale is a better fit.
>> > 
>> > That will translate into:
>> > 
>> >   latency-nice=0 : default (current mainline) behaviour, all "biasing"
>> >   policies are disabled and we wakeup up as fast as possible
>> > 
>> >   latency-nice=1024 : maximum niceness, where for example we can imaging
>> >   to turn switch a CFS task to be SCHED_IDLE?
>> 
>> There's a few things wrong there; I really feel that if we call it nice,
>> it should be like nice. Otherwise we should call it latency-bias and not
>> have the association with nice to confuse people.
>> 
>> Secondly; the default should be in the middle of the range. Naturally
>> this would be a signed range like nice [-(x+1),x] for some x. but if you
>> want [0,1024], then the default really should be 512, but personally I
>> like 0 better as a default, in which case we need negative numbers.
>> 
>> 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.

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.

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

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

-- 
#include <best/regards.h>

Patrick Bellasi