linux-kernel - Re: [RFC] Documentation/scheduler/schedutil.txt

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Date:   Wed, 2 Dec 2020 16:45:31 +0000
From:   Mel Gorman <mgorman@...e.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        dietmar.eggemann@....com, patrick.bellasi@...bug.net,
        lenb@...nel.org, linux-kernel@...r.kernel.org,
        valentin.schneider@....com, ionela.voinescu@....com,
        qperret@...gle.com, viresh.kumar@...aro.org
Subject: Re: [RFC] Documentation/scheduler/schedutil.txt

On Wed, Dec 02, 2020 at 04:54:52PM +0100, Peter Zijlstra wrote:
> > IIRC, this 32ms is tied to the value of LOAD_AVG_PERIOD and the length
> > of the ewma_sum series below. Might be worth expanding a little further.
> 
> It is LOAD_AVG_PERIOD. Some people (re)generate the PELT tables with a
> different period (16 and 64 are common).
> 
> Not sure what there is to expand; the whole of it is: y^32=0.5. We had
> to pick some half-life period, 32 seemed like a good number.
> 

No issue with the number other than the y^32 is tied to LOAD_AVG_PERIOD.
Again, it's something that someone looking at the source would eventually
figure out so it's probably for the best.

> > > To alleviate this (a default enabled option) UTIL_EST drives an (IIR) EWMA
> > 
> > Expand IIR -- Immediate Impulse Reponse?
> 
> Infinite Impuse Response
> 

Sorry, yes, still worth an expansion.

> > > with the 'running' value on dequeue -- when it is highest. A further default
> > > enabled option UTIL_EST_FASTUP modifies the IIR filter to instantly increase
> > > and only decay on decrease.
> > > 
> > > A further runqueue wide sum (of runnable tasks) is maintained of:
> > > 
> > >   util_est := \Sum_t max( t_running, t_util_est_ewma )
> > > 
> > > For more detail see: kernel/sched/fair.h:util_est_dequeue()
> > > 
> > 
> > It's less obvious what the consequence is unless the reader manages to
> > tie the IO-wait comment in "Schedutil / DVFS" to this section.
> 
> I'm not entirely sure I follow. The purpose of UTIL_EST is to avoid
> ramp-up issues and isn't related to IO-wait boosting.
> 

I mixed up the example. Historically io-wait boosting was one way of
avoiding DVFS ramp-up issues but now that I reread it, it's best to leave
it general like you already have in your current version.

> > Is it worth explicitly mentioning that a key advantage over
> > hardware-based approaches is that schedutil carries utilisation state on
> > CPU migration? You say that it is tracked but it's less obvious why that
> > matters as a pure hardware based approach loses utilisation information
> > about a task once it migrates.
> 
> Not sure that was the exact goal of the document; I set out to describe
> schedutil.
> 

Fair enough, it would simply lead to documentation creep.

> > Even moving note 3 below into this section and expanding it with an
> > example based on HWP would be helpful.
> 
> I might not be the best person to talk about HWP; even though I work for
> Intel I know remarkably little of it. I don't even think I've got a
> machine that has it on.
> 
> Latest version below... I'll probably send it as a patch soon and get it
> merged. We can always muck with it more later.
> 

True. At least any confusion can then be driven by specific questions :)

FWIW, after reading the new version I'll ack the patch when it shows up.

Thanks!

-- 
Mel Gorman
SUSE Labs