linux-kernel - Re: [RFC PATCH 1/2] sched/rt: add utilization tracking

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Date:   Wed, 31 May 2017 11:41:11 +0100
From:   Juri Lelli <juri.lelli@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, rjw@...ysocki.net,
        dietmar.eggemann@....com, Morten.Rasmussen@....com
Subject: Re: [RFC PATCH 1/2] sched/rt: add utilization tracking

On 31/05/17 12:30, Peter Zijlstra wrote:
> On Wed, May 31, 2017 at 11:40:47AM +0200, Peter Zijlstra wrote:
> > On Wed, May 24, 2017 at 11:00:51AM +0200, Vincent Guittot wrote:
> > > schedutil governor relies on cfs_rq's util_avg to choose the OPP when cfs
> > > tasks are running. When the CPU is overloaded by cfs and rt tasks, cfs tasks
> > > are preempted by rt tasks and in this case util_avg reflects the remaining
> > > capacity that is used by cfs tasks but not what cfs tasks want to use. In such
> > > case, schedutil can select a lower OPP when cfs task runs whereas the CPU is
> > > overloaded. In order to have a more accurate view of the utilization of the
> > > CPU, we track the utilization that is used by RT tasks.
> > > DL tasks are not taken into account as they have their own utilization
> > > tracking mecanism.
> > 
> > Well, the DL tracking is fairly pessimistic; it assumes all DL tasks
> > will consume their total budget, which will rarely, if ever, happen.
> > 
> > So I suspect it might well be worth it to also track DL activity for the
> > purpose of compensating CFS.
> 
> Again, it seems I have this CPPC/HWP crud firmly stuck in my brain.
> Because I was thinking:
> 
> 	min_freq = dl_util
> 	avg_freq = dl_avg + rt_avg + cfs_util
> 
> 
> But given we don't actually have that split... meh.
> 

Right, interesting. So, I guess the question is: should we have it? :)

IMHO, it makes sense and seems to benefit mobile use-cases I'm looking
at.

rt_avg though it also seems to build up very slowly (at least with
default configs). I'm experimenting with Vincent proposal and it looks
better (w.r.t. using rt_avg). Also summing up signals that behave
similarly doesn't seem the wrong thing to do.