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

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Message-ID: <20170531103017.3voouif2ixwbpbyn@hirez.programming.kicks-ass.net>
Date:   Wed, 31 May 2017 12:30:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, rjw@...ysocki.net,
        juri.lelli@....com, dietmar.eggemann@....com,
        Morten.Rasmussen@....com
Subject: Re: [RFC PATCH 1/2] sched/rt: add utilization tracking

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.

> In fact, I don't think you particularly care about RT here, as anything
> !CFS that preempts it, including those interrupts you mentioned. Which
> gets us back to what rt_avg is.
> 
> > We don't use rt_avg which doesn't have the same dynamic as PELT and which
> > can include IRQ time that are also accounted in cfs task utilization
> 
> Well, if rt_avg includes IRQ time, then that IRQ time is not part of
> the task clock.