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

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Date:   Wed, 31 May 2017 11:40:47 +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 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.

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.

> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
> 
> If the changes are reasonnable, it might worth moving the PELT function in a
> dedicated pelt.c file and the ugly
> extern int update_rt_rq_load_avg(u64 now, int cpu, struct rt_rq *rt_rq, int running);
> in a pelt.h header
> 
> 
>  kernel/sched/fair.c  | 21 +++++++++++++++++++++
>  kernel/sched/rt.c    |  9 +++++++++
>  kernel/sched/sched.h |  3 +++
>  3 files changed, 33 insertions(+)

Also, and I didn't check this, it is important that the windows are
aligned if you want to sum the values.