linux-kernel - Re: [RFC/RFT][PATCH 2/2] cpufreq: schedutil: Utilization aggregation

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Date:   Tue, 11 Apr 2017 08:00:44 +0100
From:   Juri Lelli <juri.lelli@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC/RFT][PATCH 2/2] cpufreq: schedutil: Utilization aggregation

On 10/04/17 23:13, Rafael J. Wysocki wrote:
> On Mon, Apr 10, 2017 at 1:26 PM, Juri Lelli <juri.lelli@....com> wrote:

[...]

> > Given that for RT (and still for DL as well) the next event is a
> > periodic tick, couldn't happen that the required frequency transition
> > for an RT task, that unfortunately woke up before the end of a throttling
> > period, gets delayed of a tick interval (at least 4ms on ARM)?
> 
> No, that won't be an entire tick unless it wakes up exactly at the
> update time AFAICS.
> 

Right. I was trying to think about worst case, as I'm considering RT
type of tasks.

> > Don't we need to treat such wake up events (RT/DL) in a special way and
> > maybe set a timer to fire and process them as soon as the current
> > throttling period elapses? Might be a patch on top of this I guess.
> 
> Setting a timer won't be a good idea at all, as it would need to be a
> deferrable one and Thomas would not like that (I'm sure).
> 

Why deferrable? IMHO, we should be servicing RT requestes as soon as the
HW is capable of. Even a small delay of, say, a couple of ms could be
causing deadline misses.

> We could in principle add some special casing around that, like for
> example pass flags to sugov_should_update_freq() and opportunistically
> ignore freq_update_delay_ns if SCHED_CPUFREQ_RT_DL is set in there,
> but that would lead to extra overhead on systems where frequency
> updates happen in-context.
> 

Also, it looks still event driven to me. If the RT task is the only
thing running, nothing will trigger a potential frequency change
re-evaluation before the next tick.

> Also the case looks somewhat corner to me to be honest.
> 

Sure. Only thinking about potential problems here. However, playing with
my DL patches I noticed that this can be actually a problem, as for DL,
for example, we trigger a frequency switch when the task wakes up, but
then we don't do anything during the tick (because it doesn't seem to
make sense to do anything :). So, if we missed the opportunity to
increase frequency at enqueue time, the task is hopelessly done. :(

Anyway, since this looks anyway something that we might want on top of
your patches, I'll play with the idea when refreshing my set and see
what I get.

Thanks,

- Juri