Re: [PATCH] cpufreq: schedutil: rate limits for SCHED_DEADLINE

[Juri Lelli]

On 09/02/18 12:37, Rafael J. Wysocki wrote:
> On Fri, Feb 9, 2018 at 12:26 PM, Juri Lelli <juri.lelli@xxxxxxxxxx> wrote:
> > On 09/02/18 12:04, Rafael J. Wysocki wrote:
> >> On Fri, Feb 9, 2018 at 11:53 AM, Juri Lelli <juri.lelli@xxxxxxxxxx> wrote:
> >> > Hi,
> >> >
> >> > On 09/02/18 11:36, Rafael J. Wysocki wrote:
> >> >> On Friday, February 9, 2018 9:02:34 AM CET Claudio Scordino wrote:
> >> >> > Hi Viresh,
> >> >> >
> >> >> > Il 09/02/2018 04:51, Viresh Kumar ha scritto:
> >> >> > > On 08-02-18, 18:01, Claudio Scordino wrote:
> >> >> > >> When the SCHED_DEADLINE scheduling class increases the CPU utilization,
> >> >> > >> we should not wait for the rate limit, otherwise we may miss some deadline.
> >> >> > >>
> >> >> > >> Tests using rt-app on Exynos5422 have shown reductions of about 10% of deadline
> >> >> > >> misses for tasks with low RT periods.
> >> >> > >>
> >> >> > >> The patch applies on top of the one recently proposed by Peter to drop the
> >> >> > >> SCHED_CPUFREQ_* flags.
> >> >> > >>
> >> >>
> >> >> [cut]
> >> >>
> >> >> >
> >> >> > >
> >> >> > > Is it possible to (somehow) check here if the DL tasks will miss
> >> >> > > deadline if we continue to run at current frequency? And only ignore
> >> >> > > rate-limit if that is the case ?
> >> >
> >> > Isn't it always the case? Utilization associated to DL tasks is given by
> >> > what the user said it's needed to meet a task deadlines (admission
> >> > control). If that task wakes up and we realize that adding its
> >> > utilization contribution is going to require a frequency change, we
> >> > should _theoretically_ always do it, or it will be too late. Now, user
> >> > might have asked for a bit more than what strictly required (this is
> >> > usually the case to compensate for discrepancies between theory and real
> >> > world, e.g.  hw transition limits), but I don't think there is a way to
> >> > know "how much". :/
> >>
> >> You are right.
> >>
> >> I'm somewhat concerned about "fast switch" cases when the rate limit
> >> is used to reduce overhead.
> >
> > Mmm, right. I'm thinking that in those cases we could leave rate limit
> > as is. The user should then be aware of it and consider it as proper
> > overhead when designing her/his system.
> >
> > But then, isn't it the same for "non fast switch" platforms? I mean,
> > even in the latter case we can't go faster than hw limits.. mmm, maybe
> > the difference is that in the former case we could go as fast as theory
> > would expect.. but we shouldn't. :)
> 
> Well, in practical terms that means "no difference" IMO. :-)
> 
> I can imagine that in some cases this approach may lead to better
> results than reducing the rate limit overall, but the general case I'm
> not sure about.
> 
> I mean, if overriding the rate limit doesn't take place very often,
> then it really should make no difference overhead-wise.  Now, of
> course, how to define "not very often" is a good question as that
> leads to rate-limiting the overriding of the original rate limit and
> that scheme may continue indefinitely ...

:)

My impression is that rate limit helps a lot for CFS, where the "true"
utilization is not known in advance, and being too responsive might
actually be counterproductive.

For DEADLINE (and RT, with differences) we should always respond as
quick as we can (and probably remember that a frequency transition was
requested if hw was already performing one, but that's another patch)
because, if we don't, a task belonging to a lower priority class might
induce deadline misses in highest priority activities. E.g., a CFS task
that happens to trigger a freq switch right before a DEADLINE task wakes
up and needs an higher frequency to meet its deadline: if we wait for
the rate limit of the CFS originated transition.. deadline miss!