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

[Juri Lelli <juri.lelli@redhat.com>]

On 09/02/18 13:08, Rafael J. Wysocki wrote:
> On Fri, Feb 9, 2018 at 12:51 PM, Juri Lelli <juri.lelli@redhat.com> wrote:
> > On 09/02/18 12:37, Rafael J. Wysocki wrote:
> >> On Fri, Feb 9, 2018 at 12:26 PM, Juri Lelli <juri.lelli@redhat.com> wrote:
> >> > On 09/02/18 12:04, Rafael J. Wysocki wrote:
> >> >> On Fri, Feb 9, 2018 at 11:53 AM, Juri Lelli <juri.lelli@redhat.com> 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!
> 
> Fair enough, but if there's too much overhead as a result of this, you
> can't guarantee the deadlines to be met anyway.

Indeed. I guess this only works if corner cases as the one above don't
happen too often.