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

[Claudio Scordino]

Il 09/02/2018 13:56, Rafael J. Wysocki ha scritto:
> On Fri, Feb 9, 2018 at 1:52 PM, Juri Lelli <juri.lelli@xxxxxxxxxx> wrote:
> > On 09/02/18 13:08, Rafael J. Wysocki wrote:
> > > On Fri, Feb 9, 2018 at 12:51 PM, Juri Lelli <juri.lelli@xxxxxxxxxx> wrote:
> > > > 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!
> > >
> > > 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.
>
> Well, that's the point.
>
> So there is a tradeoff: do we want to allow deadlines to be missed
> because of excessive overhead or do we want to allow deadlines to be
> missed because of the rate limit.

For a very few tasks, the tests have indeed shown that the approach pays off: we get a significant reduction of misses with a negligible increase of energy consumption.
I still need to check what happens for a high amount of tasks, trying to reproduce the  "ramp up" pattern (in which DL keeps increasing the utilization, ignoring the rate limit and adding overhead)

Thanks,

              Claudio