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

[Rafael J. Wysocki]

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.