Messages in this thread | | | Subject | Re: [PATCH] cpufreq: schedutil: rate limits for SCHED_DEADLINE | From | Claudio Scordino <> | Date | Fri, 9 Feb 2018 14:20:32 +0100 |
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Il 09/02/2018 13:56, Rafael J. Wysocki ha scritto: > On Fri, Feb 9, 2018 at 1:52 PM, Juri Lelli <juri.lelli@redhat.com> wrote: >> 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. > > 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
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