Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Fri, 9 Feb 2018 13:08:22 +0100 | Subject | Re: [PATCH] cpufreq: schedutil: rate limits for SCHED_DEADLINE |
| |
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.
| |