On 09/02/18 13:08, Rafael J. Wysocki wrote: > On Fri, Feb 9, 2018 at 12:51 PM, Juri Lelli <juri.le...@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.le...@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.le...@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.