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 ...