Hi Juri, On Thu, 18 Oct 2018 12:10:08 +0200 Juri Lelli <juri.le...@redhat.com> wrote: [...] > > Yes, a HZ related limit sounds like something we'd want. But if > > we're going to do a minimum sysctl, we should also consider adding > > a maximum, if you set a massive period/deadline, you can, even with > > a relatively low u, incur significant delays. > > > > And do we want to put the limit on runtime or on period ? > > > > That is, something like: > > > > TICK_NSEC/2 < period < 10*TICK_NSEC > > > > and/or > > > > TICK_NSEC/2 < runtime < 10*TICK_NSEC > > > > Hmm, for HZ=1000 that ends up with a max period of 10ms, that's far > > too low, 24Hz needs ~41ms. We can of course also limit the runtime > > by capping u for users (as we should anyway). > > I also thought of TICK_NSEC/2 as a reasonably safe lower limit
I tend to think that something larger than "2" should be used (maybe 10? I mean: even if HZ = 100, it might make sense to allow a runtime equal to 1ms...) > that will implicitly limit period as well since > > runtime <= deadline <= period I agree that if we end up with TICK_NSEC/2 for the runtime limit then explicitly enforcing a minimum period is not needed. > Not sure about the upper limit, though. Lower limit is something > related to the inherent granularity of the platform/config, upper > limit is more to do with highest prio stuff with huge period delaying > everything else; doesn't seem to be related to HZ? I agree Luca