Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Fri, 15 Nov 2019 11:03:20 +0100 | Subject | Re: [PATCH v4] sched/freq: move call to cpufreq_update_util |
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On Fri, Nov 15, 2019 at 10:55 AM Peter Zijlstra <peterz@infradead.org> wrote: > > On Thu, Nov 14, 2019 at 06:07:31PM +0100, Vincent Guittot wrote: > > update_cfs_rq_load_avg() calls cfs_rq_util_change() everytime pelt decays, > > which might be inefficient when cpufreq driver has rate limitation. > > > > When a task is attached on a CPU, we have call path: > > > > update_load_avg() > > update_cfs_rq_load_avg() > > cfs_rq_util_change -- > trig frequency update > > attach_entity_load_avg() > > cfs_rq_util_change -- > trig frequency update > > > > The 1st frequency update will not take into account the utilization of the > > newly attached task and the 2nd one might be discard because of rate > > limitation of the cpufreq driver. > > Doesn't this just show that a dumb rate limit in the driver is broken? > > > update_cfs_rq_load_avg() is only called by update_blocked_averages() > > and update_load_avg() so we can move the call to > > cfs_rq_util_change/cpufreq_update_util() into these 2 functions. It's also > > interesting to notice that update_load_avg() already calls directly > > cfs_rq_util_change() for !SMP case. > > > > This changes will also ensure that cpufreq_update_util() is called even > > when there is no more CFS rq in the leaf_cfs_rq_list to update but only > > irq, rt or dl pelt signals. > > I don't think it does that; that is, iirc the return value of > ___update_load_sum() is 1 every time a period lapses. So even if the avg > is 0 and doesn't change, it'll still return 1 on every period. > > Which is what that dumb rate-limit thing wants of course. But I'm still > thinking that it's stupid to do. If nothing changes, don't generate > events. > > If anything, update_blocked_avgerages() should look at > @done/others_have_blocked() to emit events for rt,dl,irq. > > So why are we making the scheduler code more ugly instead of fixing that > driver?
I guess we could "fix" the driver by making it rate limit MSR writes only, but I'm not sure if that would help.
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