Re: [PATCH] Revert "sched/fair: Fix O(nr_cgroups) in the load balancing path"

[Vincent Guittot <vincent.guittot@linaro.org>]

On Tue, 29 Oct 2019 at 16:36, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Oct 29, 2019 at 07:55:26AM -0700, Doug Smythies wrote:
>
> > I only know that the call to the intel_pstate driver doesn't
> > happen, and that it is because cfs_rq_is_decayed returns TRUE.
> > So, I am asserting that the request is not actually decayed, and
> > should not have been deleted.
>
> So what cfs_rq_is_decayed() does is allow a cgroup's cfs_rq to be
> removed from the list.
>
> Once it is removed, that cfs_rq will no longer be checked in the
> update_blocked_averages() loop. Which means done has less chance of
> getting false. Which in turn means that it's more likely
> rq->has_blocked_load becomes 0.
>
> Which all sounds good.
>
> Can you please trace what keeps the CPU awake?

I think that the sequence below is what intel pstate driver was using

rt/dl task wakes up and run for some times
rt/dl pelt signal is no more null so periodic decay happens.

before optimization update_cfs_rq_load_avg() for root cfs_rq was
called even if pelt was null,
which calls cfs_rq_util_change,  which calls intel pstate

after optimization its no more called.

The patch that i just sent will check that sequence but it's more a
hack than a clean fix because
it uses cfs notification to cpufreq for update that is not related to cfs.

I will look at a proper solution if the test confirms my assumption

>
> > Now, if we also look back at the comments for the original commit:
> >
> >       "In an edge case where temporary cgroups were leaking, this
> >       caused the kernel to consume good several tens of percents of
> >       CPU cycles running update_blocked_averages(), each run taking
> >       multiple millisecs."
> >
> > To my way of thinking: Fix the leak, don't program around it; The
> > commit breaks something else, so revert it.
>
> The leak was fixed, but it still doesn't make sense to keep idle cgroups
> on that list. Some people have a stupid amount of cgroups, most of which
> are pointless and unused, so being able to remove them is good.
>
> Which is why it got added back, once list management issues were sorted.