Re: [PATCH v3 4/8] sched/deadline: Rebuild root domain accounting after every update

[Waiman Long]

On 3/11/25 7:59 AM, Juri Lelli wrote:
> On 10/03/25 20:16, Waiman Long wrote:
> > On 3/10/25 3:18 PM, Waiman Long wrote:
> > > On 3/10/25 2:54 PM, Dietmar Eggemann wrote:
> > > > On 10/03/2025 10:37, Juri Lelli wrote:
> > > > > Rebuilding of root domains accounting information (total_bw) is
> > > > > currently broken on some cases, e.g. suspend/resume on aarch64. Problem
> > > > Nit: Couldn't spot any arch dependency here. I guess it was just tested
> > > > on Arm64 platforms so far.
> > > >
> > > > [...]
> > > >
> > > > > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > > > > index 44093339761c..363ad268a25b 100644
> > > > > --- a/kernel/sched/topology.c
> > > > > +++ b/kernel/sched/topology.c
> > > > > @@ -2791,6 +2791,7 @@ void partition_sched_domains_locked(int
> > > > > ndoms_new, cpumask_var_t doms_new[],
> > > > >        ndoms_cur = ndoms_new;
> > > > >          update_sched_domain_debugfs();
> > > > > +    dl_rebuild_rd_accounting();
> > > > Won't dl_rebuild_rd_accounting()'s lockdep_assert_held(&cpuset_mutex)
> > > > barf when called via cpuhp's:
> > > >
> > > > sched_cpu_deactivate()
> > > >
> > > >     cpuset_cpu_inactive()
> > > >
> > > >       partition_sched_domains()
> > > >
> > > >         partition_sched_domains_locked()
> > > >
> > > >           dl_rebuild_rd_accounting()
> > > >
> > > > ?
> Good catch. Guess I didn't notice while testing with LOCKDEP as I was
> never able to hit this call path on my systems.
>
> > > Right. If cpuhp_tasks_frozen is true, partition_sched_domains() will be
> > > called without holding cpuset mutex.
> > >
> > > Well, I think we will need an additional wrapper in cpuset.c that
> > > acquires the cpuset_mutex first before calling partition_sched_domains()
> > > and use the new wrapper in these cases.
> > Actually, partition_sched_domains() is called with the special arguments (1,
> > NULL, NULL) to reset the domain to a single one. So perhaps something like
> > the following will be enough to avoid this problem.
> I think this would work, as we will still rebuild the accounting after
> last CPU comes back from suspend. The thing I am still not sure about is
> what we want to do in case we have DEADLINE tasks around, since with
> this I belive we would be ignoring them and let suspend proceed.

That is the current behavior. You can certainly create a test case to 
trigger such condition and see what to do about it. Alternatively, you 
can document that and come up with a follow-up patch later on.

Cheers,
Longman