Re: [PATCH v2] sched: cpuset: Don't rebuild sched domains on suspend-resume

[Waiman Long]

On 1/31/23 14:22, Qais Yousef wrote:
> On 01/30/23 14:57, Waiman Long wrote:
> > On 1/30/23 14:48, Qais Yousef wrote:
> > > On 01/30/23 11:29, Waiman Long wrote:
> > > > On 1/30/23 08:00, Qais Yousef wrote:
> > > >
> > > >           just skip the call here if the condition is right? Like
> > > >
> > > >                   /* rebuild sched domains if cpus_allowed has changed */
> > > >                   if (cpus_updated || (force_rebuild && !cpuhp_tasks_frozen)) {
> > > >                           force_rebuild = false;
> > > >                           rebuild_sched_domains();
> > > >                   }
> > > >
> > > >           Still, we will need to confirm that cpuhp_tasks_frozen will be cleared
> > > >           outside of the suspend/resume cycle.
> > > >
> > > >       I think it's fine to use this variable from the cpuhp callback context only.
> > > >       Which I think this cpuset workfn is considered an extension of.
> > > >
> > > >       But you're right, I can't use cpuhp_tasks_frozen directly in
> > > >       rebuild_root_domains() as I did in v1 because it doesn't get cleared after
> > > >       calling the last _cpu_up().
> > > >
> > > > That is what I suspect. So we can't use that cpuhp_tasks_frozen variable here
> > > > in cpuset.
> > > >
> > > >        force_rebuild will only be set after the last cpu
> > > >       is brought online though - so this should happen once at the end.
> > > >
> > > > Perhaps you can add another tracking variable for detecting if suspend/resume
> > > > is in progress.
> > > I think cpuhp_tasks_frozen is meant for that. All users who cared so far
> > > belonged to the cpuhp callback. I think reading it from cpuset_hotplug_workfn()
> > > is fine too as this function will only run as a consequence of the cpuhp
> > > callback AFAICS. cpuset_cpu_active() takes care of not forcing a rebuild of
> > > sched_domains until the last cpu becomes active - so the part of it being done
> > > once at the end at resume is handled too.
> > Well we will have to add code to clear cpuhp_tasks_frozen at the end of
> > resume then. We don't want to affect other callers unless we are sure that
> > it won't affect them.
> Actually I think since the cpuset_hotplug_workfn() is called later, there's
> a chance to race with another cpuhp operation just after resume.
>
> Anyway. I think we don't have to use this flag. But we'd have to better distill
> the reasons of why we force_rebuild.
>
> Your 2 new users are tripping me so far - do they handle errors where the shape
> of cpuset changes? If yes, then we must take dl accounting update into
> consideration for these errors.
The 2 new users is for the cpuset cpu partition which is used to create 
a secondary scheduling domain and hence have to call 
rebuilds_sched_domains() to set it up. Those should not be used that 
frequently.

> Juri, I'd still would appreciate a confirmation from you that I'm not
> understanding things completely wrong.
>
> > > It's just rebuild_sched_domains() will always assume it needs to clear and
> > > rebuild deadline accounting - which is not true for suspend/resume case. But
> > > now looking at other users of rebuild_sched_domains(), others might be getting
> > > the hit too. For example rebuild_sched_domains_locked() is called on
> > > update_relax_domain_level() which AFAIU should not impact dl accounting.
> > >
> > > FWIW, I did capture a worst case scenario of 21ms because of
> > > rebuild_root_domains().
> > >
> > > /me thinks rebuild_root_domains() is a misleading name too as it just fixes
> > > dl accounting but not rebuild the rd itself.
> > >
> > > What makes sense to me now is to pass whether dl accounting requires updating
> > > to rebuild_sched_domains() as an arg so that the caller can decide whether the
> > > reason can affect dl accounting.
> > >
> > > Or maybe pull rebuild_root_domains() out of the chain and let the caller call
> > > it directly. And probably rename it to update_do_rd_accounting() or something.
> > >
> > > I'll continue to dig more..
> > Looking forward to see that.
> Another thought I had is maybe worth trying to optimize the rebuild root domain
> process. Interestingly in my system there are no dl tasks but
>
> 	rebuilds_sched_domains()
> 	  cpuset_for_each_descendant_pre()
> 	    update_tasks_root_domain()
> 	      css_task_iter_next()
> 	        dl_add_task_root_domain()
>
> seems to be going through every task in the hierarchy anyway which would
> explain the slow down. We can have special variants to iterate through
> hierarchies that ever seen a dl task attached to them and a special variant to
> iterate through dl tasks only in a css - but I'm not sure if I'm brave enough
> to go down this rabbit hole :D

Yes, it seems like we have to check every tasks in the system to see if 
they are dl tasks. It can be expensive if there are a large number of 
tasks. Maybe we should track the # of dl tasks in each cgroup and skip 
this operation if there is none.

Cheers,
Longman