Re: [PATCH v3 1/2] cgroup: Show # of subsystem CSSes in cgroup.stat

[Waiman Long]

On 7/11/24 17:00, Waiman Long wrote:
> On 7/11/24 15:59, Johannes Weiner wrote:
> > On Thu, Jul 11, 2024 at 03:13:12PM -0400, Waiman Long wrote:
> > > On 7/11/24 14:59, Tejun Heo wrote:
> > > > On Thu, Jul 11, 2024 at 02:51:38PM -0400, Waiman Long wrote:
> > > > > On 7/11/24 14:44, Tejun Heo wrote:
> > > > > > Hello,
> > > > > >
> > > > > > On Thu, Jul 11, 2024 at 01:39:38PM -0400, Waiman Long wrote:
> > > > > > > On 7/11/24 13:18, Tejun Heo wrote:
> > > > > > ...
> > > > > > > Currently, I use the for_each_css() macro for iteration. If you 
> > > > > > > mean
> > > > > > > displaying all the possible cgroup subsystems even if they are 
> > > > > > > not enabled
> > > > > > > for the current cgroup, I will have to manually do the iteration.
> > > > > > Just wrapping it with for_each_subsys() should do, no? 
> > > > > > for_each_css() won't
> > > > > > iterate anything if css doesn't exist for the cgroup.
> > > > > OK, I wasn't sure if you were asking to list all the possible 
> > > > > cgroup v2
> > > > > cgroup subsystems even if they weren't enabled in the current cgroup.
> > > > > Apparently, that is the case. I prefer it that way too.
> > > > Yeah, I think listing all is better. If the list corresponded 
> > > > directly to
> > > > cgroup.controllers, it may make sense to only show enabled ones but 
> > > > we can
> > > > have dying ones and implicitly enabled memory and so on, so I think 
> > > > it'd be
> > > > cleaner to just list them all.
> > > That will means cgroup subsystems that are seldomly used like rdma, 
> > > misc
> > > or even hugetlb will always be shown in all the cgroup.stat output. I
> > > actually prefer just showing those that are enabled. As for dying 
> > > memory
> > > cgroups, they will only be shown in its online ancestors. We currently
> > > don't know how many level down are each of the dying ones.
> > It seems odd to me to not show dead ones after a cgroup has disabled
> > the controller again. They still consume memory, after all, and so
> > continue to be property of that cgroup afterwards.
> >
> > Instead of doing for_each_css(), would it make more sense to have
> >
> >     struct cgroup {
> >         ...
> >         int nr_dying_subsys[CGROUP_SUBSYS_COUNT];
>
> What exactly does new this array for? Is this for copying out 
> css->nr_dying_descendants before disabling a controller? The number 
> may be out of date when it is used. I would think we should store the 
> actual css and clearing it again once the css is ready to be freed.
>
> Anyway, I would suggest doing it as a separate add-on patch if we 
> decide to do it instead of adding it to the current patch.

Alternatively, we could delay the clearing of cgroup->subsys[] entry 
from offline time to until the css is ready to be freed. We do need to 
add check about the CSS_ONLINE flag when we only want to deal with 
online csses.

Cheers,
Longman