Re: kernfs memcg accounting

Roman Gushchin
2022-05-11T17:46:23Z

On Wed, May 11, 2022 at 09:01:40AM +0300, Vasily Averin wrote:
> On 5/11/22 06:06, Roman Gushchin wrote:
> > On Wed, May 04, 2022 at 12:00:18PM +0300, Vasily Averin wrote:
> >> From my point of view it is most important to account allocated memory
> >> to any cgroup inside container. Select of proper memcg is a secondary goal here.
> >> Frankly speaking I do not see a big difference between memcg of current process,
> >> memcg of newly created child and memcg of its parent.
> >>
> >> As far as I understand, Roman chose the parent memcg because it was a special
> >> case of creating a new memory group. He temporally changed active memcg
> >> in mem_cgroup_css_alloc() and properly accounted all required memcg-specific
> >> allocations.
> >
> > My primary goal was to apply the memory pressure on memory cgroups with a lot
> > of (dying) children cgroups. On a multi-cpu machine a memory cgroup structure
> > is way larger than a page, so a cgroup which looks small can be really large
> > if we calculate the amount of memory taken by all children memcg internals.
> >
> > Applying this pressure to another cgroup (e.g. the one which contains systemd)
> > doesn't help to reclaim any pages which are pinning the dying cgroups.
> >
> > For other controllers (maybe blkcg aside, idk) it shouldn't matter, because
> > there is no such problem there.
> >
> > For consistency reasons I'd suggest to charge all *large* allocations
> > (e.g. percpu) to the parent cgroup. Small allocations can be ignored.
>
> I showed in [1] other large allocation:
> "
> number bytes $1*$2 sum note call_site
> of alloc
> allocs
> ------------------------------------------------------------
> 1 14448 14448 14448 = percpu_alloc_percpu:
> 1 8192 8192 22640 ++ (mem_cgroup_css_alloc+0x54)
> 49 128 6272 28912 ++ (__kernfs_new_node+0x4e)
> 49 96 4704 33616 ? (simple_xattr_alloc+0x2c)
> 49 88 4312 37928 ++ (__kernfs_iattrs+0x56)
> 1 4096 4096 42024 ++ (cgroup_mkdir+0xc7)
> 1 3840 3840 45864 = percpu_alloc_percpu:
> 4 512 2048 47912 + (alloc_fair_sched_group+0x166)
> 4 512 2048 49960 + (alloc_fair_sched_group+0x139)
> 1 2048 2048 52008 ++ (mem_cgroup_css_alloc+0x109)
> "
> [1] https://lore.kernel.org/all/1aa4cd22-fcb6-0e8d-a1c6-23661d618864-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org/
> = already accounted
> ++ to be accounted first
> + to be accounted a bit later
>
> There is no problems with objects allocated in mem_cgroup_alloc(),
> they will be accounted to parent's memcg.
> However I do not understand how to handle other large objects?
>
> We could move set_active_memcg(parent) call from mem_cgroup_css_alloc()
> to cgroup_apply_control_enable() and handle allocation in all .css_alloc()
>
> However I need to handle allocations called from cgroup_mkdir() too and
> badly understand how to do it properly.

I don't think there is a better alternative than just having several
set_active_memcg(parent);...set_active_memcg(old); places in the cgroup.c.

Nesting is fine here, so it shouldn't be a big issue.