Re: kernfs memcg accounting

Michal Koutný
2022-05-11T16:49:45Z

On Wed, May 11, 2022 at 09:01:40AM +0300, Vasily Averin <vvs-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org> wrote:
> 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)

This requires just adding GFP_KERNEL_ACCOUNT (no new active memcg
switch).

> 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.

If we consent to charge to the creator, the change would be just passing
GFP_ACCOUNT at fewer (right) places, wouldn't it?

Also, my undertanding of memcgs is that they're not hermetically tight,
so I think charging just kernfs_nodes (for dirs and files) provides
sufficient bound.

Except for, the xattrs, my older notes say: "make kernfs simple_xattr kernel
accounted, up to KERNFS_USER_XATTR_SIZE_LIMIT*KERNFS_MAX_USER_XATTRS =
128k * 128 = 16M / inode".

HTH,
Michal