Re: kernfs memcg accounting

[Vasily Averin <vvs@openvz.org>]

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@openvz.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.

Thank you,
	Vasily Averin