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Date:   Wed, 11 May 2022 10:46:23 -0700
From:   Roman Gushchin <roman.gushchin@linux.dev>
To:     Vasily Averin <vvs@openvz.org>
Cc:     Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Shakeel Butt <shakeelb@google.com>, kernel@openvz.org,
        Florian Westphal <fw@strlen.de>, linux-kernel@vger.kernel.org,
        Michal Hocko <mhocko@suse.com>, cgroups@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>
Subject: Re: kernfs memcg accounting
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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@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.

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.