linux-kernel - Re: kernfs memcg accounting

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Message-ID: <20220511164945.GA31592@blackbody.suse.cz>
Date:   Wed, 11 May 2022 18:49:45 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Vasily Averin <vvs@...nvz.org>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Vlastimil Babka <vbabka@...e.cz>,
        Shakeel Butt <shakeelb@...gle.com>, kernel@...nvz.org,
        Florian Westphal <fw@...len.de>, linux-kernel@...r.kernel.org,
        Michal Hocko <mhocko@...e.com>, cgroups@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>
Subject: Re: kernfs memcg accounting

On Wed, May 11, 2022 at 09:01:40AM +0300, Vasily Averin <vvs@...nvz.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@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.

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