Date:   Tue, 20 Sep 2022 16:15:25 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     Tejun Heo <tj@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Cgroups <cgroups@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH bpf-next v3 00/13] bpf: Introduce selectable memcg for
 bpf map

On Tue, Sep 20, 2022 at 08:42:36PM +0800, Yafang Shao wrote:
> On Tue, Sep 20, 2022 at 10:40 AM Roman Gushchin
> <roman.gushchin@...ux.dev> wrote:
> >
> > On Sun, Sep 18, 2022 at 11:44:48AM +0800, Yafang Shao wrote:
> > > On Sat, Sep 17, 2022 at 12:53 AM Roman Gushchin
> > > <roman.gushchin@...ux.dev> wrote:
> > > >
> > > > On Tue, Sep 13, 2022 at 02:15:20PM +0800, Yafang Shao wrote:
> > > > > On Fri, Sep 9, 2022 at 12:13 AM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
> > > > > >
> > > > > > On Thu, Sep 08, 2022 at 10:37:02AM +0800, Yafang Shao wrote:
> > > > > > > On Thu, Sep 8, 2022 at 6:29 AM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
> > > > > > > >
> > > > > > > > On Wed, Sep 07, 2022 at 05:43:31AM -1000, Tejun Heo wrote:
> > > > > > > > > Hello,
> > > > > > > > >
> > > > > > > > > On Fri, Sep 02, 2022 at 02:29:50AM +0000, Yafang Shao wrote:
> > > > > > > > > ...
> > > > > > > > > > This patchset tries to resolve the above two issues by introducing a
> > > > > > > > > > selectable memcg to limit the bpf memory. Currently we only allow to
> > > > > > > > > > select its ancestor to avoid breaking the memcg hierarchy further.
> > > > > > > > > > Possible use cases of the selectable memcg as follows,
> > > > > > > > >
> > > > > > > > > As discussed in the following thread, there are clear downsides to an
> > > > > > > > > interface which requires the users to specify the cgroups directly.
> > > > > > > > >
> > > > > > > > >  https://lkml.kernel.org/r/YwNold0GMOappUxc@slm.duckdns.org
> > > > > > > > >
> > > > > > > > > So, I don't really think this is an interface we wanna go for. I was hoping
> > > > > > > > > to hear more from memcg folks in the above thread. Maybe ping them in that
> > > > > > > > > thread and continue there?
> > > > > > > >
> > > > > > >
> > > > > > > Hi Roman,
> > > > > > >
> > > > > > > > As I said previously, I don't like it, because it's an attempt to solve a non
> > > > > > > > bpf-specific problem in a bpf-specific way.
> > > > > > > >
> > > > > > >
> > > > > > > Why do you still insist that bpf_map->memcg is not a bpf-specific
> > > > > > > issue after so many discussions?
> > > > > > > Do you charge the bpf-map's memory the same way as you charge the page
> > > > > > > caches or slabs ?
> > > > > > > No, you don't. You charge it in a bpf-specific way.
> > > > > >
> > > > >
> > > > > Hi Roman,
> > > > >
> > > > > Sorry for the late response.
> > > > > I've been on vacation in the past few days.
> > > > >
> > > > > > The only difference is that we charge the cgroup of the processes who
> > > > > > created a map, not a process who is doing a specific allocation.
> > > > >
> > > > > This means the bpf-map can be indepent of process, IOW, the memcg of
> > > > > bpf-map can be indepent of the memcg of the processes.
> > > > > This is the fundamental difference between bpf-map and page caches, then...
> > > > >
> > > > > > Your patchset doesn't change this.
> > > > >
> > > > > We can make this behavior reasonable by introducing an independent
> > > > > memcg, as what I did in the previous version.
> > > > >
> > > > > > There are pros and cons with this approach, we've discussed it back
> > > > > > to the times when bpf memcg accounting was developed. If you want
> > > > > > to revisit this, it's maybe possible (given there is a really strong and likely
> > > > > > new motivation appears), but I haven't seen any complaints yet except from you.
> > > > > >
> > > > >
> > > > > memcg-base bpf accounting is a new feature, which may not be used widely.
> > > > >
> > > > > > >
> > > > > > > > Yes, memory cgroups are not great for accounting of shared resources, it's well
> > > > > > > > known. This patchset looks like an attempt to "fix" it specifically for bpf maps
> > > > > > > > in a particular cgroup setup. Honestly, I don't think it's worth the added
> > > > > > > > complexity. Especially because a similar behaviour can be achieved simple
> > > > > > > > by placing the task which creates the map into the desired cgroup.
> > > > > > >
> > > > > > > Are you serious ?
> > > > > > > Have you ever read the cgroup doc? Which clearly describe the "No
> > > > > > > Internal Process Constraint".[1]
> > > > > > > Obviously you can't place the task in the desired cgroup, i.e. the parent memcg.
> > > > > >
> > > > > > But you can place it into another leaf cgroup. You can delete this leaf cgroup
> > > > > > and your memcg will get reparented. You can attach this process and create
> > > > > > a bpf map to the parent cgroup before it gets child cgroups.
> > > > >
> > > > > If the process doesn't exit after it created bpf-map, we have to
> > > > > migrate it around memcgs....
> > > > > The complexity in deployment can introduce unexpected issues easily.
> > > > >
> > > > > > You can revisit the idea of shared bpf maps and outlive specific cgroups.
> > > > > > Lof of options.
> > > > > >
> > > > > > >
> > > > > > > [1] https://www.kernel.org/doc/Documentation/cgroup-v2.txt
> > > > > > >
> > > > > > > > Beatiful? Not. Neither is the proposed solution.
> > > > > > > >
> > > > > > >
> > > > > > > Is it really hard to admit a fault?
> > > > > >
> > > > > > Yafang, you posted several versions and so far I haven't seen much of support
> > > > > > or excitement from anyone (please, fix me if I'm wrong). It's not like I'm
> > > > > > nacking a patchset with many acks, reviews and supporters.
> > > > > >
> > > > > > Still think you're solving an important problem in a reasonable way?
> > > > > > It seems like not many are convinced yet. I'd recommend to focus on this instead
> > > > > > of blaming me.
> > > > > >
> > > > >
> > > > > The best way so far is to introduce specific memcg for specific resources.
> > > > > Because not only the process owns its memcg, but also specific
> > > > > resources own their memcgs, for example bpf-map, or socket.
> > > > >
> > > > > struct bpf_map {                                 <<<< memcg owner
> > > > >     struct memcg_cgroup *memcg;
> > > > > };
> > > > >
> > > > > struct sock {                                       <<<< memcg owner
> > > > >     struct mem_cgroup *sk_memcg;
> > > > > };
> > > > >
> > > > > These resources already have their own memcgs, so we should make this
> > > > > behavior formal.
> > > > >
> > > > > The selectable memcg is just a variant of 'echo ${proc} > cgroup.procs'.
> > > >
> > > > This is a fundamental change: cgroups were always hierarchical groups
> > > > of processes/threads. You're basically suggesting to extend it to
> > > > hierarchical groups of processes and some other objects (what's a good
> > > > definition?).
> > >
> > > Kind of, but not exactly.
> > > We can do it without breaking the cgroup hierarchy. Under current
> > > cgroup hierarchy, the user can only echo processes/threads into a
> > > cgroup, that won't be changed in the future. The specific resources
> > > are not exposed to the user, the user can only control these specific
> > > resources by controlling their associated processes/threads.
> > > For example,
> > >
> > >                 Memcg-A
> > >                        |---- Memcg-A1
> > >                        |---- Memcg-A2
> > >
> > > We can introduce a new file memory.owner into each memcg. Each bit of
> > > memory.owner represents a specific resources,
> > >
> > >  memory.owner: | bit31 | bitN | ... | bit1 | bit0 |
> > >                                          |               |
> > > |------ bit0: bpf memory
> > >                                          |
> > > |-------------- bit1: socket memory
> > >                                          |
> > >                                          |---------------------------
> > > bitN: a specific resource
> > >
> > > There won't be too many specific resources which have to own their
> > > memcgs, so I think 32bits is enough.
> > >
> > >                 Memcg-A : memory.owner == 0x1
> > >                        |---- Memcg-A1 : memory.owner == 0
> > >                        |---- Memcg-A2 : memory.owner == 0x1
> > >
> > > Then the bpf created by processes in Memcg-A1 will be charged into
> > > Memcg-A directly without charging into Memcg-A1.
> > > But the bpf created by processes in Memcg-A2 will be charged into
> > > Memcg-A2 as its memory.owner is 0x1.
> > > That said, these specific resources are not fully independent of
> > > process, while they are still associated with the processes which
> > > create them.
> > > Luckily memory.move_charge_at_immigrate is disabled in cgroup2, so we
> > > don't need to care about the possible migration issue.
> > >
> > > I think we may also apply it to shared page caches.  For example,
> > >       struct inode {
> > >           struct mem_cgroup *memcg;          <<<< add a new member
> > >       };
> > >
> > > We define struct inode as a memcg owner, and use scope-based charge to
> > > charge its pages into inode->memcg.
> > > And then put all memcgs which shared these resources under the same
> > > parent. The page caches of this inode will be charged into the parent
> > > directly.
> >
> > Ok, so it's something like premature selective reparenting.
> >
> 
> Right. I think it  may be a good way to handle the resources which may
> outlive the process.
> 
> > > The shared page cache is more complicated than bpf memory, so I'm not
> > > quite sure if it can apply to shared page cache, but it can work well
> > > for bpf memory.
> >
> > Yeah, this is the problem. It feels like it's a problem very specific
> > to bpf maps and an exact way you use them. I don't think you can successfully
> > advocate for changes of these calibre without a more generic problem. I might
> > be wrong.
> >
> 
> What is your concern about this method? Are there any potential issues?

The issue is simple: nobody wants to support a new non-trivial cgroup interface
to solve a specific bpf accounting issue in one particular setup. Any new
interface will become an API and has to be supported for many many years,
so it has to be generic and future-proof.

If you want to go this direction, please, show that it solves a _generic_
problem, not limited to a specific way how you use bpf maps in your specific
setup. Accounting of a bpf map shared by many cgroups, which should outlive
the original memory cgroups... Idk, maybe it's how many users are using bpf
maps, but I don't hear it yet.

There were some patches from Google folks about the tmpfs accounting, _maybe_
it's something to look at in order to get an idea about a more generic problem
and solution.

Thanks!

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