Re: [PATCH bpf-next v3 00/13] bpf: Introduce selectable memcg for bpf map

[Yafang Shao <laoar.shao@gmail.com>]

On Wed, Sep 21, 2022 at 7:15 AM Roman Gushchin <roman.gushchin@linux.dev> wrote:
>
> 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@linux.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@linux.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@linux.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@linux.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.
>

Hmm...
It seems that we are in a dilemma now.
We can't fix it in memcg way, because the issue we are fixing it a
bpf-specific issue.
But we can't fix it in a bpf-specific way neither...

-- 
Regards
Yafang