On Wed, Sep 21, 2022 at 7:15 AM Roman Gushchin <roman.gushchin@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxxxxxxxx > > > > > > > > > > > > > > > > > > > > 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
