On Thu, Dec 12, 2024 at 1:35 PM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote: > > On Thu, Dec 12, 2024 at 01:30:12PM -0500, Johannes Weiner wrote: > > On Thu, Dec 12, 2024 at 09:06:25AM -0800, Yosry Ahmed wrote: > > > On Thu, Dec 12, 2024 at 8:58 AM Rik van Riel <riel@xxxxxxxxxxx> wrote: > > > > > > > > A task already in exit can get stuck trying to allocate pages, if its > > > > cgroup is at the memory.max limit, the cgroup is using zswap, but > > > > zswap writeback is enabled, and the remaining memory in the cgroup is > > > > not compressible. > > > > > > > > This seems like an unlikely confluence of events, but it can happen > > > > quite easily if a cgroup is OOM killed due to exceeding its memory.max > > > > limit, and all the tasks in the cgroup are trying to exit simultaneously. > > > > > > > > When this happens, it can sometimes take hours for tasks to exit, > > > > as they are all trying to squeeze things into zswap to bring the group's > > > > memory consumption below memory.max. > > > > > > > > Allowing these exiting programs to push some memory from their own > > > > cgroup into swap allows them to quickly bring the cgroup's memory > > > > consumption below memory.max, and exit in seconds rather than hours. > > > > > > > > Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx> > > > > > > Thanks for sending a v2. > > > > > > I still think maybe this needs to be fixed on the memcg side, at least > > > by not making exiting tasks try really hard to reclaim memory to the > > > point where this becomes a problem. IIUC there could be other reasons > > > why reclaim may take too long, but maybe not as pathological as this > > > case to be fair. I will let the memcg maintainers chime in for this. > > > > > > If there's a fundamental reason why this cannot be fixed on the memcg > > > side, I don't object to this change. > > > > > > Nhat, any objections on your end? I think your fleet workloads were > > > the first users of this interface. Does this break their expectations? > > > > Yes, I don't think we can do this, unfortunately :( There can be a > > variety of reasons for why a user might want to prohibit disk swap for > > a certain cgroup, and we can't assume it's okay to make exceptions. > > > > There might also not *be* any disk swap to overflow into after Nhat's > > virtual swap patches. Presumably zram would still have the issue too. > > Very good points. > > > > > So I'm also inclined to think this needs a reclaim/memcg-side fix. We > > have a somewhat tumultous history of policy in that space: > > > > commit 7775face207922ea62a4e96b9cd45abfdc7b9840 > > Author: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> > > Date: Tue Mar 5 15:46:47 2019 -0800 > > > > memcg: killed threads should not invoke memcg OOM killer > > > > allowed dying tasks to simply force all charges and move on. This > > turned out to be too aggressive; there were instances of exiting, > > uncontained memcg tasks causing global OOMs. This lead to that: > > > > commit a4ebf1b6ca1e011289677239a2a361fde4a88076 > > Author: Vasily Averin <vasily.averin@xxxxxxxxx> > > Date: Fri Nov 5 13:38:09 2021 -0700 > > > > memcg: prohibit unconditional exceeding the limit of dying tasks > > > > which reverted the bypass rather thoroughly. Now NO dying tasks, *not > > even OOM victims*, can force charges. I am not sure this is correct, > > either: > > > > If we return -ENOMEM to an OOM victim in a fault, the fault handler > > will re-trigger OOM, which will find the existing OOM victim and do > > nothing, then restart the fault. This is a memory deadlock. The page > > allocator gives OOM victims access to reserves for that reason. > > > > Actually, it looks even worse. For some reason we're not triggering > > OOM from dying tasks: > > > > ret = task_is_dying() || out_of_memory(&oc); > > > > Even though dying tasks are in no way privileged or allowed to exit > > expediently. Why shouldn't they trigger the OOM killer like anybody > > else trying to allocate memory? > > This is a very good point and actually out_of_memory() will mark the > dying process as oom victim and put it in the oom reaper's list which > should help further in such situation. > > > > > As it stands, it seems we have dying tasks getting trapped in an > > endless fault->reclaim cycle; with no access to the OOM killer and no > > access to reserves. Presumably this is what's going on here? > > > > I think we want something like this: > > The following patch looks good to me. Let's test this out (hopefully Rik > will be able to find a live impacted machine) and move forward with this > fix. I agree with this too. As Shakeel mentioned, this seemed like a stopgap and not an actual fix for the underlying problem. Johannes further outlined how the stopgap can be problematic. Let's try to fix this on the memcg/reclaim/OOM side, and properly treat dying tasks instead of forcing them into potentially super slow reclaim paths. Hopefully Johannes's patch fixes this.
