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From: Shakeel Butt To: Michal Hocko Cc: Andrew Morton , David Rientjes , Johannes Weiner , Roman Gushchin , Muchun Song , Thomas Gleixner , Ingo Molnar , Suren Baghdasaryan , Usama Arif , Peter Zijlstra , Darren Hart , Davidlohr Bueso , =?utf-8?B?QW5kcsOp?= Almeida , "Liam R . Howlett" , Yosry Ahmed , Rik van Riel , Nhat Pham , Meta kernel team , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC] Robust futex causing memcg OOM storm on exit Message-ID: References: <20260723001908.4046643-1-shakeel.butt@linux.dev> Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Migadu-Flow: FLOW_OUT On Tue, Jul 28, 2026 at 09:23:41PM +0200, Michal Hocko wrote: > On Tue 28-07-26 09:55:21, Shakeel Butt wrote: > > On Tue, Jul 28, 2026 at 09:27:39AM +0200, Michal Hocko wrote: > > > On Mon 27-07-26 13:48:58, Shakeel Butt wrote: > > > > On Mon, Jul 27, 2026 at 08:38:37AM -0700, Shakeel Butt wrote: > > > > > On Mon, Jul 27, 2026 at 04:36:23PM +0200, Michal Hocko wrote: > > > > > > On Wed 22-07-26 17:19:07, Shakeel Butt wrote: > > > > > > > > [...] > > > > > > > > > > > > > > > > TBH I am not entirely happy about this approach. It effectivelly reverts > > > > > > a75ffa26122b ("memcg, oom: do not bypass oom killer for dying tasks"). > > > > > > It just makes it lockless. Assumption that a dying task will do so > > > > > > quickly and with bounded resources has turned wrong on several > > > > > > occasions. > > > > > > > > > > That's why I kept it as RFC :) > > > > > > > > > > > > > > > > > On the other hand I do undestand the contention issues and I can imagine > > > > > > that the existing solution doesn't really work well for huge thread > > > > > > groups that all end up lining up on the oom_lock just to learn there is > > > > > > nothing really killable anymore because they are the oom victim... > > > > > > > > > > > > Would it be just safer to bail out only for oom victim threads. This > > > > > > would narrow down potential runaways for oom victims which should be > > > > > > more limited than any killed/exiting task. It would also give the oom > > > > > > killer/reaper chance to work. WDYT? > > > > > > > > > > I will give the following patch a try with the reproducer I have. I am still > > > > > improving the reproducer as I am still not able to recreate multi hour slowdown > > > > > yet. I will report back once I have some results. > > > > > > > > > > > > > I have some results with the reproducer I (AI) wrote. > > > > > > > > Setup: 20k threads, each with its robust_list head on its own page in a > > > > large anon region pushed into a zswap pool (writeback off, so it's > > > > unreclaimable and pins the memcg at the limit). On the OOM-group-kill > > > > every thread faults that page in futex_cleanup() -> do_swap_page() -> > > > > swapin charge -> memcg OOM. A separate thread holds mmap_lock for write > > > > in a loop so the oom_reaper can't reap the pool -- matching the host, > > > > where the reaper was blocked and the memcg never fell below the limit. > > > > > > > > I tested on next-20260726 as baseline and then tested your suggestion over it. > > > > For baseline ~48k no-victim OOM invocations serialized on oom_lock; ~40s to > > > > drain the 20k threads in a VM (hours on the host, where each invocation > > > > was a dump_header over netconsole). With the patch the dying tasks stop > > > > entering out_of_memory and the storm is gone. > > > > > > > > With the following suggestion, the threads of dying tasks stop entering > > > > out_of_memory but the exit time doubled as each thread has to decompress the > > > > page from zswap. > > > > > > > > if (tsk_is_oom_victim(current)) > > > > goto force; > > > > > > > > I also tested by replacing goto force with goto nomem and the exit time halved > > > > i.e. ~20 seconds. Looking deeper it seems like each thread is still going > > > > through the reclaim loops. > > > > > > > > Just for testing purpose I moved this check along with goto nomem just before > > > > the reclaim fuction i.e. try_to_free_mem_cgroup_pages() and the exit time falls > > > > below 1 second. I think what we need is to detect early that the cgroup is > > > > unreclaimable and the threads of exiting/reaped process should just return with > > > > enomem. > > > > > > Do you mean, something like this? > > > --- > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > index 6dc4888a90f3..66f1eebf8c22 100644 > > > --- a/mm/memcontrol.c > > > +++ b/mm/memcontrol.c > > > @@ -2648,6 +2648,18 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask, > > > if (!gfpflags_allow_blocking(gfp_mask)) > > > goto nomem; > > > > > > + /* > > > + * OOM victim still needs to charge memory to exit. OOM reaper should > > > + * help but it might fail on mmap_lock contention. If the victim is a > > > + * large thread group then all exiting threads might compete on oom_lock > > > + * just to learn that there is nothing really killable anymore. Bail > > > + * out early and fail the charge to expedite their exit. They are > > > + * considered fully reclaimed by the oom reaper and they shouldn't contribute > > > + * further charges. > > > + */ > > > + if (tsk_is_oom_victim(current) && mm_flags_test(current->signal->oom_mm)) > > > > I simply had tsk_is_oom_victim(current) check here. > > I was hoping for a more specific treatment. > > > I assume you want > > MMF_OOM_REAP_QUEUED in the mm_flags_test() above. Do you want me to test with > > this additional mm_flags_test() as well? > > Sorry I meant MMF_OOM_SKIP there. Here are the results with the reproducer I have. Ran the test 5 times and showing avg and std below: Baseline (next-20260726) 36.47 ± 0.78 s early nomem (tsk_is_oom_victim + MMF_OOM_SKIP) 20.83 ± 4.19 s early nomem (tsk_is_oom_victim) 0.59 ± 0.22 s Since MMF_OOM_SKIP is set after multiple retries by the oom_reaper the result in between baseline and simple tsk_is_oom_victim check makes sense. I think we can proceed with the early nomem (tsk_is_oom_victim + MMF_OOM_SKIP) change as it is more conservative unless you have concerns. Overall I want to deploy this in prod sooner to see if this specific issues is resolved or we catch something else.