From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-170.mta1.migadu.com (out-170.mta1.migadu.com [95.215.58.170]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 64EF03914E4 for ; Tue, 28 Jul 2026 16:55:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.170 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785257754; cv=none; b=qFUdc9i99i1EbDDbsfpkrc1Hv3l/oMPSJpat68nCLW/5jCNy9Q9Rr8n58CQYstYnjcUPJ5S1wwRgmIVqdWVlTukuWF3wA1uishEbO9KPlLoDHEWDM4MFotJ8jymnArb6QrwaIBnxP7g74ksIGEpufVnwEx7m2YuQziL2cslgXug= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785257754; c=relaxed/simple; bh=0mjpuZRpQxogwTizt2C3lDicCpVlWT9TOUP3k/i9ziU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=YHu1Kf4ZCqU1JG9AX2rXY/ChSSzQR+5Brgj7wVDuCFOv8oFtvmlSLz3fy6Zg7EMk72BRmDVhuj0A9ghaW0g1l3eoUugFwBH2aCNTWOxDT6v8bD+c2apAhwnqSV2qt1dwS3YwxMiNyIXndQ0CJSHG01UwCyn1FvX4rp5JB0R1nr4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=nzriWRMo; arc=none smtp.client-ip=95.215.58.170 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="nzriWRMo" Date: Tue, 28 Jul 2026 09:55:21 -0700 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1785257740; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=r5JlP6ioUX+/TbxbJL0DU8jGeR7d++HxnQJa+9ff9G4=; b=nzriWRMoPg/hlP/vXaFpykit9EB3DnLvs8Agjmn21AqnYWpvMInnIAWS8vM5GTeHOWuVBA GIjg2BePJN+1JkLrhCPwC3yKVQVWuo3/+KZhKYvXxcAsZnmqH6tLTUSwLSG+LlT3DPP9f5 EG9bfe3zO9W22+H2NrI3/c7wWCR0gro= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. 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=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT 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 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? > + goto nomem; > + > __memcg_memory_event(mem_over_limit, MEMCG_MAX, allow_spinning); > raised_max_event = true; > > > -- > Michal Hocko > SUSE Labs