From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-173.mta0.migadu.com (out-173.mta0.migadu.com [91.218.175.173]) (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 8C5602BCF5 for ; Sun, 2 Nov 2025 21:36:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.173 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762119399; cv=none; b=AH2BotFnEc9WkBDNhSzxIMkZA0sn9JICoeN70dZXJ/o1d3VBl1xJ7yLk+N/nX8ZoYVsfYM1VoeJe3YxTB0mufkP5+ZvxYXw+HyrrhJc6v94RSG6B30WbxPa+ndzOwat0N3yhC07JuyvECkad7et+vfek0h2OdwdiP2pO46oiQ9g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762119399; c=relaxed/simple; bh=i3EklB5KO1R4QxKOcGYeVk40OcIAul9sYhoKiHZt5NM=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=kpHnBfQO/dea5S0dkivYvaFpid9Ojj3C5tLRju7Fvg2YChuJCh4BV8UqJYCa4dXfLmXgKRzxrIxI3dH+uvvMhgrFJgDbRk6t1TS7Z2dNIIjQMkFIMqjRVx1VHjS04AIx3a6+KcfPz9CzPTmcR/T/OafP/PxEQBWc72kRim6ZwPY= 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=uWjkOvIW; arc=none smtp.client-ip=91.218.175.173 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="uWjkOvIW" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1762119394; 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=qcGx6i/+++ufNscO90BKEs/b8++kUKzaqmiZ//H9ISU=; b=uWjkOvIWS/CmQ5V4hKpdOf06Eo4ZAkvv91DlTySCJ+m53/jNAeeaZdCBfaDTh6/k+AtkyR OVFVRg96tGQ0GJMt7tJOycDaK3A5j0R8batwvzteEfJiSGDHZ8F1jonFIuRsADZ6tdXsD3 F1obi2np1FqjXVQ+3cEKvINCpn4xOk4= From: Roman Gushchin To: Michal Hocko Cc: Andrew Morton , linux-kernel@vger.kernel.org, Alexei Starovoitov , Suren Baghdasaryan , Shakeel Butt , Johannes Weiner , Andrii Nakryiko , JP Kobryn , linux-mm@kvack.org, cgroups@vger.kernel.org, bpf@vger.kernel.org, Martin KaFai Lau , Song Liu , Kumar Kartikeya Dwivedi , Tejun Heo Subject: Re: [PATCH v2 06/23] mm: introduce BPF struct ops for OOM handling In-Reply-To: (Michal Hocko's message of "Fri, 31 Oct 2025 10:02:20 +0100") References: <20251027231727.472628-1-roman.gushchin@linux.dev> <20251027231727.472628-7-roman.gushchin@linux.dev> Date: Sun, 02 Nov 2025 13:36:25 -0800 Message-ID: <875xbsglra.fsf@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 X-Migadu-Flow: FLOW_OUT Michal Hocko writes: > On Mon 27-10-25 16:17:09, Roman Gushchin wrote: >> Introduce a bpf struct ops for implementing custom OOM handling >> policies. >> >> It's possible to load one bpf_oom_ops for the system and one >> bpf_oom_ops for every memory cgroup. In case of a memcg OOM, the >> cgroup tree is traversed from the OOM'ing memcg up to the root and >> corresponding BPF OOM handlers are executed until some memory is >> freed. If no memory is freed, the kernel OOM killer is invoked. > > Do you have any usecase in mind where parent memcg oom handler decides > to not kill or cannot kill anything and hand over upwards in the > hierarchy? I believe that in most cases bpf handlers will handle ooms themselves, but because strictly speaking I don't have control over what bpf programs do or do not, the kernel should provide the fallback mechanism. This is a common practice with bpf, e.g. sched_ext falls back to CFS/EEVDF in case something is wrong. Specifically to OOM case, I believe someone might want to use bpf programs just for monitoring/collecting some information, without trying to actually free some memory. >> The struct ops provides the bpf_handle_out_of_memory() callback, >> which expected to return 1 if it was able to free some memory and 0 >> otherwise. If 1 is returned, the kernel also checks the bpf_memory_freed >> field of the oom_control structure, which is expected to be set by >> kfuncs suitable for releasing memory. If both are set, OOM is >> considered handled, otherwise the next OOM handler in the chain >> (e.g. BPF OOM attached to the parent cgroup or the in-kernel OOM >> killer) is executed. > > Could you explain why do we need both? Why is not bpf_memory_freed > return value sufficient? Strictly speaking, bpf_memory_freed should be enough, but because bpf programs have to return an int and there is no additional cost to add this option (pass to next or in-kernel oom handler), I thought it's not a bad idea. If you feel strongly otherwise, I can ignore the return value on rely on bpf_memory_freed only. > >> The bpf_handle_out_of_memory() callback program is sleepable to enable >> using iterators, e.g. cgroup iterators. The callback receives struct >> oom_control as an argument, so it can determine the scope of the OOM >> event: if this is a memcg-wide or system-wide OOM. > > This could be tricky because it might introduce a subtle and hard to > debug lock dependency chain. lock(a); allocation() -> oom -> lock(a). > Sleepable locks should be only allowed in trylock mode. Agree, but it's achieved by controlling the context where oom can be declared (e.g. in bpf_psi case it's done from a work context). > >> The callback is executed just before the kernel victim task selection >> algorithm, so all heuristics and sysctls like panic on oom, >> sysctl_oom_kill_allocating_task and sysctl_oom_kill_allocating_task >> are respected. > > I guess you meant to say and sysctl_panic_on_oom. Yep, fixed. > >> BPF OOM struct ops provides the handle_cgroup_offline() callback >> which is good for releasing struct ops if the corresponding cgroup >> is gone. > > What kind of synchronization is expected between handle_cgroup_offline > and bpf_handle_out_of_memory? You mean from a user's perspective? E.g. can these two callbacks run in parallel? Currently yes, but it's a good question, I haven't thought about it, maybe it's better to synchronize them. Internally both rely on srcu to pin bpf_oom_ops in memory. > >> The struct ops also has the name field, which allows to define a >> custom name for the implemented policy. It's printed in the OOM report >> in the oom_policy= format. "default" is printed if bpf is not >> used or policy name is not specified. > > oom_handler seems like a better fit but nothing I would insist on. Also > I would just print it if there is an actual handler so that existing > users who do not use bpf oom killers do not need to change their > parsers. Sure, works for me too. > > Other than that this looks reasonable to me. Sound great, thank you for taking a look!