From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-171.mta1.migadu.com (out-171.mta1.migadu.com [95.215.58.171]) (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 4DFA0147C9B for ; Thu, 30 Oct 2025 23:15:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.171 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761866114; cv=none; b=gYQCDgWrb41yIwkGcFyIBOoi2EFNnQ0clSxU/UzJ5vEF+cCmo7KA082GtDispPZlGlGyqSRzk4amArrsYhW03Av8cuoP4CdL4LCUPYvr45fuNobK287H92LyOSxkMEZuTSTV4jXKQrpvJgSTkIqWuvJ0KU7WCIxh66ji1/Op7lg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761866114; c=relaxed/simple; bh=Dn7XJpP4rtwOB8NIOvQUmEMjKhFFZhahMNdfR72i7xQ=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=myjAzTV9MBxq3+OTdKHJp1CR+4/ph7i1rdVuvCZFVm+Zyxam2o2h4/DgRFHFVcVqGY+hNuLEX4QY5+vmJgKDX1SndbyOv4ivFdTBE1nyxnIc4chB6cyS4zY4DeR/XcyAgG0Fvde/9h6PG2ddbhrfGaRTS7RlFbl/n8tU/JgrmqM= 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=BqYnhxem; arc=none smtp.client-ip=95.215.58.171 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="BqYnhxem" 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=1761866108; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=nbsSEZaCMc7K7Atlywq1UA86pTshfXfbD+JgyxyXRuw=; b=BqYnhxemjKDpfH7eW6N6jRHjYMwxVKsjcN/cG4hJ/T6pdPe4keRlC9cXgEVIkTtv5p+oKQ GnmPbl0/rkAOQXBte6goaO5WKU8xpmSSgoKdLnR8zZsR2QT+6Mz336obgwm5/+sF49C/wu lUk4ZnQnJUDPb4AYlGp8YqKaJXOl8rk= From: Roman Gushchin To: Martin KaFai Lau Cc: Song Liu , Amery Hung , Andrew Morton , linux-kernel@vger.kernel.org, Alexei Starovoitov , Suren Baghdasaryan , Michal Hocko , Shakeel Butt , Johannes Weiner , Andrii Nakryiko , JP Kobryn , linux-mm@kvack.org, cgroups@vger.kernel.org, bpf@vger.kernel.org, Martin KaFai Lau , Kumar Kartikeya Dwivedi , Tejun Heo Subject: Re: [PATCH v2 02/23] bpf: initial support for attaching struct ops to cgroups In-Reply-To: (Martin KaFai Lau's message of "Thu, 30 Oct 2025 15:42:12 -0700") References: <20251027231727.472628-1-roman.gushchin@linux.dev> <20251027231727.472628-3-roman.gushchin@linux.dev> <87zf98xq20.fsf@linux.dev> <877bwcus3h.fsf@linux.dev> Date: Thu, 30 Oct 2025 16:14:59 -0700 Message-ID: <87bjloj824.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; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Migadu-Flow: FLOW_OUT Martin KaFai Lau writes: > On 10/30/25 2:34 PM, Song Liu wrote: >> Hi Roman, >> On Thu, Oct 30, 2025 at 12:07=E2=80=AFPM Roman Gushchin >> wrote: >> [...] >>>> In TCP congestion control and BPF qdisc's model: >>>> >>>> During link_create, both adds the struct_ops to a list, and the >>>> struct_ops can be indexed by name. The struct_ops are not "active" by >>>> this time. >>>> Then, each has their own interface to 'apply' the struct_ops to a >>>> socket or queue: setsockopt() or netlink. >>>> >>>> But maybe cgroup-related struct_ops are different. >>> >>> Both tcp congestion and qdisk cases are somewhat different because >>> there already is a way to select between multiple implementations, bpf >>> just adds another one. In the oom case, it's not true. As of today, >>> there is only one (global) oom killer. Of course we can create >>> interfaces to allow a user make a choice. But the question is do we want >>> to create such interface for the oom case specifically (and later for >>> each new case separately), or there is a place for some generalization? >> Agreed that this approach requires a separate mechanism to attach >> the struct_ops to an entity. >>=20 >>> Ok, let me summarize the options we discussed here: >> Thanks for the summary! >>=20 >>> >>> 1) Make the attachment details (e.g. cgroup_id) the part of struct ops >>> itself. The attachment is happening at the reg() time. >>> >>> +: It's convenient for complex stateful struct ops'es, because a >>> single entity represents a combination of code and data. >>> -: No way to attach a single struct ops to multiple entities. >>> >>> This approach is used by Tejun for per-cgroup sched_ext prototype. >>> >>> 2) Make the attachment details a part of bpf_link creation. The >>> attachment is still happening at the reg() time. >>> >>> +: A single struct ops can be attached to multiple entities. >>> -: Implementing stateful struct ops'es is harder and requires passing >>> an additional argument (some sort of "self") to all callbacks. >>> I'm using this approach in the bpf oom proposal. >>> >> I think both 1) and 2) have the following issue. With cgroup_id in >> struct_ops or the link, the cgroup_id works more like a filter. The >> cgroup doesn't hold any reference to the struct_ops. The bpf link >> holds the reference to the struct_ops, so we need to keep the >> the link alive, either by keeping an active fd, or by pinning the >> link to bpffs. When the cgroup is removed, we need to clean up >> the bpf link separately. > > The link can be detached (struct_ops's unreg) by the user space. > > The link can also be detached from the subsystem (cgroup) here. > It was requested by scx: > https://lore.kernel.org/all/20240530065946.979330-7-thinker.li@gmail.com/ > > Not sure if scx has started using it. > >>=20 >>> 3) Move the attachment out of .reg() scope entirely. reg() will register >>> the implementation system-wide and then some 3rd-party interface >>> (e.g. cgroupfs) should be used to select the implementation. >>> >>> +: ? >>> -: New hard-coded interfaces might be required to enable bpf-driven >>> kernel customization. The "attachment" code is not shared between >>> various struct ops cases. >>> Implementing stateful struct ops'es is harder and requires passing >>> an additional argument (some sort of "self") to all callbacks. >>> >>> This approach works well for cases when there is already a selection >>> of implementations (e.g. tcp congestion mechanisms), and bpf is adding >>> another one. >> Another benefit of 3) is that it allows loading an OOM controller in >> a >> kernel module, just like loading a file system in a kernel module. This >> is possible with 3) because we paid the cost of adding a new select >> attach interface. >> A semi-separate topic, option 2) enables attaching a BPF program >> to a kernel object (a cgroup here, but could be something else). This >> is an interesting idea, and we may find it useful in other cases (attach >> a BPF program to a task_struct, etc.). Yep, task_struct is an attractive target for something like mm-related policies (THP, NUMA, memory tiers etc). > > Does it have plan for a pure kernel module oom implementation? I highly doubt. > I think the link-to-cgrp support here does not necessary stop the > later write to cgroupfs support if a kernel module oom is indeed needed > in the future. > > imo, cgroup-bpf has a eco-system around it, so it is sort of special. bpf= user > has expectation on how a bpf prog is attached to a cgroup. The introspect= ion, > auto detachment from the cgroup when the link is gone...etc. > > If link-to-cgrp is used, I prefer (2). Stay with one way to attach > to a cgrp. It is also consistent with the current way of attaching a sing= le > bpf prog to a cgroup. It is now attaching a map/set of bpf prog to a cgro= up. > The individual struct_ops implementation can decide if it should > allow a struct_ops be attached multiple times. +1