From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lgeamrelo07.lge.com (lgeamrelo07.lge.com [156.147.51.103]) (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 89804257AF2 for ; Fri, 13 Jun 2025 06:56:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=156.147.51.103 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749797777; cv=none; b=L1mG0vVjpuBEDTNn83yGvkKQ1Mh6FiQdpg2zsfYgsr8f7zI2JXHWfhTYHw10pwzpFFetFxrxQrQbzc3XekvuV0Vt7fh/gXA4Gf+WSOk2gVFAYzR2PBDvmskcRkVMlWBbPiK2PwNiO397KE0JQss+vb3++wfySkZwet5qkyySyAc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749797777; c=relaxed/simple; bh=i4du00HGA9zb0ohT1cC/b0VFC6Vw65Lig03aEVDZji8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=N5fMzo5lPOBvD27FUZnJz0LDGHPrbUgqkzRdb4emaWrka0lFdnxNbyUSHTEtpRSjEPiYSLofn/QlyGcVcurE/tj7bZhBNANBG7AEytKqiACqVaTAXP37GOo5bFn+w/SxH940E9vqHBqi2LTQ/K5yWbWBDUby5oF8tpAy5yTzjUY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=lge.com; spf=pass smtp.mailfrom=lge.com; arc=none smtp.client-ip=156.147.51.103 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=lge.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=lge.com Received: from unknown (HELO yjaykim-PowerEdge-T330) (10.177.112.156) by 156.147.51.103 with ESMTP; 13 Jun 2025 15:56:12 +0900 X-Original-SENDERIP: 10.177.112.156 X-Original-MAILFROM: youngjun.park@lge.com Date: Fri, 13 Jun 2025 15:56:12 +0900 From: YoungJun Park To: Kairui Song Cc: linux-mm@kvack.org, akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, shikemeng@huaweicloud.com, nphamcs@gmail.com, bhe@redhat.com, baohua@kernel.org, chrisl@kernel.org, muchun.song@linux.dev, iamjoonsoo.kim@lge.com, taejoon.song@lge.com, gunho.lee@lge.com Subject: Re: [RFC PATCH 0/2] mm/swap, memcg: Support per-cgroup swap device prioritization Message-ID: References: <20250612103743.3385842-1-youngjun.park@lge.com> 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: On Thu, Jun 12, 2025 at 08:24:08PM +0800, Kairui Song wrote: > On Thu, Jun 12, 2025 at 6:38 PM wrote: > > > > From: Youngjun Park > > > > Introduction > > ============ > > I am a kernel developer working on platforms deployed on commercial consumer devices. > > Due to real-world product requirements, needed to modify the Linux kernel to support > > a new swap management mechanism. The proposed mechanism allows assigning different swap > > priorities to swap devices per cgroup. > > I believe this mechanism can be generally useful for similar constrained-device scenarios > > and would like to propose it for upstream inclusion and solicit feedback from the community. > > > > Motivation > > ========== > > Core requirement was to improve application responsiveness and loading time, especially > > for latency critical applications, without increasing RAM or storage hardware resources. > > Device constraints: > > - Linux-based embedded platform > > - Limited system RAM > > - Small local swap > > - No option to expand RAM or local swap > > To mitigate this, we explored utilizing idle RAM and storage from nearby devices as remote > > swap space. To maximize its effectiveness, we needed the ability to control which swap devices > > were used by different cgroups: > > - Assign faster local swap devices to latency critical apps > > - Assign remote swap devices to background apps > > However, current Linux kernel swap infrastructure does not support per-cgroup swap device > > assignment. > > To solve this, I propose a mechanism to allow each cgroup to specify its own swap device > > priorities. > > > > Evaluated Alternatives > > ====================== > > 1. **Per-cgroup dedicated swap devices** > > - Previously proposed upstream [1] > > - Challenges in managing global vs per-cgroup swap state > > - Difficult to integrate with existing memory.limit / swap.max semantics > > 2. **Multi-backend swap device with cgroup-aware routing** > > - Considered sort of layering violation (block device cgroup awareness) > > - Swap devices are commonly meant to be physical block devices. > > - Similar idea mentioned in [2] > > 3. **Per-cgroup swap device enable/disable with swap usage contorl** > > - Expand swap.max with zswap.writeback usage > > - Discussed in context of zswap writeback [3] > > - Cannot express arbitrary priority orderings > > (e.g. swap priority A-B-C on cgroup C-A-B impossible) > > - Less flexible than per-device priority approach > > 4. **Per-namespace swap priority configuration** > > - In short, make swap namespace for swap device priority > > - Overly complex for our use case > > - Cgroups are the natural scope for this mechanism > > > > Based on these findings, we chose to prototype per-cgroup swap priority configuration > > as the most natural, least invasive extension of the existing kernel mechanisms. > > > > Design and Semantics > > ==================== > > - Each swap device gets a unique ID at `swapon` time > > - Each cgroup has a `memory.swap.priority` interface: > > - Show unique ID by memory.swap.priority interface > > - Format: `unique_id:priority,unique_id:priority,...` > > - All currently-active swap devices must be listed > > - Priorities follow existing swap infrastructure semantics > > - The interface is writeable and updatable at runtime > > - A priority configuration can be reset via `echo "" > memory.swap.priority` > > - Swap on/off events propagate to all cgroups with priority configurations > > > > Example Usage > > ------------- > > # swap device on > > $ swapon > > NAME TYPE SIZE USED PRIO > > /dev/sdb partition 300M 0B 10 > > /dev/sdc partition 300M 0B 5 > > > > # assign custom priorities in a cgroup > > $ echo "1:5,2:10" > memory.swap.priority > > $ cat memory.swap.priority > > Active > > /dev/sdb unique:1 prio:5 > > /dev/sdc unique:2 prio:10 > > > > # adding new swap device later > > $ swapon /dev/sdd --priority -1 > > $ cat memory.swap.priority > > Active > > /dev/sdb unique:1 prio:5 > > /dev/sdc unique:2 prio:10 > > /dev/sdd unique:3 prio:-2 > > > > # reset cgroup priority > > $ echo "" > memory.swap.priority > > $ cat memory.swap.priority > > Inactive > > /dev/sdb unique:1 prio:10 > > /dev/sdc unique:2 prio:5 > > /dev/sdd unique:3 prio:-2 > > > > Implementation Notes > > ==================== > > The items mentioned below are to be considered during the next patch work. > > > > - Workaround using per swap cpu cluster as before > > - Priority propgation of child cgroup > > - And other TODO, XXX > > - Refactoring for reviewability and maintainability, comprehensive testing > > and performance evaluation > > Hi Youngjun, > > Interesting idea. For your current approach, I think all we need is > per-cgroup swap meta info structures (and infrastures for maintaining > and manipulating them). > > So we have a global version and a cgroup version of "plist, next > cluster list, and maybe something else", right? And then > once the allocator is folio aware it can just prefer the cgroup ones > (as I mentioned in another reply) reusing all the same other > routines. Changes are minimal, the cgroup swap meta infos > and control plane are separately maintained. > > It seems aligned quite well with what I wanted to do, and can be done > in a clean and easy to maintain way. > > Meanwhile with virtual swap, things could be even more flexible, not > only changing the priority at swapout time, it will also provide > capabilities to migrate and balance devices adaptively, and solve long > term issues like mTHP fragmentation and min-order swapout etc.. > > Maybe they can be combined, like maybe cgroup can be limited to use > the virtual device or physical ones depending on priority. Seems all > solvable. Just some ideas here. I had been thinking about the work related to vswap and alignment, so I'm glad to hear that they can harmonize. > Vswap can cover the priority part too. I think we might want to avoid > duplicated interfaces. > > So I'm just imagining things now, will it be good if we have something > like (following your design): > > $ cat memcg1/memory.swap.priority > Active > /dev/vswap:(zram/zswap? with compression params?) unique:0 prio:5 > > $ cat memcg2/memory.swap.priority > Active > /dev/vswap:/dev/nvme1 unique:1 prio:5 > /dev/vswap:/dev/nvme2 unique:2 prio:10 > /dev/vswap:/dev/vda unique:3 prio:15 > /dev/sda unique:4 prio:20 > > $ cat memcg3/memory.swap.priority > Active > /dev/vda unique:3 prio:5 > /dev/sda unique:4 prio:15 > > Meaning memcg1 (high priority) is allowed to use compressed memory > only through vswap, and memcg2 (mid priority) uses disks through vswap > and fallback to HDD. memcg3 (low prio) is only allowed to use slow > devices. > > Global fallback just uses everything the system has. It might be over > complex though? Just looking at the example usage which you mention, it seems flexible and good. I will think more about this in relation to it.