From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 3688F40628E for ; Mon, 11 May 2026 15:53:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778514815; cv=none; b=Lp2jDYXkSR53ZLH1PW+M/S9flyW7uLAACtvsaadj0p6jA9Kk5wKhu4d0F4zkRE2FX8h9oIRS37NFydbt/mJj4SzttLQhXYbNHGWQY6S7vDfoonSrCrh4X4WZfccxZZ5XesEhiT0QJ8tHQ61FlADjwL8D/gXqYsDZOJyGWGIqzLs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778514815; c=relaxed/simple; bh=SnAkfE24ca2jtyTZMDfreShDIAAPWeFO+kOTNCDv/TA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=FyjxP91EO5Ld1UdkbCJWlmG7FMRrFXxL0Y2aBX8SGwGZBcoNra7+zh5p5W0tVTjNfXdonDFAcmgFSShuvLAzLEayMec6lpeHhY3X/WKkL0+zLWLP0Wmmjx2/2S8GT4gljMFouYdAxclZveOlF+cFvD/7DBzTwnsj6erHwTCP2yI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=F8DWqg7r; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="F8DWqg7r" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DA717C2BCB0; Mon, 11 May 2026 15:53:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778514814; bh=SnAkfE24ca2jtyTZMDfreShDIAAPWeFO+kOTNCDv/TA=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=F8DWqg7rI3nnQzIVpolksEhrFcuOfoRLRo4DwLE0wdYu7zfm7gJCefP9avj2VvDNY KVGYpZg9045/pEsr4UtSdGK5HavzTS+ibvxS6OYH8GHrcjbqSsHcjONr8mVjJ5KNAk vsBnzMpc8TXTbMAaxCLyz4huBftufwbddNfS1NWalEs281F9xsmuGfdsIHLZXP5/QJ qmxoX6x/AMggefDmFgmRtFWvIvG5Kwj+SjEswlLZJOkfMx191RoLwACg0h0yOJ7O9z T8xN9KiLgjbJP2Uo4tCHqIR+hrPNPrb1mDM5rSrTIadrIXFG8ddj/xc0hlbc9Vlz1f JANtLwqyhJdsg== Message-ID: <98a16642-35b7-4cf9-9ee2-8de15e877920@kernel.org> Date: Mon, 11 May 2026 17:53:24 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/7] Accelerate page migration with batch copying and hardware offload To: Shivank Garg , akpm@linux-foundation.org Cc: kinseyho@google.com, weixugc@google.com, ljs@kernel.org, Liam.Howlett@oracle.com, vbabka@kernel.org, willy@infradead.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, ziy@nvidia.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, dave@stgolabs.net, Jonathan.Cameron@huawei.com, rkodsara@amd.com, vkoul@kernel.org, bharata@amd.com, sj@kernel.org, rientjes@google.com, xuezhengchu@huawei.com, yiannis@zptcorp.com, dave.hansen@intel.com, hannes@cmpxchg.org, jhubbard@nvidia.com, peterx@redhat.com, riel@surriel.com, shakeel.butt@linux.dev, stalexan@redhat.com, tj@kernel.org, nifan.cxl@gmail.com, jic23@kernel.org, aneesh.kumar@kernel.org, nathan.lynch@amd.com, Frank.li@nxp.com, djbw@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20260428155043.39251-2-shivankg@amd.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/28/26 17:50, Shivank Garg wrote: > This is the fifth RFC of the patchset to enhance page migration by Ah, this is an RFC ... ... I suggest b4 for patch series management :P That also explains why patch #7 is still in there. > batching folio-copy operations and enabling acceleration via DMA offload. > > Single-threaded, folio-by-folio copying bottlenecks page migration in > modern systems with deep memory hierarchies, especially for large folios > where copy overhead dominates, leaving significant hardware potential > untapped. > > By batching the copy phase, we create an opportunity for hardware > acceleration. This series builds the framework and provides a DMA > offload driver (dcbm) as a reference implementation, targeting bulk > migration workloads where offloading the copy improves throughput > and latency while freeing the CPU cycles. > > See the RFC V3 cover letter [2] for motivation. > > Changelog since V4: > ------------------- > > 1. Renamed PAGE_* migration state flags to FOLIO_*. (David) > 2. Use the new folio->migrate_info field instead of folio->private > for migration state. (David) > 3. Fold folios_mc_copy patch in batch-copy implementation patch. (David) > 3. Renamed migrate_offload_start()/stop() to register()/unregister(). > (Huang, Ying) > 4. Dropped should_batch() callback from struct migrator. Reason-based > policy now lives in migrate_pages_batch(). Migrators can still skip > a batch they don't want (size based policy). (Huang, Ying) > 5. CONFIG_MIGRATION_COPY_OFFLOAD is now hidden and selected by the > migrator driver. CONFIG_DCBM_DMA is tristate. (Huang Ying, Gregory Price). > 6. Wrapped the SRCU + static_call dispatch in a small helper. (Huang, Ying) > 7. Requir m->owner in migrate_offload_register(), SRCU sync at > unregister relies on it. Counters are atomic_long_t to avoid lock-order > issue. > 9. Moved DCBM sysfs from /sys/kernel/dcbm to /sys/module/dcbm (Huang, Ying) > 10. Rebased on v7.1-rc1. > [...] > > OPEN QUESTIONS: > --------------- > > 1. Should the batch path run without a registered migrator? Patches 1-4 > are self-contained and use folios_mc_copy() (CPU). I have several > options like making batch path always-on for eligible folios, or > giving admin an option to flip the static branch, or keep the gate. > I'm leaning toward always-on. Hiding that detail from migrate.c sounds interesting. > > 2. Carrying already_copied via folio->migrate_info vs changing the > migrate_folio() callback signature (Huang, Ying). I went with the > field for now to avoid touching every fs callback before the design > settles. Happy to revisit. > > 3. Per-caller offload selection: Today eligibility is by migrate_reason > only. Some are latency-tolerant, others may be not. Is reason the > right granularity, or do we want a per-caller hint? Isn't it sufficient to just do it based on the #folios or sth like that? If someone migrates a handful of folios, latency is likely more important (and batching less beneficial). I'd assume when migrating many folios, batching could just always be done. Or what's the concern? > > 4. Cgroup integration: How should per-cgroup be accounted for different > migrators (e.g.: any accounting for DMA-busy time)? Oh. Do we even have to mess with that? > > 5. Tuning migrate_pages callers for offloading. For instance, in > compaction COMPACT_CLUSTER_MAX = 32 caps DMA's payoff for compaction > (V4 experiment). Is that HW dependent? > > 6. Where do batch-size thresholds live, and how are they tuned? Per > Huang Ying's split, that policy lives in the migrator. DCBM has no > threshold today. Open whether it should later be a per-migrator > sysfs knob or hard-coded; probably clearer once a second migrator > (SDXI, mtcopy) shows the trade-off. Again, sounds like being HW dependent, no? -- Cheers, David