From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-119.freemail.mail.aliyun.com (out30-119.freemail.mail.aliyun.com [115.124.30.119]) (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 309D825B30D for ; Thu, 30 Apr 2026 01:23:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.119 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777512231; cv=none; b=ulglC4ADKCJUBr66kqLBMCpepfyFIOZihutHyOOCdKrRSotRKpBsTPgfCj8//0RAoffvFWCjy/48xUXAqDUhTyzhTIzyYFsNKCtv2F7MFgI3c38MoMJXrH3SAsOfTle+ucaqkPtPcq3Ybqf4mo74miFXY+HOe6GcWKLun7YPuSc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777512231; c=relaxed/simple; bh=baPJ3x4XSiYrEcwrN9IRT9ft3qeWH5j1uTypQHd28+A=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=K2eCVbUw4lgMblJD81p5/IMwhWDzyrw3HwkR41enRwDaiX12Nd/t4jGcRfGiNbXo0nBAfwgL6AEdIqOx53Kxe77fWu43fXlwNg02LS/j2oVF4N5RkDUmLBKBQ/qQur+GfIsfumrMRVX2NdCh+JdO1WGUeOhFBaY1s21hrKwVDHo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=ROMd2Hzs; arc=none smtp.client-ip=115.124.30.119 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="ROMd2Hzs" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1777512221; h=From:To:Subject:Date:Message-ID:MIME-Version:Content-Type; bh=1hGcplYVwQABxYlLF+wndCBgicnUwEbPp4jkVLwNR6Q=; b=ROMd2Hzs5rvU2/uO7mcU/jChA/XWH5J2tlLnvBsJHPUO4nV+2FLdlQijN6Y8UwlE9ov9UcPgNBLIvp8el56o0XYqwFwtyovMYxBGpF7UW0hUIk0YP4CoOE3DS42Q1efUZajkwNmjenTUS/X6i9sBZy6HlFjqhPM+3IC/x7qMNCM= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R101e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033037009110;MF=ying.huang@linux.alibaba.com;NM=1;PH=DS;RN=40;SR=0;TI=SMTPD_---0X1zKUK7_1777512216; Received: from DESKTOP-5N7EMDA(mailfrom:ying.huang@linux.alibaba.com fp:SMTPD_---0X1zKUK7_1777512216 cluster:ay36) by smtp.aliyun-inc.com; Thu, 30 Apr 2026 09:23:37 +0800 From: "Huang, Ying" To: "Garg, Shivank" Cc: akpm@linux-foundation.org, david@kernel.org, lorenzo.stoakes@oracle.com, 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, apopple@nvidia.com, dave@stgolabs.net, Jonathan.Cameron@huawei.com, rkodsara@amd.com, vkoul@kernel.org, bharata@amd.com, sj@kernel.org, weixugc@google.com, dan.j.williams@intel.com, 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, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Mike Day Subject: Re: [RFC PATCH v4 4/6] mm/migrate: add copy offload registration infrastructure In-Reply-To: <4652f10e-1993-4151-a0f2-1d1623de79f7@amd.com> (Shivank Garg's message of "Tue, 28 Apr 2026 17:40:53 +0530") References: <20260309120725.308854-3-shivankg@amd.com> <20260309120725.308854-12-shivankg@amd.com> <87o6kdzeei.fsf@DESKTOP-5N7EMDA> <70414c31-d928-41a6-89db-eeb0f34ec07d@amd.com> <4652f10e-1993-4151-a0f2-1d1623de79f7@amd.com> Date: Thu, 30 Apr 2026 09:23:36 +0800 Message-ID: <875x59w89z.fsf@DESKTOP-5N7EMDA> User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=ascii "Garg, Shivank" writes: > On 4/3/2026 4:41 PM, Garg, Shivank wrote: >> >> >> On 3/24/2026 4:24 PM, Huang, Ying wrote: >>> Shivank Garg writes: >>> >>>> Introduce CONFIG_MIGRATION_COPY_OFFLOAD, which lets offload driver >>> >>> Do we really need a new kconfig option? IMHO, we have too many now. >>> Because we have a jump label already, the performance difference should >>> be trivial. Can you measure the size difference? >> >> BASELINE (offload=n) >> text data bss dec filename >> 23577 1632 32 25241 mm/migrate.o >> 39202900 14159750 6502152 59864802 vmlinux >> >> WITH OFFLOAD (offload=y) >> text data bss dec filename >> 24444 2568 32 27044 mm/migrate.o >> 676 64 8 748 mm/migrate_copy_offload.o >> 39208218 14163942 6498120 59870280 vmlinux >> >> WITHOUT CONFIG (always-on) >> text data bss dec filename >> 24444 2568 32 27044 mm/migrate.o >> 676 64 8 748 mm/migrate_copy_offload.o >> 39208405 14163942 6498120 59870467 vmlinux >> >> It saves around 5.5KB of size, when offload support is disabled. >> Is it meaningful savings? What do you think? >> >>> >>>> +#ifdef CONFIG_MIGRATION_COPY_OFFLOAD >>>> +extern struct static_key_false migrate_offload_enabled; >>>> +extern struct srcu_struct migrate_offload_srcu; >>>> +bool migrate_should_batch_default(int reason); >>>> +int migrate_offload_start(struct migrator *m); >>>> +int migrate_offload_stop(struct migrator *m); >>> >>> Why not naming the function migrate_offload_register/unregister()? >>> IMHO, that sounds more natural. >> >> Ack. I'll rename to migrate_offload_register/unregister(). >> >>> >>>> >>>> +#ifdef CONFIG_MIGRATION_COPY_OFFLOAD >>>> + /* Check if the offload driver wants to batch for this reason */ >>>> + if (static_branch_unlikely(&migrate_offload_enabled)) >>>> + do_batch = static_call(migrate_should_batch)(reason); >>> >>> Should batching based on "reason" be determined by the general migrate >>> code instead of the migrator implementation? For example, if we only >>> batch copying for ASYNC migration, we should determine that in >>> migrate_pages_batch() instead of the migreation implementation. Or am I >>> missed something? If so, can you provide an example? >>> >> >> My idea was that different drivers may have different cost/benefit >> profiles(e.g. setup cost, migrate batch-size, etc..) >> >> For instance, a DMA driver may want to target only bulk migration usecase. >> And a CPU-thread based driver can be used more broadly, without worrying >> about setup-costs. >> >> But I agree it's premature with only one-driver. >> I'll move the reason check with target usecases into migrate_pages_batch() >> and drop the should_batch() callback. If a future driver needs different >> filtering, we can add it back then. >> >>>> >>>> +#ifdef CONFIG_MIGRATION_COPY_OFFLOAD >>>> /* Batch-copy eligible folios before the move phase */ >>>> if (!list_empty(&src_batch)) { >>> >>> Guard with "static_branch_unlikely(&migrate_offload_enabled)" first? >>> Better to define a inline function to shorten the expression. >>> >> >> Sure, will add the static_branch_unlikely guard and wrap in a helper >> function. Thanks. > > Coming back to this while reworking the patch. > I think the static branch guard here is actually redundant. We already check it at > the per-folio classification that builds src_batch, > so when offload is disabled src_batch stays empty and the list_empty() check > short-circuits. I'll still wrap the SRCU + static_call into a helper at this call > site, as you suggested. Sorry for the flip-flop. Yes. It's functionally redundant. I just want to know whether it can benefit performance (in a minor way). --- Best Regards, Huang, Ying