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 6A9233630B9; Sun, 3 May 2026 18:41:33 +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=1777833693; cv=none; b=hA/hhw2ajb6qytzNLrYgdG3O6vc/o8n8MzHVBKhRKavrt91KYhtse/+xiXUjTkw71dimxems4jQihG9MTiYutlD59g2PfGv0+GSc8R1ej2xJU8oD1u1N//+4zg/PmVE6pM4P1OFRqLCb6oRNY+0WBmHpT+Wo1RtcUEx+/hpNSTg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777833693; c=relaxed/simple; bh=vgELmPGv0ejDx+rBC6rv02xJDSy6NaM+0xejX18ULhQ=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=OMTUu+jGDL+Bgu3xLDE/dPcwgjMPye49IXvkg0CXV1RGLStgEzYBQnKWPgISEjdOgkW7Laj/e00zO5GRS0TDjqkmqunArT0GC9msnij3Lucn4FKinWXCrzaSX7Eeuh7sas6eFA9QGSxLHGsDdyNeGrSxPNmWGb31NehppO7fg3E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=UbFRXW0j; 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="UbFRXW0j" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 98CA0C2BCB4; Sun, 3 May 2026 18:41:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777833693; bh=vgELmPGv0ejDx+rBC6rv02xJDSy6NaM+0xejX18ULhQ=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=UbFRXW0jwghfQEq7bGe33xnAR+3XoLtEJYQE2Dhzgz6JUuo2Ahocggd8xX+yTfigv DyWSXPq/HGQ8QUx3hjAy3YYZFmXjbL/b+XRGRh8/Ni8X7yPmHW7/sxCbdhffTTtEI0 NEtelalzlQZ61vvxklNdpnE/iL/4+hxmJ1P/1jWFz7RP1HKAieF5I5VRxhNfEWQnDp uL79Nvghs3eCHVGROr32/OOsv4PXH7fOVPlm4WqzhOpGb/ak7qwnkI3O5iElmrJ1XL marTVaD2y+UeYew1FUDatNo7PpU20MPOgSM29vtwc5WOp0urqpgzRT6K1Zx6vs+3OE /YxEqNbCri5fA== Message-ID: Subject: Re: [PATCH v4 2/4] mm: kick writeback flusher for IOCB_DONTCACHE with targeted dirty tracking From: Jeff Layton To: Jan Kara Cc: Alexander Viro , Christian Brauner , "Matthew Wilcox (Oracle)" , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Mike Snitzer , Jens Axboe , Ritesh Harjani , Chuck Lever , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org Date: Sun, 03 May 2026 20:41:24 +0200 In-Reply-To: References: <20260501-dontcache-v4-0-5d5e6dc71cb3@kernel.org> <20260501-dontcache-v4-2-5d5e6dc71cb3@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn 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DfX/sn9XcfoVZIxMpCRE750zvJvcCUz9HojsrMQ1NFc7MFT1z3MOW2/RlzPcog7xvR5ENPH19ojRD CHqumUHRry+RF0lH00clzX/W8OrQJZtoBPXv9ahka/Vp7kEulcBJr1cH5Wz/WprhsIM7U9pse1f1g Yy9YbXtWctUz8uvDR7shsQxAhX3qO7DilMtuGo1v97I/Kx4gXQ52syh/w6EBny71CZrOgD6kJwPVV AaM1LRC28muq91WCFhs/nzHozpbzcheyGtMUI2Ao4K6mnY+3zIuXPygZMFr9KXE6fF7HzKxKuZMJO aEZCiDOq0anx6FmOzs5E6Jqdpo/mtI8beK+BE7Va6ni7YrQlnT0i3vaTVMTiCThbqsB20VrbMjlhp f8lfK1XVNbRq/R7GZ9zHESlsa35ha60yd/j3pu5hT2xyy8krV8vGhHvnJ1XRMJBAB/UYb6FyC7S+m QZIQXVeAA+smfTT0tDrisj1U5x6ZB9b3nBg65kc= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.58.3 (3.58.3-1.fc43) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Sun, 2026-05-03 at 16:45 +0200, Jan Kara wrote: > On Fri 01-05-26 10:49:36, Jeff Layton wrote: > > The IOCB_DONTCACHE writeback path in generic_write_sync() calls > > filemap_flush_range() on every write, submitting writeback inline in > > the writer's context. Perf lock contention profiling shows the > > performance problem is not lock contention but the writeback submission > > work itself =E2=80=94 walking the page tree and submitting I/O blocks t= he writer > > for milliseconds, inflating p99.9 latency from 23ms (buffered) to 93ms > > (dontcache). > >=20 > > Replace the inline filemap_flush_range() call with a flusher kick that > > drains dirty pages in the background. This moves writeback submission > > completely off the writer's hot path. > >=20 > > To avoid flushing unrelated buffered dirty data, add a dedicated > > WB_start_dontcache bit and wb_check_start_dontcache() handler that uses > > the per-wb WB_DONTCACHE_DIRTY counter to determine how many pages to > > write back. The flusher writes back that many pages from the oldest di= rty > > inodes (not restricted to dontcache-specific inodes). This helps > > preserve I/O batching while limiting the scope of expedited writeback. > >=20 > > Like WB_start_all, the WB_start_dontcache bit coalesces multiple > > DONTCACHE writes into a single flusher wakeup without per-write > > allocations. > >=20 > > Also add WB_REASON_DONTCACHE as a new writeback reason for tracing > > visibility, and target the correct cgroup writeback domain via > > unlocked_inode_to_wb_begin(). > >=20 > > dontcache-bench results (same host, T6F_SKL_1920GBF, 251 GiB RAM, > > xfs on NVMe, fio io_uring): > >=20 > > Buffered and direct I/O paths are unaffected by this patchset. All > > improvements are confined to the dontcache path: > >=20 > > Single-stream throughput (MB/s): > > Before After Change > > seq-write/dontcache 298 897 +201% > > rand-write/dontcache 131 236 +80% > >=20 > > Tail latency improvements (seq-write/dontcache): > > p99: 135,266 us -> 23,986 us (-82%) > > p99.9: 8,925,479 us -> 28,443 us (-99.7%) > >=20 > > Multi-writer (4 jobs, sequential write): > > Before After Change > > dontcache aggregate (MB/s) 2,529 4,532 +79% > > dontcache p99 (us) 8,553 1,002 -88% > > dontcache p99.9 (us) 109,314 1,057 -99% > >=20 > > Dontcache multi-writer throughput now matches buffered (4,532 vs > > 4,616 MB/s). > >=20 > > 32-file write (Axboe test): > > Before After Change > > dontcache aggregate (MB/s) 1,548 3,499 +126% > > dontcache p99 (us) 10,170 602 -94% > > Peak dirty pages (MB) 1,837 213 -88% > >=20 > > Dontcache now reaches 81% of buffered throughput (was 35%). > >=20 > > Competing writers (dontcache vs buffered, separate files): > > Before After > > buffered writer 868 433 MB/s > > dontcache writer 415 433 MB/s > > Aggregate 1,284 866 MB/s > >=20 > > Previously the buffered writer starved the dontcache writer 2:1. > > With per-bdi_writeback tracking, both writers now receive equal > > bandwidth. The aggregate matches the buffered-vs-buffered baseline > > (863 MB/s), indicating fair sharing regardless of I/O mode. > >=20 > > The dontcache writer's p99.9 latency collapsed from 119 ms to > > 33 ms (-73%), eliminating the severe periodic stalls seen in the > > baseline. Both writers now share identical latency profiles, > > matching the buffered-vs-buffered pattern. > >=20 > > The per-bdi_writeback dirty tracking dramatically reduces peak dirty > > pages in dontcache workloads, with the 32-file test dropping from > > 1.8 GB to 213 MB. Dontcache sequential write throughput triples and > > multi-writer throughput reaches parity with buffered I/O, with tail > > latencies collapsing by 1-2 orders of magnitude. > >=20 > > Assisted-by: Claude:claude-opus-4-6 > > Signed-off-by: Jeff Layton >=20 > Nice and looks good to me now. Feel free to add: >=20 > Reviewed-by: Jan Kara >=20 > One nit below: >=20 > > +/** > > + * filemap_dontcache_kick_writeback - kick flusher for IOCB_DONTCACHE = writes > > + * @mapping: address_space that was just written to > > + * > > + * Kick the writeback flusher thread to expedite writeback of dontcach= e > > + * dirty pages. Uses a dedicated WB_start_dontcache bit so that only > > + * pages tracked by WB_DONTCACHE_DIRTY are written back, rather than > > + * flushing the entire BDI's dirty pages. >=20 > This comment is a bit confusing as in fact we write arbitrary dirty pages= . > It is only the amount of pages that is influenced by WB_DONTCACHE_DIRTY. = So > I'd rephrase the last sentence like: We queue writeback for the inode's w= b > for as many pages as there are dontcache pages but we don't restrict > writeback to dontcache pages only. This significantly improves performanc= e > over either writing all wb's pages or writing only dontcache pages. > Although it doesn't guarantee quick writeback and reclaim of dontcache > pages it keeps the amount of dirty pages in check and over longer term > dontcache pages get written and reclaimed by background writeback even wi= th > this rough heuristic. >=20 > Honza >=20 I'll add that. Thanks for the suggestion and review! --=20 Jeff Layton