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 82A421F5847; Tue, 5 May 2026 19:00:20 +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=1778007620; cv=none; b=jPbMdNNThAsJUQbyFu56UKPQ8iQwJayW/OUoVVHX1fYVhstDrQqMUPIKadKli8KtB52/dJ6+/jvAIcthhxNEQaKBwxeKsqJGunBuc+JIh0KchzRR+nshu7u78MWdxe2ZQPnvbgDn5AJ1SLDZbZkEAvrbXG9fwVoIxZC25uVLs0s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778007620; c=relaxed/simple; bh=/OcYNCE/fDC5kwoNxmn93OOTbnEEIB7Un/u2ZoKZZmM=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=MS9MDYoVIjIVP861RhZ+D8rxnu2A9elUYC7XC7YQFzr9NDkgU9i1zw4MnVrSYPeHTZK1hha2kBnuvOrsj8PCf7NPu4i7v5sWyTgTigbt0MR5BXiQQ7JES/wz2Xjln8wiyqSFXiaO6MWMk4XfOrb3MaMQIO06pgVPO62rPPNiihE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Pw/HlPtL; 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="Pw/HlPtL" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C0E64C2BCB4; Tue, 5 May 2026 19:00:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778007620; bh=/OcYNCE/fDC5kwoNxmn93OOTbnEEIB7Un/u2ZoKZZmM=; h=From:Date:Subject:References:In-Reply-To:To:Cc:From; b=Pw/HlPtLi9QZuSy0R/GnGVx/M088vN+is++AsVyK+HkM65BpmXKrjG1+FtVEufNbF 1JTwPCDtTtFASeItu0D/qS/p7Hrmo5kPk8V12o7x+VtPlFlstl0AoU1oma0wToRPKE qvaLSBSW7sZQltZopaacSX5nIxkIsQBYz1vltvTlMljHo4qJzJZK5ps13ccTFCnyWu voQfWk/C0S4JQJRq0Vn56aWoQxcFPCikGxFRGRzIJojN3iKBreYa0PeZRgr3y9nngK 3YZcffK8BS+CHZ/ln1P9+7SFkSpmhTQdnT7HP6WwLXTuPzVlsPl8sbwh8BqXTmk2Mo wYkZ3nLfCV/lw== From: Jeff Layton Date: Tue, 05 May 2026 20:59:49 +0200 Subject: [PATCH v6 2/2] mm: kick writeback flusher for IOCB_DONTCACHE with targeted dirty tracking 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="utf-8" Content-Transfer-Encoding: 8bit Message-Id: <20260505-dontcache-v6-2-66463805dd6a@kernel.org> References: <20260505-dontcache-v6-0-66463805dd6a@kernel.org> In-Reply-To: <20260505-dontcache-v6-0-66463805dd6a@kernel.org> To: Alexander Viro , Christian Brauner , Jan Kara , "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 Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, Jeff Layton X-Mailer: b4 0.14.3 X-Developer-Signature: v=1; a=openpgp-sha256; l=9615; i=jlayton@kernel.org; h=from:subject:message-id; bh=/OcYNCE/fDC5kwoNxmn93OOTbnEEIB7Un/u2ZoKZZmM=; b=owEBbQKS/ZANAwAKAQAOaEEZVoIVAcsmYgBp+j4zmzZ7nDbB+fjQU1X+H+KbEaew/Rb2QF6q/ oHNf3dZOuWJAjMEAAEKAB0WIQRLwNeyRHGyoYTq9dMADmhBGVaCFQUCafo+MwAKCRAADmhBGVaC FaZgEACq12/XKwbO+w71Vhf3mCc0WqNQSjceQThHmpkBQv9ZlF8uO/C7lf/v7idibv62hAJPRCC H3Uj4IauQdId4XWPj9SxBYQQASNuK+0qILP0XkR0YChV4RZvLPDPuT0ty4BKyyd3G76icATGdZB nKc0pdzr/YNo8RK5OI1iOpnlxM+l2jiiI182RtkmnatMQZZ5gfPqNwMSwayO16kMambplXFRMEp We13egbqmz7ayZO+RAnbdcpGbqU4doJq9GLZQCK8QNulUwxdmGsjgnB/TRwRA19/XBN1g3L4mSu ektu96BSyIIIJ1eWDHCfvNjO83FIlQ3Pal9oPTzrCm8aAUe3cLxiwlVX+YtIJE0smP5JO8x+auv iPd8j59q1eton3j/vUG/kuICFmTVHdYguxpBJ7BkbPwOMWPS9RQvj42+TzxwiJCQXqNPloimRKl JHWttx/Y0OjA4pypoGH5K8+n7MaFckzIv+S7T8rJXs85UMHci/TnuI6soZn4B7q7ypnxV164bXv ZEwwmjaXSgFoR3L/KZ9d/dQXbjr8weRraFl55IX+qcmQ4Anw1DuJqRSaYS8I57lor9WkA+u8yth TkogGYAY87faDji9bazKU3HLXprV5pSq9Agpn5Rtw5N+u/oQCY2cN9eM3X26fsP116+XoTxAF0x 79lm31kEMtSuNcQ== X-Developer-Key: i=jlayton@kernel.org; a=openpgp; fpr=4BC0D7B24471B2A184EAF5D3000E684119568215 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 — walking the page tree and submitting I/O blocks the writer for milliseconds, inflating p99.9 latency from 23ms (buffered) to 93ms (dontcache). 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. 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 dirty inodes (not restricted to dontcache-specific inodes). This helps preserve I/O batching while limiting the scope of expedited writeback. Like WB_start_all, the WB_start_dontcache bit coalesces multiple DONTCACHE writes into a single flusher wakeup without per-write allocations. Use test_and_clear_bit to atomically consume the kick request before reading the dirty counter and starting writeback, so that concurrent DONTCACHE writes during writeback can re-set the bit and schedule a follow-up flusher run. Read the dirty counter with wb_stat_sum() (aggregating per-CPU batches) rather than wb_stat() (which reads only the global counter) to ensure small writes below the percpu batch threshold are visible to the flusher. In filemap_dontcache_kick_writeback(), set the WB_start_dontcache bit inside the unlocked_inode_to_wb_begin/end section for correct cgroup writeback domain targeting, but defer the wb_wakeup() call until after the section ends, since wb_wakeup() uses spin_unlock_irq() which would unconditionally re-enable interrupts while the i_pages xa_lock may still be held under irqsave during a cgroup writeback switch. Also add WB_REASON_DONTCACHE as a new writeback reason for tracing visibility. dontcache-bench results (same host, T6F_SKL_1920GBF, 251 GiB RAM, xfs on NVMe, fio io_uring): Buffered and direct I/O paths are unaffected by this patchset. All improvements are confined to the dontcache path: Single-stream throughput (MB/s): Before After Change seq-write/dontcache 298 897 +201% rand-write/dontcache 131 236 +80% 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%) 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% Dontcache multi-writer throughput now matches buffered (4,532 vs 4,616 MB/s). 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% Dontcache now reaches 81% of buffered throughput (was 35%). 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 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. 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. 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. Assisted-by: Claude:claude-opus-4-6 Signed-off-by: Jeff Layton --- fs/fs-writeback.c | 59 ++++++++++++++++++++++++++++++++++++++++ include/linux/backing-dev-defs.h | 2 ++ include/linux/fs.h | 6 ++-- include/trace/events/writeback.h | 3 +- 4 files changed, 65 insertions(+), 5 deletions(-) diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index a65694cbfe68..faf15cf84c68 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -2373,6 +2373,27 @@ static long wb_check_start_all(struct bdi_writeback *wb) return nr_pages; } +static long wb_check_start_dontcache(struct bdi_writeback *wb) +{ + long nr_pages; + + if (!test_and_clear_bit(WB_start_dontcache, &wb->state)) + return 0; + + nr_pages = wb_stat_sum(wb, WB_DONTCACHE_DIRTY); + if (nr_pages) { + struct wb_writeback_work work = { + .nr_pages = nr_pages, + .sync_mode = WB_SYNC_NONE, + .range_cyclic = 1, + .reason = WB_REASON_DONTCACHE, + }; + + nr_pages = wb_writeback(wb, &work); + } + + return nr_pages; +} /* * Retrieve work items and do the writeback they describe @@ -2394,6 +2415,11 @@ static long wb_do_writeback(struct bdi_writeback *wb) */ wrote += wb_check_start_all(wb); + /* + * Check for dontcache writeback request + */ + wrote += wb_check_start_dontcache(wb); + /* * Check for periodic writeback, kupdated() style */ @@ -2468,6 +2494,39 @@ void wakeup_flusher_threads_bdi(struct backing_dev_info *bdi, rcu_read_unlock(); } +/** + * 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 dontcache dirty + * pages. Queue writeback for the inode's wb for as many pages as there are + * dontcache pages, but don't restrict writeback to dontcache pages only. + * + * This significantly improves performance 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. Over longer term dontcache pages get written and reclaimed by + * background writeback even with this rough heuristic. + */ +void filemap_dontcache_kick_writeback(struct address_space *mapping) +{ + struct inode *inode = mapping->host; + struct bdi_writeback *wb; + struct wb_lock_cookie cookie = {}; + bool need_wakeup = false; + + wb = unlocked_inode_to_wb_begin(inode, &cookie); + if (wb_has_dirty_io(wb) && + !test_bit(WB_start_dontcache, &wb->state) && + !test_and_set_bit(WB_start_dontcache, &wb->state)) + need_wakeup = true; + unlocked_inode_to_wb_end(inode, &cookie); + + if (need_wakeup) + wb_wakeup(wb); +} +EXPORT_SYMBOL_GPL(filemap_dontcache_kick_writeback); + /* * Wakeup the flusher threads to start writeback of all currently dirty pages */ diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h index cb660dd37286..4f1084937315 100644 --- a/include/linux/backing-dev-defs.h +++ b/include/linux/backing-dev-defs.h @@ -26,6 +26,7 @@ enum wb_state { WB_writeback_running, /* Writeback is in progress */ WB_has_dirty_io, /* Dirty inodes on ->b_{dirty|io|more_io} */ WB_start_all, /* nr_pages == 0 (all) work pending */ + WB_start_dontcache, /* dontcache writeback pending */ }; enum wb_stat_item { @@ -56,6 +57,7 @@ enum wb_reason { */ WB_REASON_FORKER_THREAD, WB_REASON_FOREIGN_FLUSH, + WB_REASON_DONTCACHE, WB_REASON_MAX, }; diff --git a/include/linux/fs.h b/include/linux/fs.h index 11559c513dfb..df72b42a9e9b 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -2624,6 +2624,7 @@ extern int __must_check file_write_and_wait_range(struct file *file, loff_t start, loff_t end); int filemap_flush_range(struct address_space *mapping, loff_t start, loff_t end); +void filemap_dontcache_kick_writeback(struct address_space *mapping); static inline int file_write_and_wait(struct file *file) { @@ -2657,10 +2658,7 @@ static inline ssize_t generic_write_sync(struct kiocb *iocb, ssize_t count) if (ret) return ret; } else if (iocb->ki_flags & IOCB_DONTCACHE) { - struct address_space *mapping = iocb->ki_filp->f_mapping; - - filemap_flush_range(mapping, iocb->ki_pos - count, - iocb->ki_pos - 1); + filemap_dontcache_kick_writeback(iocb->ki_filp->f_mapping); } return count; diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h index bdac0d685a98..13ee076ccd16 100644 --- a/include/trace/events/writeback.h +++ b/include/trace/events/writeback.h @@ -44,7 +44,8 @@ EM( WB_REASON_PERIODIC, "periodic") \ EM( WB_REASON_FS_FREE_SPACE, "fs_free_space") \ EM( WB_REASON_FORKER_THREAD, "forker_thread") \ - EMe(WB_REASON_FOREIGN_FLUSH, "foreign_flush") + EM( WB_REASON_FOREIGN_FLUSH, "foreign_flush") \ + EMe(WB_REASON_DONTCACHE, "dontcache") WB_WORK_REASON -- 2.54.0