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 07218350D7D; Fri, 1 May 2026 09:50:24 +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=1777629025; cv=none; b=lJ/BNo7Vl/edWeVg8/VIOg1it9/3qNl5ozRU/GeGrG09IcoYK1ItyGixpcasToyroXtDEDz1bWI8DYmubIoFke5Rwmpg8LhcgBhuaF/DJvoKzD87HTyRHed2mS0BjugzFaWeYPoAxZbQ19ivUbhtdufyuZFIEeikeKep5SCeo9A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777629025; c=relaxed/simple; bh=yQTt3liEqaBCXR2rlPWa8sEqJC3iU4De0vNfeKvDjTo=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=VzVqXK9GeHU3bUVzr2mCnPSWLw1+7Lvxz+JEkfbe4LL0ts75MnLebXLYe31gdpBl7i/iAz272329IjlMqbmmvxoThsa/gRev/bj7x5PzgOU4bAnZwBfzp9NFCzrVlAdcNiudUjQJGBbSOcaNwV+l0WY+u7lP6/pYFZStcjwjLcE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=G+nS8Pjb; 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="G+nS8Pjb" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 44FFFC2BCB4; Fri, 1 May 2026 09:50:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777629024; bh=yQTt3liEqaBCXR2rlPWa8sEqJC3iU4De0vNfeKvDjTo=; h=From:Date:Subject:References:In-Reply-To:To:Cc:From; b=G+nS8Pjbnmo9t/IZ57mzwBlpCRQmSurm/m1WdYyt4lnMR1H1j+7xVvyCTXLeeLbuZ cLRKLgklu+q86CKFnx3bd4R5BFOWCXb0JIOI6MAKLU1CW+//W8nAUSZT9VJy8lb6zQ ShcnX/VMJ0dzb1Cx5NHJ4LgRUIYeYT0r/amFEA17PzUtVhBaOTXKzIhBFeBUlvb9oY RF9n+TJ687D/9O7xYFGvTtcccjbeljDpZbT8XA0MW8W0N5vQ5F44W8UugIXIZbM6iR 4HqcP2jqt06D+oJ22nJNuHbbkcPqASO7nZtZ6TJQOb6D/l2aBUtHdtxmonuqfD+kqe EwnQOWE5UDTxw== From: Jeff Layton Date: Fri, 01 May 2026 10:49:36 +0100 Subject: [PATCH v4 2/4] 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: <20260501-dontcache-v4-2-5d5e6dc71cb3@kernel.org> References: <20260501-dontcache-v4-0-5d5e6dc71cb3@kernel.org> In-Reply-To: <20260501-dontcache-v4-0-5d5e6dc71cb3@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=8810; i=jlayton@kernel.org; h=from:subject:message-id; bh=yQTt3liEqaBCXR2rlPWa8sEqJC3iU4De0vNfeKvDjTo=; b=owEBbQKS/ZANAwAKAQAOaEEZVoIVAcsmYgBp9HdQEPnjarLJF089nZsNrHY1wCEnRipW9HnmX Ak9A1pY4MyJAjMEAAEKAB0WIQRLwNeyRHGyoYTq9dMADmhBGVaCFQUCafR3UAAKCRAADmhBGVaC FX3TD/9FjtZLOba+69gJe2qy6mDT7cmra2LtxLO/aVBzPzXDnPDrSmcdfM9UEygP/7hJX8zOsKH LzpeSPO1qYPIrhzqKiCv5pEtFoiPjHQFiJkl5Q4nkV5W1VFQ+ixBabI0TilzTu7Rkh0YqlObh8Q NUybJwTdLdmGzoiaZjucNOT0G92X6i+08nnVAAndvKOSNzsEIimkSOO5TnZBhoQl0SN4SSxCXiG pLtGCL6+s69HOVeJs/RKgiB6oaGuIzJXgsWFCU6wlD1/l1vXI+X5RlC8KYnH/ZeW5FK3Bb/Tczi KtLMSdDcJtB+SeNaiOmNQIU+T8+wLOG7YO1XSKi2itGHz2aLH9bsU8qFZz6DKTg7bwRI9TAc197 TNkU1dBpvTKgLw4RqjWlbAsI0awNASKAXoV8D5Ik7iDlWZR8Grn6o8ApebcrnkU867kqV/GUtlN B4IEZEqkcQXwkvUHQtMNaoRByWkl1nOFjfOrncCJhwxyKkIj1zCd+P7WI6tehiJocXZyJBchJ+L LQQlTPWUH4Dz6ASuI08fX9p3hupvNnwtaMhD4UEIwvOoMtQWIzyaA2zcTY2blc0j/xoWxFG04rM W8xJqJD59HoP3WZ9AwC+iHWNHKBASOEizv0rBSozfXzfWnjnuNZxhQrn69YJ4FX5/x5qYwmoYdk ACBMGMVwOGQLDuA== 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. 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(). 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 | 60 ++++++++++++++++++++++++++++++++++++++++ include/linux/backing-dev-defs.h | 2 ++ include/linux/fs.h | 6 ++-- include/trace/events/writeback.h | 3 +- 4 files changed, 66 insertions(+), 5 deletions(-) diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index a65694cbfe68..b06a51fb5d6c 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -1334,6 +1334,18 @@ static void wb_start_writeback(struct bdi_writeback *wb, enum wb_reason reason) wb_wakeup(wb); } +static void wb_start_dontcache_writeback(struct bdi_writeback *wb) +{ + if (!wb_has_dirty_io(wb)) + return; + + if (test_bit(WB_start_dontcache, &wb->state) || + test_and_set_bit(WB_start_dontcache, &wb->state)) + return; + + wb_wakeup(wb); +} + /** * wb_start_background_writeback - start background writeback * @wb: bdi_writback to write from @@ -2373,6 +2385,28 @@ 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_bit(WB_start_dontcache, &wb->state)) + return 0; + + nr_pages = wb_stat(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); + } + + clear_bit(WB_start_dontcache, &wb->state); + return nr_pages; +} /* * Retrieve work items and do the writeback they describe @@ -2394,6 +2428,11 @@ 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 +2507,27 @@ 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. 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. + */ +void filemap_dontcache_kick_writeback(struct address_space *mapping) +{ + struct inode *inode = mapping->host; + struct bdi_writeback *wb; + struct wb_lock_cookie cookie = {}; + + wb = unlocked_inode_to_wb_begin(inode, &cookie); + wb_start_dontcache_writeback(wb); + unlocked_inode_to_wb_end(inode, &cookie); +} +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