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 0E7083E63A4; Mon, 11 May 2026 13:24:36 +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=1778505877; cv=none; b=sncTt8ejItkd+GtLrOakNJMWUBuviOjhhCEm+pjmMz9A13THI1geMYUVPgxUrp2FTZPX6/PUYnDPjD1Xbips9AM9gBJPhs8oDL8xvKNi06dMk8YqNaD/x8colZ5E0RgcrjoK5QqLA+0v36ithqlnKqBVYRjCvWmikBkiSQFBw+M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778505877; c=relaxed/simple; bh=45Jm/PhkxV5J5go2T8hXkU65Az6gpJJzXMVog7W1o6c=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=qlNzKEtBjgxw43lQS0tC2cCcvaw54N9YYUzGkcy/iX5fUmDvjxu8RVe8kBDOilZ4mhJfqBt9zzc7FQxePWQcykm279kVCVg2yHVYpTlg6VRSn955bODzfscN817qa/bHmuOBtk/4hl/0x7725kjxLrKagSSCiHDo2n6/sHJksXM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ozKRNnPS; 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="ozKRNnPS" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CF8D1C2BCB0; Mon, 11 May 2026 13:24:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778505876; bh=45Jm/PhkxV5J5go2T8hXkU65Az6gpJJzXMVog7W1o6c=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ozKRNnPSby6dVKr1OyOOJp34eF/PUQ8m2BzMnJ3UDjBp3qh6WIVAD0AfZSzokF8vv iusOSSnCgeqQu/uiPKo8dUf/W5HXUepK2cjSA+lvfAg4adXt4btHWV19kZzxFXc1iK 66oJSJK8g2l4uJSvQsHxw6ct7Ju1Ost5dAhb0KXLomY82vQncGzhZbPJETTHpNsRFC JNQ+iYrHR7KgIVBNUuwRlfKMhM6iPqFkz26EryuL8OruTNDtNRrhl8gFhCK0bVF71e W8erm/gQ+BE9YczlLPDqyQ+eL14u0K4PUpwPgXEHLe/G9H1jLbqW6wFqj0PTzOmaas k7mb4m0tAWahg== Date: Mon, 11 May 2026 15:24:29 +0200 From: Christian Brauner To: Jeff Layton Cc: Alexander Viro , 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 , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v7 3/3] mm: kick writeback flusher for IOCB_DONTCACHE with targeted dirty tracking Message-ID: <20260511-zusieht-amputation-efe8b5058cb7@brauner> References: <20260511-dontcache-v7-0-2848ddce8090@kernel.org> <20260511-dontcache-v7-3-2848ddce8090@kernel.org> 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-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260511-dontcache-v7-3-2848ddce8090@kernel.org> On Mon, May 11, 2026 at 07:58:29AM -0400, 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 — 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. Pin the wb with > wb_get() inside the RCU critical section before calling wb_wakeup() > outside it, since cgroup bdi_writeback structures are RCU-freed and the > wb pointer could become invalid after unlocked_inode_to_wb_end() drops > the RCU read lock. > > 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 | 63 ++++++++++++++++++++++++++++++++++++++++ > include/linux/backing-dev-defs.h | 2 ++ > include/linux/fs.h | 6 ++-- > include/trace/events/writeback.h | 3 +- > 4 files changed, 69 insertions(+), 5 deletions(-) > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > index 32ecc745f5f7..77d53df97cc3 100644 > --- a/fs/fs-writeback.c > +++ b/fs/fs-writeback.c > @@ -2377,6 +2377,27 @@ 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 > @@ -2398,6 +2419,11 @@ 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 > */ > @@ -2472,6 +2498,43 @@ 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)) { Doesn't test_and_set_bit() return the old value? IOW, if it sees that WB_start_dontcache was already set it'll return true? So you can remove the test_bit() call, right? > + wb_get(wb); > + need_wakeup = true; > + } Actually, I think you can rewrite this function quite a bit: > + unlocked_inode_to_wb_end(inode, &cookie); > + > + if (need_wakeup) { > + wb_wakeup(wb); > + wb_put(wb); > + } > +} > +EXPORT_SYMBOL_GPL(filemap_dontcache_kick_writeback); 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); if (wb_has_dirty_io(wb) && !test_and_set_bit(WB_start_dontcache, &wb->state)) wb_get(wb); else wb = NULL; unlocked_inode_to_wb_end(inode, &cookie); if (wb) { wb_wakeup(wb); wb_put(wb); } } No? > + > /* > * 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 >