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 A34B63FB069; Mon, 11 May 2026 14:06:49 +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=1778508409; cv=none; b=kYE/gTMrycbG4gZOODmJoQxUMtrtWutNaieXPKYHhbAj7PEwGcrZ7OU7lDNtmg2g1RtK1CMfPpkq43IB2TB/W+nMBuo2zx2qJNW+LA51jmv1KSwfnhd5DtsJ2oLy3vhxKDSc8o0UpJ978bwXSKS8iqQk1dHUQLT/JedFOViVSTE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778508409; c=relaxed/simple; bh=6h69zeVI2LyhrFnt9fHZYf3Cs3ft+CqnX6W7eig9MNs=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=aMXXNEK3CcGGKsHgiY/C2i2A0yGgr73X6O1Uz11HQadyGp4cgdaik5xRdLFb+fSeCknyn1PlspIXg0Pzj5az6w7+UN//pImDBMCDMWkpUiEjnjnRqjhNF1pb1TlK2cSlb1QqZAkYIU8WyUcei9eiyEn4M/75HE/G/AwCYja/Qk0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mywdzTL+; 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="mywdzTL+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BF516C2BCB0; Mon, 11 May 2026 14:06:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778508409; bh=6h69zeVI2LyhrFnt9fHZYf3Cs3ft+CqnX6W7eig9MNs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=mywdzTL+czEm1AXzppL/6JUl+x83xTrdygiPs/M+z9hINXlDLPZZ4AcF2VZA0E3Vx rrcuIcrHpmEAAzUsUKnM/etzJVsgvyiLPB535ikFVhEXB5mujnyhPu3CibxoyPcBb3 2+sK7mEC5xj+NphtYHuPvjyDO+4hUzaivrU2L6v/G2MgOON4t9bukA9QJFpIstO47l t0xQYE/wdXmIMSYGFl/I5palFSsjZYEbD696ru6RyGXqysvb2Fbfm3S6XUsFwoFFp3 LIt/Uo/5w+BIb8ULYoC1ZESjljjB6xKOHuZ/pHBlammO7rbsrIXMdYBafwZM0y2MMS H+EwHjsvDcEGA== Date: Mon, 11 May 2026 16:06:42 +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-caravan-behaupten-0402c454c22d@brauner> References: <20260511-dontcache-v7-0-2848ddce8090@kernel.org> <20260511-dontcache-v7-3-2848ddce8090@kernel.org> <20260511-zusieht-amputation-efe8b5058cb7@brauner> <7c0880ee25b13f64f71319203fcd7105f54e5ad0.camel@kernel.org> Precedence: bulk X-Mailing-List: linux-nfs@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: <7c0880ee25b13f64f71319203fcd7105f54e5ad0.camel@kernel.org> On Mon, May 11, 2026 at 09:53:21AM -0400, Jeff Layton wrote: > On Mon, 2026-05-11 at 15:24 +0200, Christian Brauner wrote: > > 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? > > > > Yes. > > > > + 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? > > > > That does look much cleaner. Do you want to just make that change or > would you rather I resend? I'll just fold it. I already have 1157 mails. I don't need more. :D