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Fri, 01 May 2026 09:44:13 -0700 (PDT) Message-ID: Date: Fri, 1 May 2026 10:44:11 -0600 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 2/4] mm: kick writeback flusher for IOCB_DONTCACHE with targeted dirty tracking To: Jeff Layton , 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 , Ritesh Harjani , Chuck Lever Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org References: <20260501-dontcache-v4-0-5d5e6dc71cb3@kernel.org> <20260501-dontcache-v4-2-5d5e6dc71cb3@kernel.org> Content-Language: en-US From: Jens Axboe In-Reply-To: <20260501-dontcache-v4-2-5d5e6dc71cb3@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 1D7B61A0016 X-Stat-Signature: oqpmrxzee3ntd1ykkpqifkcz5sgc9zfw X-Rspam-User: X-HE-Tag: 1777653854-294738 X-HE-Meta: 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 UjzFkPym mgiWFC5wQ9g5gnimnC2KKrJM5/Tm0hSslG93OAKPjdxTru5IT44PbSTZScyEJa8DZfF1XkXqFlrexpHkpuFl+5FYYKPDvp5uqI2BqdaZh7mECiLtlGNhdZzX1OwiwvOPPsnRp1Hmv/GgAyXavJWIuSHVdLcK2ACbyac+4ZwbrNLAzAS+YQzuYX9qlmPbRkvKHlBKmfPt2sBXDsZBI2NYpyWSVfG/A17GPqjCjO4Xf0lZGP/FK/8GOv+TtWQmiXfn27IV9kCO+nNwdjFs0XzXoInVs+/jKFF0MtJcb/ejl7cQk+DDOnfPfqW0k8YPMmWEH6TvHDMcduq2pTzkNvu/y/5gICOzV6KV7zxlOuxgGZqTxNSOem7D1QOAKr+29+skwlfOeViq9IhgEa5swyTWUWwdyUJz89U7gUx/5TUL9KL3RajwPwpkEvo6lZTC0vmdBTDkwJjvmMgYylbhG+I8/TA1pIcxdOn4PbpFEbxOKtFMxdSdU01KDs0TDLSqLmq5GvedEib1YezKM8XSa89diqOrcCw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/1/26 3:49 AM, 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. > > 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. I like this, this is the better way to kick off the writeback. Reviewed-by: Jens Axboe -- Jens Axboe