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Sun, 9 Aug 2026 10:08:36 +0000 (GMT) From: Nilay Shroff To: linux-nvme@lists.infradead.org Cc: hare@suse.de, kbusch@kernel.org, hch@lst.de, sagi@grimberg.me, dwagner@suse.de, kanie@linux.alibaba.com, jmeneghi@redhat.com, randyj@purestorage.com, martin.petersen@oracle.com, john.g.garry@oracle.com, gjoyce@linux.ibm.com, Nilay Shroff Subject: [PATCH v7 0/9] nvme-multipath: introduce latency I/O policy Date: Sun, 9 Aug 2026 15:37:55 +0530 Message-ID: <20260809100825.2014133-1-nilay@linux.ibm.com> X-Mailer: git-send-email 2.53.0 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-GUID: aazBeIYW6pD8lqbiInMEm6D1q7g5CHEL X-Authority-Analysis: v=2.4 cv=PbDPQChd c=1 sm=1 tr=0 ts=6a7851ad cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=IkcTkHD0fZMA:10 a=Sv0fKeRqtYgA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=iQ6ETzBq9ecOQQE5vZCe:22 a=VwQbUJbxAAAA:8 a=VnNF1IyMAAAA:8 a=_zs8TyAAHTrT5RJFmqkA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwODA5MDA4OCBTYWx0ZWRfX5ZlmK4xwIhzV bTVSxOItYTXqPeaGiB6HAqzZV7ckCCEub+JjgK9T2DhVZIWrGgx3C0X77GKI6snQP/T+sk5Coqw AlxpxDl1FI/5W+fqlEHLoaYfS7qdk8EOtNlcNuS7H0CXl8KMBAmumJlo9usqJrposoSEmdxusXz YfxejsHGXL0054T4WXh8B88lrnE0xsPERKId9cwrrx0KGimY/IPiZz8r2s0wmY3Wg49R15cdLvs 3Fdi/Wgcvqus3H8BVbx1PuicUBH/peiR6VSiyalVUyY4nGph6IrqlSO4bdwL1wJyo8yHwkPlnC1 epJjH8IgZ6T144Fv20bM9800WYGRk52inWXMbtXF9g5kbXGmAhRP6njfdbr4KojTTLLlcPwRwHK Z2rFtAAZzwyQBShnk36ITMTuwHPbPiaWGo6c6nyuDuZhVdgklJ/eOo44OY7ISRr0sL+GF0xwH7j xU//2dw2NkJgzVCI8gQ== X-Proofpoint-ORIG-GUID: c-FELOvxissycWb3Jx4_RMHY__UPaxsT X-Proofpoint-Spam-Info: AW1haW4tMjYwODA5MDA4OCBTYWx0ZWRfX7S9UF0UXFDce +qKBj5EffxKwXQ/TWYGOkiVVycDK8ItJq2m6yr4l2f/isAuimWtZM1ac/n4DV0sbylYFUbANSCj tRGJfEyjVcX8RM+5y2N19R14hXD8kTE= X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-08-09_03,2026-08-07_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 bulkscore=0 impostorscore=0 malwarescore=0 adultscore=0 clxscore=1011 priorityscore=1501 suspectscore=0 phishscore=0 lowpriorityscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2608090088 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260809_030904_185175_54CE07E6 X-CRM114-Status: GOOD ( 25.31 ) X-BeenThere: linux-nvme@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Linux-nvme" Errors-To: linux-nvme-bounces+linux-nvme=archiver.kernel.org@lists.infradead.org Hi, This series introduces a new latency I/O policy for NVMe native multipath. Existing policies such as numa, round-robin, and queue-depth are static and do not adapt to real-time transport performance. The numa selects the path closest to the NUMA node of the current CPU, optimizing memory and path locality, but ignores actual path performance. The round-robin distributes I/O evenly across all paths, providing fairness but not performance awareness. The queue-depth reacts to instantaneous queue occupancy, avoiding heavily loaded paths, but does not account for actual latency, throughput, or link speed. The new latency policy addresses these gaps selecting paths dynamically based on measured I/O latency for both PCIe and fabrics. Latency is derived by passively sampling I/O completions. Each path is assigned a weight proportional to its latency score, and I/Os are then forwarded accordingly. As condition changes (e.g. latency spikes, bandwidth differences), path weights are updated, automatically steering traffic toward better-performing paths. Early results show reduced tail latency under mixed workloads and improved throughput by exploiting higher-speed links more effectively. For example, with NVMf/TCP using two paths (one throttled with ~30 ms delay), fio results with random read/write/rw workloads (direct I/O) showed: numa round-robin queue-depth adaptive ----------- ----------- ----------- --------- READ: 50.0 MiB/s 105 MiB/s 230 MiB/s 350 MiB/s WRITE: 65.9 MiB/s 125 MiB/s 385 MiB/s 446 MiB/s RW: R:30.6 MiB/s R:56.5 MiB/s R:122 MiB/s R:175 MiB/s W:30.7 MiB/s W:56.5 MiB/s W:122 MiB/s W:175 MiB/s This pathcset includes totla 8 patches: [PATCH 1/9] block: expose blk_stat_{enable,disable}_accounting() - Make blk_stat APIs available to block drivers. - Needed for per-path latency measurement. [PATCH 2/9] block: record I/O request start time for passthru request - Record I/O start time for I/O passthru requests - This is prep patch which allows measuring I/O completion latency for passthru requests [PATCH 3/9] nvme-multipath: pass I/O type to nvme_find_path() - This is the prep patch which updates nvme_find_path() signature [PATCH 4/9] nvme-multipath: add latency I/O policy - Implement path scoring based on latency (EWMA). - Distribute I/O proportionally to per-path weights. [PATCH 5/9] nvme: add generic debugfs support - Introduce generic debugfs support for NVMe module [PATCH 6/9] nvme-multipath: add debugfs attribute latency_ewma_shift - Adds a debugfs attribute to control ewma shift [PATCH 7/9] nvme-multipath: add debugfs attribute latency_batch_timeout - Adds a debugfs attribute to control latency batch window interval [PATCH 8/9] nvme-multipath: add debugfs attribute latency_stat - Add “latency_stat” under per-path and head debugfs directories to expose latency policy state and statistics. [PATCH 9/9] nvme-multipath: add documentation for latency I/O policy - Includes documentation for latency I/O multipath policy. LSFMM discussion: ================= During lsfmm 2026, it was decided to rename this I/O policy from "adaptive" to "latency". This series reflects that rename. The discussion at lsfmm also focused extensively on the latency measurement model, including whether latency should be tracked per-CPU or per-NUMA, and whether separate I/O-size buckets should be maintained for different request sizes. After detailed discussion and evaluation of throughput results, the consensus was to initially measure I/O completion latency on a per-CPU basis. The available performance data showed that the per-CPU implementation already provides sufficient averaging across CPUs while keeping the design relatively simple. The use of additional I/O-size buckets did not demonstrate meaningful throughput improvement in the general case and would introduce extra complexity into the fast path and accounting logic. As a result, the consensus was to avoid I/O-size bucketing for now and keep the policy focused on per-CPU latency measurement. If future real-world workloads demonstrate a clear benefit from I/O-size-aware latency accounting, the policy can be extended later to support it. As ususal, feedback and suggestions are most welcome! Thanks! Changes from v6: - Add patch 2/9, which records I/O start time for the passthru requests (Guixin Liu) - Correctly record the passthru I/O data direction (Guixin Liu) - Clear NVME_NS_PATH_STAT before cancelling the latency weight work. (Guixin Liu) Link to v6: https://lore.kernel.org/all/20260520182112.863076-1-nilay@linux.ibm.com/ Changes from v5: - Rename the policy from "adaptive" to "latency". The entire series updates policy names, function names, and variable names accordingly, without introducing any functional changes. (lsfmm discussion) - The second patch is now splitted into two patches: Patch #2: prep patch where we pass op_type to nvme_find_path() Patch #3: core patch which introduces latency I/O policy (Sagi) - Rename ewma_update() to calc_ewma_update() (Sagi) Link to v5: https://lore.kernel.org/all/20251105103347.86059-1-nilay@linux.ibm.com/ Changes from v4: - Added patch #7 which includes the documentation for adaptive I/O policy. (Guixin Liu) Link to v4: https://lore.kernel.org/all/20251104104533.138481-1-nilay@linux.ibm.com/ Changes from v3: - Update the adaptive APIs name (which actually enable/disable adaptive policy) to reflect the actual work it does. Also removed the misleading use of "current_path" from the adaptive policy code (Hannes Reinecke) - Move adaptive_ewma_shift and adaptive_weight_timeout attributes from sysfs to debugfs (Hannes Reinecke) Link to v3: https://lore.kernel.org/all/20251027092949.961287-1-nilay@linux.ibm.com/ Changes from v2: - Addede a new patch to allow user to configure EWMA shift through sysfs (Hannes Reinecke) - Added a new patch to allow user to configure path weight calculation timeout (Hannes Reinecke) - Distinguish between read/write and other commands (e.g. admin comamnd) and calculate path weight for other commands which is separate from read/write weight. (Hannes Reinecke) - Normalize per-path weight in the range from 0-128 instead of 0-100 (Hannes Reinecke) - Restructure and optimize adaptive I/O forwarding code to use one loop instead of two (Hannes Reinecke) Link to v2: https://lore.kernel.org/all/20251009100608.1699550-1-nilay@linux.ibm.com/ Changes from v1: - Ensure that the completion of I/O occurs on the same CPU as the submitting I/O CPU (Hannes Reinecke) - Remove adapter link speed from the path weight calculation (Hannes Reinecke) - Add adaptive I/O stat under debugfs instead of current sysfs (Hannes Reinecke) - Move path weight calculation to a workqueue from IO completion code path Link to v1: https://lore.kernel.org/all/20250921111234.863853-1-nilay@linux.ibm.com/ Nilay Shroff (9): block: expose blk_stat_{enable,disable}_accounting() to drivers block: record I/O request start time for passthru request nvme-multipath: pass I/O type to nvme_find_path() nvme-multipath: add support for latency I/O policy nvme: add generic debugfs support nvme-multipath: add debugfs attribute latency_ewma_shift nvme-multipath: add debugfs attribute latency_batch_timeout nvme-multipath: add debugfs attribute latency_stat nvme-multipath: add documentation for latency I/O policy Documentation/admin-guide/nvme-multipath.rst | 19 + block/blk-mq.c | 12 +- block/blk-stat.h | 4 - drivers/nvme/host/Makefile | 2 +- drivers/nvme/host/core.c | 18 +- drivers/nvme/host/debugfs.c | 345 ++++++++++++++ drivers/nvme/host/ioctl.c | 38 +- drivers/nvme/host/multipath.c | 460 ++++++++++++++++++- drivers/nvme/host/nvme.h | 105 ++++- drivers/nvme/host/pr.c | 6 +- drivers/nvme/host/sysfs.c | 2 +- include/linux/blk-mq.h | 4 + 12 files changed, 979 insertions(+), 36 deletions(-) create mode 100644 drivers/nvme/host/debugfs.c -- 2.53.0