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b=uBaLnPHFc6DANj63Li1yXgzWRaRREr8lo1iYH2B7TRwybVCvqRulnEw7SbKE1H9guuCQg8 iu0FcmQZcqJ3toCg== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id 6529D1339F; Tue, 10 Nov 2026 13:09:50 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id rXH7D54X82oqUgAAD6G6ig (envelope-from ); Tue, 10 Nov 2026 13:09:50 +0000 Message-ID: Date: Tue, 10 Nov 2026 14:09:49 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 2/2] nvme-tcp: allow setting per-queue io_cpu through sysfs To: Saravanan D , linux-nvme@lists.infradead.org Cc: kbusch@kernel.org, hch@lst.de, sagi@grimberg.me, axboe@kernel.dk, nilay@linux.ibm.com, dwagner@suse.de, sgogte@purestorage.com, hare@kernel.org, linux-kernel@vger.kernel.org, iyamahata@crusoe.ai, kiyer@crusoe.ai, ganbalagane@crusoe.ai, sjpark@crusoe.ai, bchen@crusoe.ai References: <394e47d486979175bc5ebb94c789db8bfacb1736.1794015622.git.saravanand@crusoe.ai> Content-Language: en-US From: Hannes Reinecke In-Reply-To: <394e47d486979175bc5ebb94c789db8bfacb1736.1794015622.git.saravanand@crusoe.ai> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Spamd-Result: default: False [-13.50 / 50.00]; SUSE_INTERNAL_BYPASS(-9.00)[]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; XM_UA_NO_VERSION(0.01)[]; MX_GOOD(-0.01)[]; RCPT_COUNT_TWELVE(0.00)[16]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; MIME_TRACE(0.00)[0:+]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; RCVD_TLS_ALL(0.00)[]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[grimberg.me:email,crusoe.ai:email,imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo,suse.de:email,suse.de:dkim]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; DKIM_TRACE(0.00)[suse.de:+] X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Rspamd-Action: no action X-Spam-Flag: NO X-Spam-Score: -13.50 X-Spam-Level: X-Rspamd-Queue-Id: B906221E39 On 11/7/26 10:53 AM, Saravanan D wrote: > nvme_tcp_set_queue_io_cpu() picks each queue's io_cpu at connect time > as the least loaded CPU in the queue's blk-mq map group, and all socket > work runs there for the connection's lifetime. This decision falls > short when the host partitions its CPUs after connect time. On a 384 > cpu multi tenant host with 128 queue controllers, blk-mq folds three > CPUs into every map group, some groups straddle two tenants' cpusets, > and 9% of nvme_tcp_io_work executions ran outside the submitting VM's > cpuset, seen by the neighbor as steal time. > > Expose the io_cpu in each I/O queue's directory > > /sys/class/nvme/nvmeX/queues//io_cpu > > so a control plane that owns CPU placement can set it directly instead > of relying on the driver's heuristic. The attribute accepts a decimal > cpu number, -1 or "unbound". A user assignment, unbound included, is > marked in the queue flags, persists across reconnects and is never > reselected by the driver. The read-only managed attribute reports 1 > while io_cpu is driver managed and 0 once the user assigned it. The > accounting of nvme_tcp_cpu_queues applies regardless of who set the > io_cpu. > > A queue's directory is added once the connect time selection has set its > io_cpu, so readers never see a placement that is about to change, and > removed when the queue is freed, before it is torn down, so a sysfs > store never runs on a freed queue. All io_cpu writes, from > sysfs and from the driver, serialize with the accounting under > nvme_tcp_cpu_queues_lock, while the I/O paths read io_cpu locklessly. > The queue stop now clears NVME_TCP_Q_IO_CPU_SET under that lock, since a > sysfs store may move io_cpu in between. > > nvme_tcp_free_queue() now clears NVME_TCP_Q_IO_CPU_SET and drops its > count, because a queue can still be counted when it is freed. A sysfs > store may have run after the queue stop, or the connect may have > failed, and the setup error path frees such a queue without calling > nvme_tcp_stop_queue(), where the count is normally dropped. > > Suggested-by: Sagi Grimberg > Link: https://lore.kernel.org/linux-nvme/13c160d0-0657-47ce-9a16-58d348267ab0@grimberg.me/ > Assisted-by: Claude:claude-opus-5-5 [Claude Code] > Signed-off-by: Saravanan D > --- > Documentation/ABI/stable/sysfs-nvme | 31 ++++++ > drivers/nvme/host/tcp.c | 167 ++++++++++++++++++++++++---- > 2 files changed, 178 insertions(+), 20 deletions(-) > Reviewed-by: Hannes Reinecke Cheers, Hannes -- Dr. Hannes Reinecke Kernel Storage Architect hare@suse.de +49 911 74053 688 SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich