On 21/10/2024 4:39, Ming Lei wrote:
On Sun, Oct 20, 2024 at 10:40:41PM +0800, zhuxiaohui wrote:From: Zhu Xiaohui <zhuxiaohui.400@xxxxxxxxxxxxx> It is observed that nvme connect to a nvme over fabric target will always fail when 'nohz_full' is set. In commit a46c27026da1 ("blk-mq: don't schedule block kworker on isolated CPUs"), it clears hctx->cpumask for all isolate CPUs, and when nvme connect to a remote target, it may fails on this stack: blk_mq_alloc_request_hctx+1 __nvme_submit_sync_cmd+106 nvmf_connect_io_queue+181 nvme_tcp_start_queue+293 nvme_tcp_setup_ctrl+948 nvme_tcp_create_ctrl+735 nvmf_dev_write+532 vfs_write+237 ksys_write+107 do_syscall_64+128 entry_SYSCALL_64_after_hwframe+118 due to that the given blk_mq_hw_ctx->cpumask is cleared with no available blk_mq_ctx on the hw queue. This patch introduce a new blk_mq_req_flags_t flag 'BLK_MQ_REQ_ARB_MQ' as well as a nvme_submit_flags_t 'NVME_SUBMIT_ARB_MQ' which are used to indicate that block layer can fallback to a blk_mq_ctx whose cpu is not isolated.blk_mq_alloc_request_hctx() ... cpu = cpumask_first_and(data.hctx->cpumask, cpu_online_mask); ... It can happen in case of non-cpu-isolation too, such as when this hctx hasn't online CPUs, both are same actually from this viewpoint. It is one long-time problem for nvme fc.
For what nvmf is using blk_mq_alloc_request_hctx() is not important. It just needs a tag from that hctx. the request execution is running where blk_mq_alloc_request_hctx() is running.
