[PATCH v1] blk-mq: add one blk_mq_req_flags_t type to support mq ctx fallback

[Ming Lei]

On Sun, Oct 20, 2024 at 10:40:41PM +0800, zhuxiaohui wrote:
> From: Zhu Xiaohui <zhuxiaohui.400 at bytedance.com>
> 
> 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.


Thanks,
Ming