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

[Sagi Grimberg]

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.