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

[Sagi Grimberg]

On 21/10/2024 11:31, Ming Lei wrote:
> On Mon, Oct 21, 2024 at 10:05:34AM +0300, Sagi Grimberg wrote:
>>
>>
>> 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 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.
>> 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.
> I am afraid that just one tag from the specified hw queue isn't enough.
>
> The connection request needs to be issued to the hw queue & completed.
> Without any online CPU for this hw queue, the request can't be completed
> in case of managed-irq.

None of the consumers of this API use managed-irqs. the networking stack
takes care of steering irq vectors to online cpus.