[PATCH 1/2] blk-mq: not deactivate hctx if the device doesn't use managed irq

[John Garry]

On 29/06/2021 08:49, Ming Lei wrote:
> hctx is deactivated when all CPU in hctx->cpumask become offline by
> draining all requests originated from this hctx and moving new
> allocation to active hctx. This way is for avoiding inflight IO when
> the managed irq is shutdown.
> 
> Some drivers(nvme fc, rdma, tcp, loop) doesn't use managed irq, so
> they needn't to deactivate hctx. Also, they are the only user of
> blk_mq_alloc_request_hctx() which is used for connecting io queue.
> And their requirement is that the connect request can be submitted
> via one specified hctx on which all CPU in its hctx->cpumask may have
> become offline.
> 
> Address the requirement for nvme fc/rdma/loop, so the reported kernel
> panic on the following line in blk_mq_alloc_request_hctx() can be fixed.
> 
> 	data.ctx = __blk_mq_get_ctx(q, cpu)
> 
> Cc: Sagi Grimberg <sagi at grimberg.me>
> Cc: Daniel Wagner <dwagner at suse.de>
> Cc: Wen Xiong <wenxiong at us.ibm.com>
> Cc: John Garry <john.garry at huawei.com>
> Signed-off-by: Ming Lei <ming.lei at redhat.com>
> ---
>   block/blk-mq.c         | 6 +++++-
>   include/linux/blk-mq.h | 1 +
>   2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index df5dc3b756f5..74632f50d969 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -494,7 +494,7 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
>   	data.hctx = q->queue_hw_ctx[hctx_idx];
>   	if (!blk_mq_hw_queue_mapped(data.hctx))
>   		goto out_queue_exit;
> -	cpu = cpumask_first_and(data.hctx->cpumask, cpu_online_mask);
> +	cpu = cpumask_first(data.hctx->cpumask);
>   	data.ctx = __blk_mq_get_ctx(q, cpu);
>   
>   	if (!q->elevator)
> @@ -2570,6 +2570,10 @@ static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
>   	    !blk_mq_last_cpu_in_hctx(cpu, hctx))
>   		return 0;
>   
> +	/* Controller doesn't use managed IRQ, no need to deactivate hctx */
> +	if (hctx->flags & BLK_MQ_F_NOT_USE_MANAGED_IRQ)
> +		return 0;
Is there anything to be gained in registering the CPU hotplug handler 
for the hctx in this case at all?

> +
>   	/*
>   	 * Prevent new request from being allocated on the current hctx.
>   	 *
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 21140132a30d..600c5dd1a069 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -403,6 +403,7 @@ enum {
>   	 */
>   	BLK_MQ_F_STACKING	= 1 << 2,
>   	BLK_MQ_F_TAG_HCTX_SHARED = 1 << 3,
> +	BLK_MQ_F_NOT_USE_MANAGED_IRQ = 1 << 4,

Many block drivers don't use managed interrupts - to be proper, why not 
set this everywhere (which doesn't use managed interrupts)? I know why, 
but it's odd.

As an alternative, if the default queue mapping was used (in 
blk_mq_map_queues()), then that's the same thing as 
BLK_MQ_F_NOT_USE_MANAGED_IRQ in reality, right? If so, could we 
alternatively check for that somehow?

Thanks,
John

>   	BLK_MQ_F_BLOCKING	= 1 << 5,
>   	BLK_MQ_F_NO_SCHED	= 1 << 6,
>   	BLK_MQ_F_ALLOC_POLICY_START_BIT = 8,
>