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

From: Hannes Reinecke <hare@suse.de>
To: Ming Lei <ming.lei@redhat.com>, Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, linux-nvme@lists.infradead.org,
	Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagi@grimberg.me>, Daniel Wagner <dwagner@suse.de>,
	Wen Xiong <wenxiong@us.ibm.com>,
	John Garry <john.garry@huawei.com>
Subject: Re: [PATCH 1/2] blk-mq: not deactivate hctx if the device doesn't use managed irq
Date: Tue, 29 Jun 2021 14:39:14 +0200	[thread overview]
Message-ID: <1a14a397-6244-928e-5aaa-85c2ccbe0e40@suse.de> (raw)
In-Reply-To: <20210629074951.1981284-2-ming.lei@redhat.com>

On 6/29/21 9:49 AM, 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.
> 

How can you submit a connect request for a hctx on which all CPUs are 
offline? That hctx will be unusable as it'll never be able to receive 
interrupts ...

> 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@grimberg.me>
> Cc: Daniel Wagner <dwagner@suse.de>
> Cc: Wen Xiong <wenxiong@us.ibm.com>
> Cc: John Garry <john.garry@huawei.com>
> Signed-off-by: Ming Lei <ming.lei@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);

I don't get it.
Doesn't this allow us to allocate a request on a dead cpu, ie the very 
thing we try to prevent?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

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