linux-kernel - Re: [PATCH v5 0/3] Handle update hardware queues and queue freeze more carefully

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Date:   Fri, 20 Aug 2021 13:55:21 +0200
From:   Daniel Wagner <dwagner@...e.de>
To:     linux-nvme@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org,
        James Smart <james.smart@...adcom.com>,
        Keith Busch <kbusch@...nel.org>,
        Ming Lei <ming.lei@...hat.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Hannes Reinecke <hare@...e.de>,
        Wen Xiong <wenxiong@...ibm.com>,
        Himanshu Madhani <himanshu.madhani@...cle.com>
Subject: Re: [PATCH v5 0/3] Handle update hardware queues and queue freeze
 more carefully

On Fri, Aug 20, 2021 at 10:48:32AM +0200, Daniel Wagner wrote:
> Then we try to do the same thing again which fails, thus we never
> make progress.
> 
> So clearly we need to update number of queues at one point. What would
> be the right thing to do here? As I understood we need to be careful
> with frozen requests. Can we abort them (is this even possible in this
> state?) and requeue them before we update the queue numbers?

After starring a bit longer at the reset path, I think there is no
pending request in any queue. nvme_fc_delete_association() calls
__nvme_fc_abort_outstanding_ios() which makes sure all queues are
drained (usage counter is 0). Also it clears the NVME_FC_Q_LIVE bit,
which prevents further request added to queues.

I start wonder why we have to do the nvme_start_freeze() in the first
place and why we want to wait for the freeze. 88e837ed0f1f ("nvme-fc:
wait for queues to freeze before calling update_hr_hw_queues") doesn't
really tell why we need wait for the freeze.

Given we know the usage counter of the queues is 0, I think we are
safe to move the blk_mq_update_nr_hw_queues() before the start queue
code. Also note nvme_fc_create_hw_io_queues() calls
blk_mq_freeze_queue() but it wont block as we are sure there is no
pending request.