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Date:   Fri, 25 Jun 2021 21:00:25 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Daniel Wagner <dwagner@suse.de>
Cc:     linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org,
        James Smart <james.smart@broadcom.com>,
        Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@fb.com>,
        Sagi Grimberg <sagi@grimberg.me>
Subject: Re: [PATCH 0/2] Handle update hardware queues and queue freeze more
 carefully
Message-ID: <YNXTaUMAFCA84jfZ@T590>
References: <20210625101649.49296-1-dwagner@suse.de>
 <20210625122156.x5yzoobuaaec5hss@beryllium.lan>
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On Fri, Jun 25, 2021 at 02:21:56PM +0200, Daniel Wagner wrote:
> On Fri, Jun 25, 2021 at 12:16:47PM +0200, Daniel Wagner wrote:
> > this is a followup on the crash I reported in
> > 
> >   https://lore.kernel.org/linux-block/20210608183339.70609-1-dwagner@suse.de/
> > 
> > By moving the hardware check up the crash was gone. Unfortuntatly, I
> > don't understand why this fixes the crash. The per-cpu access is
> > crashing but I can't see why the blk_mq_update_nr_hw_queues() is
> > fixing this problem.
> > 
> > Even though I can't explain why it fixes it, I think it makes sense to
> > update the hardware queue mapping bevore we recreate the IO
> > queues. Thus I avoided in the commit message to say it fixes
> > something.
> 
> I just discussed this with Hannes and we figured out how the crash is
> fixed by moving the blk_mq_update_nr_hw_queues() before the
> nvme_fc_create_hw_io_queues()/nvme_fc_connect_io_queues().
> 
> First of all, blk_mq_update_nr_hw_queues() operates on the normal
> tag_set and not the admin_tag_set. That means when we move the
> blk_mq_update_nr_hw_queues() before the nvme_fc_connect_io_queues(), we
> update the mapping to only CPUs and hwctx which are available. When we
> then do the connect call nvmf_connect_io_queue() we will only allocate
> tags from queues which are not in the BLK_MQ_S_INACTIVE anymore. Hence
> we skip the blk_mq_put_tag() call.

Your patch just reduces the race window, what if all cpus in
hctx->cpumask become offline when calling blk_mq_alloc_request_hctx()?


Thanks,
Ming