Re: [PATCH 0/2] Handle update hardware queues and queue freeze more carefully
From: Daniel Wagner <hidden>
Date: 2021-06-25 12:22:02
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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/ (local) 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.
Also during testing I observed the we hang indivinetly in blk_mq_freeze_queue_wait(). Again I can't explain why we get stuck there but given a common pattern for the nvme_wait_freeze() is to use it with a timeout I think the timeout should be used too :)
The nvme_wait_freeeze() is probably not needed at all, __blk_mq_update_nr_hw_queues() already calls blk_mq_freeze_queue(). So there this is not needed at all. Furthermore, if we move blk_mq_update_nr_hw_queues() in front of nvme_fc_create_hw_io_queues() there can't be any pending I/Os because there are not queues. _______________________________________________ Linux-nvme mailing list Linux-nvme@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-nvme