Re: [PATCH 2/2] blk-mq: add flag for drivers wanting blocking ->queue_rq()

On Thu, Sep 22, 2016 at 11:17 PM, Jens Axboe [off-list ref] wrote:

On 09/22/2016 09:12 AM, Christoph Hellwig wrote:

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On Thu, Sep 22, 2016 at 09:03:56AM -0600, Jens Axboe wrote:

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Having to grab a mutex, for instance. We invoke ->queue_rq() with
preemption disabled, so I'd hope that would not be the case... What
drivers block in ->queue_rq?

I though I had converted a lot of them to GFP_NOIO instead of GFP_ATOMIC
allocations, but I can't find any evidence of that.  Maybe it was just
my imagination, or an unsubmitted patch series.  Sorry for the
confusion.

OK, that makes more sense. Pretty sure we would have had complaints!

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Loop was another case that was on my radar to get rid of the queue_work
it currently has to do. Josef is currently testing the nbd driver using
this approach, so we should get some numbers there too. If we have to,
we can always bump up the concurrency to mimic more of the behavior of
having multiple workers running on the hardware queue. I'd prefer to
still do that in blk-mq, instead of having drivers reinventing their own
work offload functionality.

There should be a lot of numbers in the list archives from the time
that Ming did the loop conversion, as I've been trying to steer him
that way, and he actually implemented and benchmarked it.

We can't just increase the concurrency as a single work_struct item
can't be queued multiple times even on a high concurreny workqueue.

But we could have more work items, if we had to. Even if loop isn't a
drop-in replacement for this simpler approach, I think it'll work well
enough for nbd. The 5% number from Josef is comparing to not having any
offload at all, I suspect the number from just converting to queue_work
in the driver would be similar.

5% might be a noise.

From nbd's .queue_rq(), kthread_work should match its case because

one per-nbd mutex is required and all cmds sent to the nbd are serialized,
so using common work items may hurt performance caused by
unnecessary context switch.

thanks,