[dm-devel] blk-mq request allocation stalls [was: Re: [PATCH v3 0/8] dm: add request-based blk-mq support]

[Jens Axboe]

On 01/09/2015 12:49 PM, Mike Snitzer wrote:
> On Wed, Jan 07 2015 at  3:40pm -0500,
> Keith Busch <keith.busch at intel.com> wrote:
>
>> On Wed, 7 Jan 2015, Bart Van Assche wrote:
>>> On 01/06/15 17:15, Jens Axboe wrote:
>>>> blk-mq request allocation is pretty much as optimized/fast as it can be.
>>>> The slowdown must be due to one of two reasons:
>>>>
>>>> - A bug related to running out of requests, perhaps a missing queue run
>>>> or something like that.
>>>> - A smaller number of available requests, due to the requested queue depth.
>>>>
>>>> Looking at Barts results, it looks like it's usually fast, but sometimes
>>>> very slow. That would seem to indicate it's option #1 above that is the
>>>> issue. Bart, since this seems to wait for quite a bit, would it be
>>>> possible to cat the 'tags' file for that queue when it is stuck like that?
>>>
>>> Hello Jens,
>>>
>>> Thanks for the assistance. Is this the output you were looking for
>>
>> I'm a little confused by the later comments given the below data. It says
>> multipath_clone_and_map() is stuck at bt_get, but that doesn't block
>> unless there are no tags available. The tags should be coming from one
>> of dm-1's path queues, and I'm assuming these queues are provided by sdc
>> and sdd. All their tags are free, so that looks like a missing wake_up
>> when the queue idles.
>
> Like I said in an earlier email, I cannot reproduce Bart's hangs running
> mkfs.xfs against a multipath device that is built ontop of a virtio
> device in a KVM guest.
>
> But I can hit __bt_get() failures on the virtio-blk device that I'm
> using for the root device on this guest.  Bart I'd be interested to see
> what you get when running the attached debug patch (likely will just
> echo the same type of info you've already provided).
>
> There does appear to be something weird going on with bt_get().  With
> the debug patch I'm seeing the following when I simply run "make install"
> of the kernel (it'll run dracut to build the initramfs, etc):
>
> You'll note that in all instances where __bt_get() returns -1 nr_free isn't 0.

Yeah, that doesn't look good. Can you try with this patch? The second 
hunk is the interesting bit, the first is more of a cleanup.

-- 
Jens Axboe

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