RCU-ified dm-mpath for testing/review

[Hannes Reinecke]

On 02/12/2016 07:00 PM, Mike Snitzer wrote:
> On Fri, Feb 12 2016 at 11:04am -0500,
> Hannes Reinecke <hare at suse.de> wrote:
> 
>> On 02/12/2016 04:26 PM, Mike Snitzer wrote:
>>> On Fri, Feb 12 2016 at 10:18am -0500,
>>> Hannes Reinecke <hare at suse.de> wrote:
>>>>>
>>>> Good news is that I've managed to hit the roof for my array with the
>>>> devel2 version of those patches. (And a _heavily_ patched-up lpfc
>>>> driver :-)
>>>> So from that perspective everything's fine now; we've reached the
>>>> hardware limit for my setup.
>>>> Which in itself is quite impressive; beating Intel P3700 with 16FC
>>>> is not bad methinks :-)
>>>>
>>>> So thanks for all your work here.
>>>
>>> Ah, that's really good news!  But devel2 is definitely _not_ destined
>>> for upstream.  'devel3' is much closer to "ready".  But your testing and
>>> review would really push it forward.
>>>
>>> Please see/test:
>>> http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=devel3
>>>
>>> Also, please read this header:
>>> http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=devel3&id=65a01b76502dd68e8ca298ee6614c0151b677f4a
>>>
>>> Even with devel2 I hacked it such that repeat_count > 1 is effectively
>>> broken.  I'm now _seriously_ considering deprecating repeat_count
>>> completely (adding a DMWARN that will inform the user. e.g.:
>>> "repeat_count > 1 is no longer supported").  I see no point going to
>>> great lengths to maintain a dm-mpath feature that was only a hack for
>>> when dm-mpath was bio-based.  What do you think?
>>
>> Drop it, and make setting of which a no-op.
>> Never liked it anyway, and these decisions should really be
>> delegated to the path selector.
> 
> Sure, but my point is DM-mpath will no longer be able to provide the
> ability to properly handle repeat_count > 1 (because updating the
> ->current_pgpath crushes the write-side of the RCU).
> 
Fully understood. But as I said, the _need_ for repeat_count has
essentially vanished with the move to request-based multipath.

> As such I've rebased 'devel3' to impose repeat_count = 1 (both in
> dm-mpath.c and the defaults in each path selector).

Kewl.
I'll give it a go.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare at suse.de			               +49 911 74053 688
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