Re: RCU-ified dm-mpath for testing/review

[Hannes Reinecke <hare@suse.de>]

On 02/12/2016 04:26 PM, Mike Snitzer wrote:
> On Fri, Feb 12 2016 at 10:18am -0500,
> Hannes Reinecke <hare@suse.de> wrote:
>
>> On 02/11/2016 04:34 PM, Mike Snitzer wrote:
>>> On Wed, Feb 10 2016 at  8:50pm -0500,
>>> Mike Snitzer <snitzer@redhat.com> wrote:
>>>
>>>> On Tue, Feb 09 2016 at  7:45pm -0500,
>>>> Mike Snitzer <snitzer@redhat.com> wrote:
>>>>
>>>>>
>>>>> OK, I took a crack at embracing RCU.  Only slightly better performance
>>>>> on my single NUMA node testbed.  (But I'll have to track down a system
>>>>> with multiple NUMA nodes to do any justice to the next wave of this
>>>>> optimization effort)
>>>>>
>>>>> This RCU work is very heavy-handed and way too fiddley (there could
>>>>> easily be bugs).  Anyway, please see:
>>>>> http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=devel2&id=d80a7e4f8b5be9c81e4d452137623b003fa64745
>>>>>
>>>>> But this might give you something to build on to arrive at something
>>>>> more scalable?
>>>>
>>>> I've a bit more polished version of this work (broken up into multiple
>>>> commits, with some fixes, etc) here:
>>>> http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=devel3
>>>>
>>>> Hannes and/or Sagi, if you get a chance to try this on your NUMA system
>>>> please let me know how it goes.
>>>
>>> Initial review has uncovered some locking problems with the current code
>>> (nothing that caused crashes or hangs in my testing but...) so please
>>> hold off on testing until you hear from me (hopefully tomorrow).
>>>
>> 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.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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