Re: [PATCH v5 3/3] locking/qrwlock: Don't contend with readers when setting _QW_WAITING

[Will Deacon <will.deacon@arm.com>]

On Thu, Jun 25, 2015 at 09:33:36PM +0100, Waiman Long wrote:
> On 06/25/2015 02:35 PM, Peter Zijlstra wrote:
> > On Fri, Jun 19, 2015 at 11:50:02AM -0400, Waiman Long wrote:
> >> The current cmpxchg() loop in setting the _QW_WAITING flag for writers
> >> in queue_write_lock_slowpath() will contend with incoming readers
> >> causing possibly extra cmpxchg() operations that are wasteful. This
> >> patch changes the code to do a byte cmpxchg() to eliminate contention
> >> with new readers.
> >>
> >> A multithreaded microbenchmark running 5M read_lock/write_lock loop
> >> on a 8-socket 80-core Westmere-EX machine running 4.0 based kernel
> >> with the qspinlock patch have the following execution times (in ms)
> >> with and without the patch:
> >>
> >> With R:W ratio = 5:1
> >>
> >> 	Threads	   w/o patch	with patch	% change
> >> 	-------	   ---------	----------	--------
> >> 	   2	     990 	    895		  -9.6%
> >> 	   3	    2136 	   1912		 -10.5%
> >> 	   4	    3166	   2830		 -10.6%
> >> 	   5	    3953	   3629		  -8.2%
> >> 	   6	    4628	   4405		  -4.8%
> >> 	   7	    5344	   5197		  -2.8%
> >> 	   8	    6065	   6004		  -1.0%
> >> 	   9	    6826	   6811		  -0.2%
> >> 	  10	    7599	   7599		   0.0%
> >> 	  15	    9757	   9766		  +0.1%
> >> 	  20	   13767	  13817		  +0.4%
> >>
> >> With small number of contending threads, this patch can improve
> >> locking performance by up to 10%. With more contending threads,
> >> however, the gain diminishes.
> >>
> >> With the extended qrwlock structure defined in asm-generic/qrwlock,
> >> the queue_write_unlock() function is also simplified to a
> >> smp_store_release() call.
> >>
> >> Signed-off-by: Waiman Long<Waiman.Long@hp.com>
> > This one does not in fact apply, seeing how I applied a previous
> > version.
> >
> > Please send an incremental patch if you still want to change things to
> > this form.
> 
> I saw that Ingo has merged a previous version of the patch. I am fine 
> with that version. As Will is working on a qrwlock patch to enable ARM 
> to use it, I will let him make the structure move to qrwlock.h if he 
> choose to do so.

Sure, I'll rework my series when -rc1 lands and take this change into
account.

Will