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

Will Deacon
2015-06-26T11:14:22Z

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