Re: [RFC PATCH 0/3] arm64: queued spinlocks and rw-locks

[Will Deacon]

On Thu, Apr 13, 2017 at 01:33:09PM +0300, Yury Norov wrote:
> On Wed, Apr 12, 2017 at 01:04:55PM -0400, Adam Wallis wrote:
> > On 4/10/2017 5:35 PM, Yury Norov wrote:
> > > The patch of Jan Glauber enables queued spinlocks on arm64. I rebased it on
> > > latest kernel sources, and added a couple of fixes to headers to apply it 
> > > smoothly.
> > > 
> > > Though, locktourture test shows significant performance degradation in the
> > > acquisition of rw-lock for read on qemu:
> > > 
> > >                           Before           After
> > > spin_lock-torture:      38957034        37076367         -4.83
> > > rw_lock-torture W:       5369471        18971957        253.33
> > > rw_lock-torture R:       6413179         3668160        -42.80
> > > 
> > 
> > On our 48 core QDF2400 part, I am seeing huge improvements with these patches on
> > the torture tests. The improvements go up even further when I apply Jason Low's
> > MCS Spinlock patch: https://lkml.org/lkml/2016/4/20/725
> 
> It sounds great. So performance issue is looking like my local
> problem, most probably because I ran tests on Qemu VM.
> 
> I don't see any problems with this series, other than performance,
> and if it looks fine now, I think it's good enough for upstream.

I would still like to understand why you see such a significant performance
degradation, and whether or not you also see that on native hardware (i.e.
without Qemu involved).

Will