On Wed, Feb 03, 2016 at 01:32:10PM +0000, Will Deacon wrote: > On Wed, Feb 03, 2016 at 09:33:39AM +0100, Ingo Molnar wrote: > > In fact I'd suggest to test this via a quick runtime hack like this in > > rcupdate.h: > > > > extern int panic_timeout; > > > > ... > > > > if (panic_timeout) > > smp_load_acquire(p); > > else > > typeof(*p) *________p1 = (typeof(*p) > > *__force)lockless_dereference(p); > > > > (or so) > > So the problem with this is that a LOAD <ctrl> LOAD sequence isn't an > ordering hazard on ARM, so you're potentially at the mercy of the branch > predictor as to whether you get an acquire. That's not to say it won't > be discarded as soon as the conditional is resolved, but it could > screw up the benchmarking. > > I'd be better off doing some runtime patching, but that's not something > I can knock up in a couple of minutes (so I'll add it to my list).
... so I actually got that up and running, believe it or not. Filthy stuff. The good news is that you're right, and I'm now seeing ~1% difference between the runs with ~0.3% noise for either of them. I still think that's significant, but it's a lot more reassuring than 4%. Will
