Re: [PATCH 00/31 v2] PTI support for x86_32

[Ingo Molnar]

* Joerg Roedel <jroedel@xxxxxxx> wrote:

> Hi Andy,
> 
> On Fri, Feb 09, 2018 at 05:47:43PM +0000, Andy Lutomirski wrote:
> > One thing worth noting is that performance of this whole series is
> > going to be abysmal due to the complete lack of 32-bit PCID.  Maybe
> > any kernel built with this option set that runs on a CPU that has the
> > PCID bit set in CPUID should print a big fat warning like "WARNING:
> > you are using 32-bit PTI on a 64-bit PCID-capable CPU.  Your
> > performance will increase dramatically if you switch to a 64-bit
> > kernel."
> 
> Thanks for your review. I can add this warning, but I just hope that not
> a lot of people will actually see it :)

Could you please measure the PTI kernel vs. vanilla kernel?

Nothing complex, just perf's built-in scheduler and syscall benchmark should be 
enough:

   perf stat --null --sync --repeat 10 perf bench sched messaging -g 20

this should give us a pretty good worst-case overhead figure for process 
workloads.

Add '-t' to test threaded workloads as well:

   perf stat --null --sync --repeat 10 perf bench sched messaging -g 20 -t

The 10 runs used should be enough to reach good stability in practice:

 Performance counter stats for 'perf bench sched messaging -g 20 -t' (10 runs):

       0.380742219 seconds time elapsed                                          ( +-  0.73% )

Maybe do the same on the 64-bit kernel as well, so that we have 4 good data points 
on the same hardware?

Thanks,

	Ingo