Re: [PATCH 00/11] Use global pages with PTI

Dave Hansen
2018-03-27T20:19:43Z

On 03/27/2018 01:07 PM, Ingo Molnar wrote:
> - To see at minimum stddev numbers, to make sure we are not looking at some weird
> statistical artifact. (I also outlined a more robust measurement method.)
>
> - If the numbers are right, a CPU engineer should have a look if possible,
> because frankly this effect is not expected and is not intuitive. Where global
> pages can be used safely they are almost always an unconditional win.
> Maybe we are missing some limitation or some interaction with PCID.
>
> Since we'll be using PCID even on Meltdown-fixed hardware, maybe the same negative
> performance effect already exists on non-PTI kernels as well, we just never
> noticed?

Yep, totally agree. I'll do the more robust collection and also explore
on "real" !PCID hardware. I also know the right CPU folks to go ask
about this, I just want to do the second round of robust data collection
before I bug them.