Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops -5.5% regression

[Feng Tang]

On Mon, Feb 24, 2020 at 10:19:15AM +0800, Feng Tang wrote:
> > 
> > > No, it's not the biggest, I tried another machine 'Xeon Phi(TM) CPU 7295',
> > > which has 72C/288T, and the regression is not seen. This is the part
> > > confusing me :)
> > 
> > Hmm.
> > 
> > Humor me - what  happens if you turn off SMT on that Cascade Lake
> > system?  Maybe it's about the thread ID bit in the L1? Although again,
> > I'd have expected things to get _worse_ if it's the two fields that
> > are now in the same cachline thanks to alignment.
> 
> I'll try it and report back.

I added "nosmt=force" on the 2S 4 nodes 96C/192T machine, and tested
both 96 and 192 processes, and the regression still exists.

Also for Ying's suggestion about separate 'sigpending' to another cache
line than '__refcount', it can not heal the regression either.

Thanks,
Feng