Re: [x86/signal] 3aac3ebea0: will-it-scale.per_thread_ops -11.9% regression

[Dave Hansen]

On 12/6/21 5:21 PM, kernel test robot wrote:
> 
> 1bdda24c4af64cd2 3aac3ebea08f2d342364f827c89 
> ---------------- --------------------------- 
>          %stddev     %change         %stddev
>              \          |                \  
>     980404 ±  3%     -10.2%     880436 ±  2%  will-it-scale.16.threads
>      61274 ±  3%     -10.2%      55027 ±  2%  will-it-scale.per_thread_ops
>     980404 ±  3%     -10.2%     880436 ±  2%  will-it-scale.workload
>    9745749 ± 18%     +26.8%   12356608 ±  4%  meminfo.DirectMap2M

Something else funky is going on here.  Why would there all of a sudden
be so many more 2M pages in the direct map?  I also see gunk like
interrupts on the network card going up.  I can certainly see that
happening if something else on the network was messing around.

Granted, this was seen across several systems, but it's really odd.  I
guess I'll go try to dig up one of the actual ones where this was seen.

I tried on a smaller Skylake system and I don't see any regression at
all or any interesting delta in a perf profile.

Oliver or Chang, could you try to reproduce this by hand on one of the
suspect systems?  Build:

  1bdda24c4a ("signal: Add an optional check for altstack size")

then run will-it-scale by hand.  Then build:

  3aac3ebea0 ("x86/signal: Implement sigaltstack size validation")

and run it again.  Also, do we see any higher core-count regressions?
These all seem to happen with:

	mode=thread
	nr_task=16

That's really odd to see that for these systems with probably ~50 cores
each.  I'd expect to see it get worse at higher core counts.