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

Dave Hansen
2021-12-07T23:14:57Z

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.