Re: [PATCH -tip] x86/locking/atomic: Use asm_inline for atomic locking insns

Dirk Gouders
2025-03-06T10:53:23Z

Hi Ingo,

my interest comes, because I just started to try to better understand
PCL and am reading the perf manual pages. Perhaps I should therefore
keep my RO-bit permanent for some more months, but:

> And if the benchmark is context-switching heavy, you'll want to use
> 'perf stat -a' option to not have PMU context switching costs, and the

I'm sure you know what you are talking about so I don't doubt the above
is correct but perhaps, the manual page should also clarify -a:

-a::
--all-cpus::
system-wide collection from all CPUs (default if no target is specified)

In the last example -a is combined with -C 2 which is even more irritating when
you just started with the manual pages.

But the main reason why I thought it might be OK to once toggle my
RO-bit is that I tried your examples and with the first one I have way
higher numbers than yours and I thought that must be, because you just
own the faster machine (as I would have expected):

> starship:~> perf bench sched pipe
> # Running 'sched/pipe' benchmark:
> # Executed 1000000 pipe operations between two processes
>
> Total time: 6.939 [sec]
>
> 6.939128 usecs/op
> 144110 ops/sec

lena:~> perf bench sched pipe
# Running 'sched/pipe' benchmark:
# Executed 1000000 pipe operations between two processes

Total time: 11.129 [sec]

11.129952 usecs/op
89847 ops/sec

And I expected this to continue throughout the examples.

But -- to keep this short -- with the last example, my numbers are
suddenly significantly lower than yours:

> starship:~> taskset 0x4 perf stat -a -C 2 -e instructions --repeat 5 perf bench sched pipe
> 5.808068 usecs/op
> 5.843716 usecs/op
> 5.826543 usecs/op
> 5.801616 usecs/op
> 5.793129 usecs/op
>
> Performance counter stats for 'system wide' (5 runs):
>
> 32,244,691,275 instructions ( +- 0.21% )
>
> 5.81624 +- 0.00912 seconds time elapsed ( +- 0.16% )

lena:~> taskset 0x4 perf stat -a -C 2 -e instructions --repeat 5 perf bench sched pipe
4.204444 usecs/op
4.169279 usecs/op
4.186812 usecs/op
4.217039 usecs/op
4.208538 usecs/op

Performance counter stats for 'system wide' (5 runs):

14,196,762,588 instructions ( +- 0.04% )

4.20203 +- 0.00854 seconds time elapsed ( +- 0.20% )

Of course, I don't want to waste anyones time if this is a so obvious
thing that only newbies don't understand. So, feel free to just ignore this.

Regards

Dirk