Re: [PATCH v2 0/6] futex: Use RCU-based per-CPU reference counting

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 7/15/25 22:01, Sebastian Andrzej Siewior wrote:
> On 2025-07-15 21:29:34 [+0530], Shrikanth Hegde wrote:
>> Hi. Sorry for not stumble upon this earlier. Saw these now.
>>
>> Since perf bench had shown a significant regression last time around, and
>> for which immutable option was added, gave perf futex a try again.
>>
>> Below are the results: Ran on 5 core LPAR(VM) on power. perf was compiled from tools/perf.
> Thank you.
> 
> If you use perf-bench with -b then the buckets are applied
> "immediately". It mostly works also with auto scaling. The problem is
> that perf creates the threads and immediately after it starts the test.
> While the RCU kicks in shortly after there is no transition happening
> until after all the test completes/ the threads terminate. The reason is
> that several private-hash references are in use because a some threads
> are always in the futex() syscall.
> 
> It would require something like commit
>      a255b78d14324 ("selftests/futex: Adapt the private hash test to RCU related changes")
> 
> to have this transition before the test starts.
> Your schbench seems not affected?

Yes. schbench shows similar number.

> 
> If you use -b, is it better than or equal compared to the immutable
> option? This isn't quite clear.
> n


I did try again by going to baseline, removed BROKEN and ran below. Which gives us immutable numbers.
./perf bench futex hash -Ib512
Averaged 1536035 operations/sec (+- 0.11%), total secs = 10
Futex hashing: 512 hash buckets (immutable)

So, with -b 512 option, it is around 8-10% less compared to immutable.