Overhead of arm64 LSE per-CPU atomics?

[Breno Leitao]

On Tue, Nov 04, 2025 at 07:08:19PM +0100, Willy Tarreau wrote:
> Hello Breno,
> 
> On Tue, Nov 04, 2025 at 07:59:38AM -0800, Breno Leitao wrote:
> > I found that the LSE case (__percpu_add_case_64_lse) has a huge variation,
> > while LL/SC case is stable.
> > In some case, LSE function runs at the same latency as LL/SC function and
> > slightly faster on p50, but, something happen to the system and LSE operations
> > start to take way longer than LL/SC.
> > 
> > Here are some interesting output coming from the latency of the functions above>
> > 
> > 	CPU: 47 - Latency Percentiles:
> > 	====================
> > 	LL/SC:   p50: 5.69 ns      p95: 5.71 ns      p99: 5.80 ns
> > 	LSE  :   p50: 45.53 ns     p95: 54.06 ns     p99: 55.18 ns
> (...)
> 
> Very interesting. I've run them here on a 80-core Ampere Altra made
> of Neoverse-N1 (armv8.2) and am getting very consistently better timings
> with LSE than LL/SC:

<snip>

> It now gives me much better LSE performance on the ARMv9:

I also see a stable latency for ldadd in my test case, also, better than LL/SC.

	CPU: 0 - Latency Percentiles:
	====================
	LL/SC:   p50: 5.74 ns      p95: 5.81 ns      p99: 7.13 ns
	LSE  :   p50: 4.34 ns      p95: 4.36 ns      p99: 4.40 ns

	CPU: 1 - Latency Percentiles:
	====================
	LL/SC:   p50: 5.74 ns      p95: 5.77 ns      p99: 5.82 ns
	LSE  :   p50: 4.35 ns      p95: 4.37 ns      p99: 4.42 ns

	CPU: 2 - Latency Percentiles:
	====================
	LL/SC:   p50: 5.74 ns      p95: 5.81 ns      p99: 6.76 ns
	LSE  :   p50: 4.35 ns      p95: 4.80 ns      p99: 5.55 ns

	...

	CPU: 71 - Latency Percentiles:
	====================
	LL/SC:   p50: 5.72 ns      p95: 5.75 ns      p99: 5.91 ns
	LSE  :   p50: 4.33 ns      p95: 4.35 ns      p99: 4.38 ns

> PS: thanks Breno for sharing your test code, that's super useful!

Glad you liked it. I tried to narrow down the problem as much as I could, so, I
could could follow up the discussion. :-)