Re: [RFC 12/32] stack: always use C11 memory model implementation

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 31 Jul 2026 16:53:45 +0200
Morten Brørup <mb@smartsharesystems.com> wrote:

> +TO: x86 maintainers, ThunderX maintainers
> 
> > From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> > Sent: Wednesday, 29 July 2026 19.54
> > 
> > The generic and C11 lock-free stack implementations differ only in
> > memory ordering. The generic version uses a full barrier where its
> > own comments state an acquire fence is sufficient, and seq_cst for
> > all length counter operations.
> > 
> > Only x86 and ThunderX still used the generic version. On x86 the
> > switch removes a locked add per CAS attempt in push and pop; TSO
> > provides the acquire semantics. On ThunderX the pop fence weakens
> > from dmb ish to dmb ishld and the push fence goes away. Unlike the
> > ring, no platform selected the generic stack for measured
> > performance reasons.
> > 
> > Remove it and use the C11 implementation everywhere.  
> 
> The lack of measured performance difference documentation is not a valid reason to remove the generic version!
> 
> It would be reasonable to assume that x86 (and ThunderX) use the generic version for non-insignificant performance reasons.
> 
> If there is no performance difference, I agree with this patch. Otherwise not.
> This could be verified by providing the missing measurements.
> 

Surprisingly, the performance of the C11 version is better than the old generic
version that had smp_mb.  That is because C11 code generates no locked prefixes.
Gets speedup of upto 60%.

Between main (with rte_smp_mb) and the unified C11 version on the 32-core x86 machine:

Test	main (n=9)	unified C11 (n=9)	delta
single push/pop	46.62 ±0.30	33.41 ±0.10	-28%
empty pop	1.47 ±0.01	0.98 ±0.01	-33%
1 lcore, bulk 8	9.06 ±0.05	8.20 ±0.08	-10%
1 lcore, bulk 32	6.09 ±0.02	6.15 ±0.03	+1%
2 HT, bulk 8	42.05 ±0.31	39.24 ±0.52	-7%
2 HT, bulk 32	11.92 ±0.13	11.89 ±0.10	0
2 cores, bulk 8	78.90 ±0.60	72.96 ±1.11	-7%
2 cores, bulk 32	20.74 ±1.56	7.70 ±0.13	-63%
32 cores, bulk 8	6126 ±72	6121 ±89	0
32 cores, bulk 32	1953.9 ±2.9	1984.6 ±13.3	+1.6%