Re: [PATCH v2 02/11] this_cpu_cmpxchg: ARM64: switch this_cpu_cmpxchg to locked, add _local function

[Peter Xu]

On Thu, Feb 09, 2023 at 12:01:52PM -0300, Marcelo Tosatti wrote:
> Goal is to have vmstat_shepherd to transfer from
> per-CPU counters to global counters remotely. For this, 
> an atomic this_cpu_cmpxchg is necessary.
> 
> Following the kernel convention for cmpxchg/cmpxchg_local,
> change ARM's this_cpu_cmpxchg_ helpers to be atomic,
> and add this_cpu_cmpxchg_local_ helpers which are not atomic.

I can follow on the necessity of having the _local version, however two
questions below.

> 
> Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> 
> Index: linux-vmstat-remote/arch/arm64/include/asm/percpu.h
> ===================================================================
> --- linux-vmstat-remote.orig/arch/arm64/include/asm/percpu.h
> +++ linux-vmstat-remote/arch/arm64/include/asm/percpu.h
> @@ -232,13 +232,23 @@ PERCPU_RET_OP(add, add, ldadd)
>  	_pcp_protect_return(xchg_relaxed, pcp, val)
>  
>  #define this_cpu_cmpxchg_1(pcp, o, n)	\
> -	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> +	_pcp_protect_return(cmpxchg, pcp, o, n)
>  #define this_cpu_cmpxchg_2(pcp, o, n)	\
> -	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> +	_pcp_protect_return(cmpxchg, pcp, o, n)
>  #define this_cpu_cmpxchg_4(pcp, o, n)	\
> -	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> +	_pcp_protect_return(cmpxchg, pcp, o, n)
>  #define this_cpu_cmpxchg_8(pcp, o, n)	\
> +	_pcp_protect_return(cmpxchg, pcp, o, n)

This makes this_cpu_cmpxchg_*() not only non-local, but also (especially
for arm64) memory barrier implications since cmpxchg() has a strong memory
barrier, while the old this_cpu_cmpxchg*() doesn't have, afaiu.

Maybe it's not a big deal if the audience of this helper is still limited
(e.g. we can add memory barriers if we don't want strict ordering
implication), but just to check with you on whether it's intended, and if
so whether it may worth some comments.

> +
> +#define this_cpu_cmpxchg_local_1(pcp, o, n)	\
>  	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> +#define this_cpu_cmpxchg_local_2(pcp, o, n)	\
> +	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> +#define this_cpu_cmpxchg_local_4(pcp, o, n)	\
> +	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
> +#define this_cpu_cmpxchg_local_8(pcp, o, n)	\
> +	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)

I think cmpxchg_relaxed()==cmpxchg_local() here for aarch64, however should
we still use cmpxchg_local() to pair with this_cpu_cmpxchg_local_*()?

Nothing about your patch along since it was the same before, but I'm
wondering whether this is a good time to switchover.

The other thing is would it be good to copy arch-list for each arch patch?
Maybe it'll help to extend the audience too.

Thanks,

-- 
Peter Xu