Messages in this thread | | | Date | Thu, 2 Mar 2023 11:42:57 +0100 | From | David Hildenbrand <> | Subject | Re: [PATCH v2 02/11] this_cpu_cmpxchg: ARM64: switch this_cpu_cmpxchg to locked, add _local function |
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On 09.02.23 16:01, 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. > > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com> > > 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) > + > +#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) > +
Call me confused (not necessarily your fault :) ).
We have cmpxchg_local, cmpxchg_relaxed and cmpxchg. this_cpu_cmpxchg_local_* now calls ... *drumroll* ... cmpxchg_relaxed.
IIUC, cmpxchg_local is only guaranteed to be atomic WRO the current CPU (especially, protection against interrupts when the operation is implemented using multiple instructions). We do have a generic implementation that disables/enables interrupts.
IIUC, cmpxchg_relaxed an atomic update without any memory ordering guarantees (in contrast to cmpxchg, cmpxchg_acquire, cmpxchg_acquire). We default to arch_cmpxchg if we don't have arch_cmpxchg_relaxed. arch_cmpxchg defaults to arch_cmpxchg_local, if not supported.
Naturally I wonder:
(a) Should these new variants be rather called this_cpu_cmpxchg_relaxed_* ?
(b) Should these new variants rather call the "_local" variant?
Shedding some light on this would be great.
-- Thanks,
David / dhildenb
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