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@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)
+
+#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
