On Thu, Mar 02, 2023 at 03:53:12PM -0500, Peter Xu wrote: > 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. A later patch changes users of this_cpu_cmpxchg to this_cpu_cmpxchg_local, which maintains behaviour. > 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 > >
