linux-kernel - Re: [PATCH v2 02/11] this_cpu_cmpxchg: ARM64: switch this_cpu_cmpxchg to locked, add

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Date:   Fri, 3 Mar 2023 12:47:02 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 02/11] this_cpu_cmpxchg: ARM64: switch
 this_cpu_cmpxchg to locked, add _local function

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@...hat.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)
> 
> 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
> 
>