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Fri, 03 Mar 2023 11:39:43 -0500 X-MC-Unique: wZQY2SDpPNujwrXpzEKAQg-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 16E8A101A521; Fri, 3 Mar 2023 16:39:43 +0000 (UTC) Received: from tpad.localdomain (ovpn-112-2.gru2.redhat.com [10.97.112.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B62FD40C6EC4; Fri, 3 Mar 2023 16:39:42 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 8DE8B403EA21C; Fri, 3 Mar 2023 12:47:02 -0300 (-03) Date: Fri, 3 Mar 2023 12:47:02 -0300 From: Marcelo Tosatti To: Peter Xu Cc: Christoph Lameter , Aaron Tomlin , Frederic Weisbecker , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2 02/11] this_cpu_cmpxchg: ARM64: switch this_cpu_cmpxchg to locked, add _local function Message-ID: References: <20230209150150.380060673@redhat.com> <20230209153204.683821550@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 8B94B20024 X-Stat-Signature: g9wr1nziireu7ncp1eo17pzr17ymm83s X-Rspam-User: X-HE-Tag: 1677861587-849262 X-HE-Meta: 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 exCer6Hd TWiq+oLegy5SIMYr6uaF4i1nMRUsm7DdlTNPhbEEhwb1X5jYyv7gOTFYds/yTQi7ttLZOHOBlafEAi0Fa498+DJfKd2PfZdCjN095o2ZeJWfE2pcJTraY1b3VavxjTm76B7W8G2HSXIZF+pf5Y44m2hgl0zWYqRNLpKzQb5kRxVZmExrn6+8UFWXDQmrAdTCMWOhXYt78jnmh7sXq7pbIJIIRkCsI0a6Tw5A+nqOwbP8c+bYNjtOiWZeX5w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 > > > > 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 > >