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Thu, 02 Mar 2023 09:48:48 -0500 X-MC-Unique: 1PGCLXHvPm-glmSZu0fXJA-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 8117485A5B1; Thu, 2 Mar 2023 14:48:47 +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 4C6BF492C3E; Thu, 2 Mar 2023 14:48:47 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id D78F7403CC770; Thu, 2 Mar 2023 11:32:06 -0300 (-03) Date: Thu, 2 Mar 2023 11:32:06 -0300 From: Marcelo Tosatti To: David Hildenbrand 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 10.11.54.10 X-Stat-Signature: 94oyb6xpqpfbbtmn93s81ta3enpak5nd X-Rspam-User: X-Rspamd-Queue-Id: 1DB3A10001E X-Rspamd-Server: rspam06 X-HE-Tag: 1677768529-543673 X-HE-Meta: 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 yHsLWGXt 2V70PPkPuEQxrg0PN9FOb8qQmLhJsAiGDNU+jAggCSjLfc0CnC2xbpP5QQiUaQ3VNNoFNFNPvh+f90dPBxj1MtEI1PuwfPZS9x0u5OOB97R2Nj6jd09q9tKuwq8n/J3WdHQuEczMiO2K4YI7cQcBFueLwHTqk1eHx4pDr6svwsD8JlmsFVvpSCyhibx8FTPUYfMZT5JSfGX0gk4MY5JyckLbz9KAgwvu4v6+gRua1i8viPHz9NjC9XR0C0g== 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 11:42:57AM +0100, David Hildenbrand wrote: > 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 > > > > 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_* ? No: it happens that on ARM-64 cmpxchg_local == cmpxchg_relaxed. See cf10b79a7d88edc689479af989b3a88e9adf07ff. > (b) Should these new variants rather call the "_local" variant? They probably should. But this patchset maintains the current behaviour of this_cpu_cmpxch (for this_cpu_cmpxch_local), which was: #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) Thanks.