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Thu, 02 Mar 2023 16:17:19 -0500 X-MC-Unique: VAq8u0t7MzKbczVyVcrBkQ-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id EA74A185A78B; Thu, 2 Mar 2023 21:17:18 +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 A5FA740C83B6; Thu, 2 Mar 2023 21:17:18 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 37D79400E055B; Thu, 2 Mar 2023 18:04:25 -0300 (-03) Date: Thu, 2 Mar 2023 18:04:25 -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 10.11.54.1 X-Rspam-User: X-Rspamd-Queue-Id: 2E7B91C0020 X-Rspamd-Server: rspam01 X-Stat-Signature: qmnwdm48fnnyfrn7ikbkqbz5q73ckkhm X-HE-Tag: 1677791841-839163 X-HE-Meta: U2FsdGVkX1/MBC06cCv2F0wK33DQT5VdBD+17h1oeueipq5rtSdyo5GbrOC+WPqZGpK/woF4z6Ix3UJvzCCLlMYQ1SC7a4yHoRkXKBg7keOYpnxdgeY/5w4vdhgCdpzG/Uf8sBi2i+BXtqdwhydWPeciINju9n+IEXTj5ViBFgny7Wd7KhClFVjSlpgLZvYiAO3UUkD9HD56usKukDtiz/1GpGXg0Pu5nmHx0DiMJPwaeZ+eJXsuBHF6sJzgmuuyUl9z2HQomrilyWiusUE/eDEitNFH2l+50HN2KcJvWNGAox4Xq/vmn4V5Of+wKbD/MLJ9dpSbTTUqiDekZzOEi/QVoLd7UfkWCkmhgwKvTcx6eJ6JFgEHjpfpM9tvLUCWkj4fuDENejr3Wk6cqgr1qq0P1hhkDuNHnjxzS18S77rFekSZ2TVmKIQC75RkdPX/WZWQCBMaaPi+ySOxDN7GOKIZBBT72xQP7onvBuY3fzIiPmLHaNHPkLimRmq/knI36b0O3gDcx3iuUqBqR8qdrzqr5eWgG/gMpp0qGcuBVaGftS8ZFWmtNeO1Nd3Ys7BIj3F3DCcIP54mrkAb75lBBAxcvevOKfyB8390bqMUP/cxi50W49Sx2DLYpL4jy7grY9XIv6ei/yuzQG4lyH08YMC0VsrSFJxy2EaQSW91TK8LsBU6RIv5CQkD4k4u6WkvI+lnTvRx+B2NXu2pv7H9ii1XSspw23imLiefjWI2N17YD4O4MOMUFgXJfkHd8z+ouBlGKmAmXHMaA9jtjH9FFOFOfkMbWNL8gZKBT4Mf7lfnxLxGazNXyEK5M3u7VYyOcz/4qgp+p4sp2N5HYsKW0oXT3sxjj7kBcyWlUfhRuQ2GXDTSMHy7zhjaaGAjM8Fm2HfBuJlXLtl+yxEACOf0QbX5OGE02/eKS8pHTZaHJaMCtHNFI+fOgf/3bleh10yw0n+RBlt+2eq6I1DzvO9 upHb+NaF ra8Lg/GcYga1xSRVaFAiY6+V4A/drw/Hax/GMrAYfMRYDOUmCDj2dtx0yM2YQDqrx+xfJxzKRvzpVo9Ji0rU+gUoTZRAjUQUC5eFEO8wD48IIlNltQQgHhr3lNZWZdtJ0RpevjSm+Djqv8VCs78WdMLHIsgwsPpvkqxltA9Gs1PyKxOzWd6k+5lEsI0vkjsr3ejqQ8Qly41VZyYLY2E6yr2DgzBPRWcz2TPUEsvjSI+EOHF4BwjOL052b7kRxlMFXZyRswLXy5a+qI/9ALXpZ1NcT/A== 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. > > 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. It happens that on ARM-64 cmpxchg_local == cmpxchg_relaxed. See cf10b79a7d88edc689479af989b3a88e9adf07ff. 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) > > + > > +#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_*()? Since cmpxchg_local = cmpxchg_relaxed, seems like this is not necessary. > Nothing about your patch along since it was the same before, but I'm > wondering whether this is a good time to switchover. I would say that another patch is more appropriate to change this, if desired. > 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. Yes, should have done that (or CC each individual maintainer). Will do on next version. Thanks.