From mboxrd@z Thu Jan 1 00:00:00 1970 From: Luis Chamberlain Subject: [PATCH] mm: expland documentation over __read_mostly Date: Wed, 6 May 2020 23:13:53 +0000 Message-ID: <20200506231353.32451-1-mcgrof@kernel.org> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: Sender: linux-kernel-owner@vger.kernel.org To: cl@linux.com, akpm@linux-foundation.org Cc: arnd@arndb.de, willy@infradead.org, aquini@redhat.com, keescook@chromium.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Luis Chamberlain List-Id: linux-arch.vger.kernel.org __read_mostly can easily be misused by folks, its not meant for just read-only data. There are performance reasons for using it, but we also don't provide any guidance about its use. Provide a bit more guidance over it use. Acked-by: Christoph Lameter Signed-off-by: Luis Chamberlain --- I sent this 2 years ago, but it fell through the cracks. This time I'm adding Andrew Morton now, the fix0r-of-falling-through-the-cracks. Resending as I just saw a patch which doesn't clearly justifiy the merits of the use of __read_mostly on it. include/linux/cache.h | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/include/linux/cache.h b/include/linux/cache.h index 750621e41d1c..8106fb304fa7 100644 --- a/include/linux/cache.h +++ b/include/linux/cache.h @@ -15,8 +15,14 @@ /* * __read_mostly is used to keep rarely changing variables out of frequently - * updated cachelines. If an architecture doesn't support it, ignore the - * hint. + * updated cachelines. Its use should be reserved for data that is used + * frequently in hot paths. Performance traces can help decide when to use + * this. You want __read_mostly data to be tightly packed, so that in the + * best case multiple frequently read variables for a hot path will be next + * to each other in order to reduce the number of cachelines needed to + * execute a critial path. We should be mindful and selective of its use. + * ie: if you're going to use it please supply a *good* justification in your + * commit log */ #ifndef __read_mostly #define __read_mostly -- 2.25.1 From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: Luis Chamberlain Subject: [PATCH] mm: expland documentation over __read_mostly Date: Wed, 6 May 2020 23:13:53 +0000 Message-ID: <20200506231353.32451-1-mcgrof@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org To: cl@linux.com, akpm@linux-foundation.org Cc: arnd@arndb.de, willy@infradead.org, aquini@redhat.com, keescook@chromium.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Luis Chamberlain List-ID: Message-ID: <20200506231353.VtXNt0JsngsJR9WNY951WEFT-1kSrLFAkghH8lFdR94@z> __read_mostly can easily be misused by folks, its not meant for just read-only data. There are performance reasons for using it, but we also don't provide any guidance about its use. Provide a bit more guidance over it use. Acked-by: Christoph Lameter Signed-off-by: Luis Chamberlain --- I sent this 2 years ago, but it fell through the cracks. This time I'm adding Andrew Morton now, the fix0r-of-falling-through-the-cracks. Resending as I just saw a patch which doesn't clearly justifiy the merits of the use of __read_mostly on it. include/linux/cache.h | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/include/linux/cache.h b/include/linux/cache.h index 750621e41d1c..8106fb304fa7 100644 --- a/include/linux/cache.h +++ b/include/linux/cache.h @@ -15,8 +15,14 @@ =20 /* * __read_mostly is used to keep rarely changing variables out of freque= ntly - * updated cachelines. If an architecture doesn't support it, ignore the - * hint. + * updated cachelines. Its use should be reserved for data that is used + * frequently in hot paths. Performance traces can help decide when to u= se + * this. You want __read_mostly data to be tightly packed, so that in th= e + * best case multiple frequently read variables for a hot path will be n= ext + * to each other in order to reduce the number of cachelines needed to + * execute a critial path. We should be mindful and selective of its use= . + * ie: if you're going to use it please supply a *good* justification in= your + * commit log */ #ifndef __read_mostly #define __read_mostly --=20 2.25.1