From: "Luis R. Rodriguez" Subject: [PATCH v2] mm: expland documentation over __read_mostly Date: Tue, 8 May 2018 11:19:24 -0700 Message-ID: <20180508181924.19939-1-mcgrof@kernel.org> Return-path: Sender: linux-kernel-owner@vger.kernel.org To: tglx@linutronix.de, arnd@arndb.de, cl@linux.com Cc: keescook@chromium.org, luto@amacapital.net, longman@redhat.com, viro@zeniv.linux.org.uk, dhowells@redhat.com, willy@infradead.org, ebiederm@xmission.com, rdunlap@infradead.org, joel.opensrc@gmail.com, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Luis R. Rodriguez" 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 R. Rodriguez --- include/linux/cache.h | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/include/linux/cache.h b/include/linux/cache.h index 750621e41d1c..4967566ed08c 100644 --- a/include/linux/cache.h +++ b/include/linux/cache.h @@ -15,8 +15,16 @@ /* * __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. + * + * If an architecture doesn't support it, ignore the hint. */ #ifndef __read_mostly #define __read_mostly -- 2.17.0