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authorLuis Chamberlain <mcgrof@kernel.org>2020-06-08 21:35:07 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2020-06-09 09:39:16 -0700
commit4fa7252338a56fbc90220e6330f136a379175a7a (patch)
tree5061c6346a7723a4970f8d49f13dbf10fcb0087d /include/linux/cache.h
parent2a71e81d32198785387c8bc0f2cc5d78f84e2e78 (diff)
include/linux/cache.h: expand documentation over __read_mostly
__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 its use. Signed-off-by: Luis Chamberlain <mcgrof@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Reviewed-by: Kees Cook <keescook@chromium.org> Acked-by: Christoph Lameter <cl@linux.com> Acked-by: Rafael Aquini <aquini@redhat.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Matthew Wilcox <willy@infradead.org> Link: http://lkml.kernel.org/r/20200507161424.2584-1-mcgrof@kernel.org Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include/linux/cache.h')
-rw-r--r--include/linux/cache.h10
1 files changed, 8 insertions, 2 deletions
diff --git a/include/linux/cache.h b/include/linux/cache.h
index 750621e41d1c..1aa8009f6d06 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 critical 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