mm: 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.

Acked-by Christoph Lameter <[email protected]>

Signed-off-by Luis Chamberlain <[email protected]>

__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.

Acked-by Christoph Lameter <[email protected]>

Signed-off-by Luis Chamberlain <[email protected]>

---

This v2 just has a few spelling fixes.

 include/linux/cache.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

include/linux/cache.h +8 -2

index 750621e41d1c..8106fb304fa7 100644
@@ -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

Signature · 1 line

2.25.1