[PATCH] mm: expland documentation over __read_mostly

[Luis R. Rodriguez]

__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.

Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
---
 include/linux/cache.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

Every now and then we get a patch suggesting to use __read_mostly for
something new or old but with no justifications. Add a bit more
verbiage to help guide its users.

Is this sufficient documentation to at least ask for a reason in the commit
log as to why its being used for new entries? Or should we be explicit and
ask for such justifications in commit logs? Taken from prior discussions
with Christoph Lameter [0] over its use.

[0] https://lkml.kernel.org/r/alpine.DEB.2.11.1504301343190.28879@xxxxxxxxxx

diff --git a/include/linux/cache.h b/include/linux/cache.h
index 750621e41d1c..62bc5adc0ed5 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 if its use.
+ *
+ * If an architecture doesn't support it, ignore the hint.
  */
 #ifndef __read_mostly
 #define __read_mostly
-- 
2.17.0