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* [PATCH v2] mm: expland documentation over __read_mostly
@ 2018-05-08 18:19 Luis R. Rodriguez
  2020-05-06 23:03 ` Luis Chamberlain
  0 siblings, 1 reply; 2+ messages in thread
From: Luis R. Rodriguez @ 2018-05-08 18:19 UTC (permalink / raw)
  To: tglx, arnd, cl
  Cc: keescook, luto, longman, viro, dhowells, willy, ebiederm,
	rdunlap, joel.opensrc, linux-arch, linux-mm, linux-kernel,
	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.

Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
---
 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

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2018-05-08 18:19 [PATCH v2] mm: expland documentation over __read_mostly Luis R. Rodriguez
2020-05-06 23:03 ` Luis Chamberlain

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