[PATCH v2] mm: expland documentation over __read_mostly

[PATCH v2] 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.

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

Re: [PATCH v2] mm: expland documentation over __read_mostly

[Luis Chamberlain]

On Tue, May 8, 2018 at 12:19 PM Luis R. Rodriguez <mcgrof@kernel.org> wrote:
>
> __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>

After 2 years, this patch was never applied... and so people can
easily keep misusing this. I'll resend now.

  Luis
> ---
>  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
>