From: Luis Chamberlain <mcgrof@xxxxxxxxxx> Subject: 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. Link: http://lkml.kernel.org/r/20200507161424.2584-1-mcgrof@xxxxxxxxxx Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx> Acked-by: Christoph Lameter <cl@xxxxxxxxx> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> Acked-by: Rafael Aquini <aquini@xxxxxxxxxx> Cc: Arnd Bergmann <arnd@xxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/cache.h | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) --- a/include/linux/cache.h~mm-expand-documentation-over-__read_mostly +++ a/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 _
