| Message ID | 20180508181924.19939-1-mcgrof@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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 >
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