| Message ID | 20200506231353.32451-1-mcgrof@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm: expland documentation over __read_mostly | expand |
On Wed, May 06, 2020 at 11:13:53PM +0000, Luis Chamberlain 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. s/it/its same goes for the subject, as I think there is a minor typo: s/expland/expand > > Acked-by: Christoph Lameter <cl@linux.com> > Signed-off-by: Luis Chamberlain <mcgrof@kernel.org> > --- > > I sent this 2 years ago, but it fell through the cracks. This time > I'm adding Andrew Morton now, the fix0r-of-falling-through-the-cracks. > > Resending as I just saw a patch which doesn't clearly justifiy the > merits of the use of __read_mostly on it. > That would be my fault! (sorry) given the rationale below, the patch I sent really doesn't need the hint. Thanks for the extra bit of education here. (not an excuse) In a glance over the source tree, though, it seems most of the hinting cases are doing it in the misguided way. > include/linux/cache.h | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/include/linux/cache.h b/include/linux/cache.h > index 750621e41d1c..8106fb304fa7 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 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 > -- > 2.25.1 > Acked-by: Rafael Aquini <aquini@redhat.com>
On Wed, May 06, 2020 at 11:13:53PM +0000, Luis Chamberlain wrote:
> + * execute a critial path. We should be mindful and selective of its use.
"critical"
diff --git a/include/linux/cache.h b/include/linux/cache.h index 750621e41d1c..8106fb304fa7 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 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