On 05/07/2018 04:15 PM, Luis R. Rodriguez 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. > > Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx> > --- > include/linux/cache.h | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > Every now and then we get a patch suggesting to use __read_mostly for > something new or old but with no justifications. Add a bit more > verbiage to help guide its users. > > Is this sufficient documentation to at least ask for a reason in the commit > log as to why its being used for new entries? Or should we be explicit and > ask for such justifications in commit logs? Taken from prior discussions > with Christoph Lameter [0] over its use. > > [0] https://lkml.kernel.org/r/alpine.DEB.2.11.1504301343190.28879@xxxxxxxxxx > > diff --git a/include/linux/cache.h b/include/linux/cache.h > index 750621e41d1c..62bc5adc0ed5 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 if its use. of its use. > + * > + * If an architecture doesn't support it, ignore the hint. > */ > #ifndef __read_mostly > #define __read_mostly > -- ~Randy
