mm: expland documentation over __read_mostly
Subsystems (2)
PATCH 1/1 mm: expland 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 it use.
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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.
include/linux/cache.h | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)
include/linux/cache.h +8 -2
@@ -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
Signature · 1 line
2.25.1
Quoted 4 lines · Luis Chamberlain
__read_mostlycan 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
Quoted 11 lines
Signed-off-by Luis Chamberlain <[email protected]>---
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_mostlyon 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.
Quoted 27 lines
include/linux/cache.h | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)include/linux/cache.h +8
-2@@ -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_mostlySignature · 1 line
2.25.1
Quoted 1 line · Luis Chamberlain
+ * execute a critial path. We should be mindful and selective of its use.
"critical"