On Thu, Aug 22, 2024 at 9:15 AM Mateusz Guzik [off-list ref] wrote:

The refcount shares the cacheline with uid, gid and other frequently
read fields.

Said counter gest modified a lot (for example every time a file is
closed or opened) in turn causing avoidable bounces when another thread
tries to do permission checks. Said bouncing can be avoided without
growing the struct by reordering the fields -- keyring (enabled by
default) is comparatively rarely used and can suffer bouncing instead.

An additional store is performed to clear the non_rcu flag. Since the
flag is rarely set (a special case in the access(2) system call) and
transitions at most once, it can get placed in a read-mostly area and is
only conditionally written to.

With this in place regular permission checks no longer bounce cachelines
in face of refcount changes.

Validated with a simple test where one thread opens and closes a file
(dirtying creds twice), while another keeps re-reading *another* file in
a loop (ops/s):
before: 4353763
after:  4742792 (+9%)

Signed-off-by: Mateusz Guzik <redacted>
---

This is a resend with a reworded commit message and added Linus since he
wrote the non_rcu thing.

Note each process has its on creds, so this is not causing bounces
globally.

Even so, there is stuff I plan to do in the area and this patch can be
considered prep (only one store to non_rcu).

I'll also note I don't see a way to *whack* non_rcu either. :)

0 rush

 fs/open.c            |  2 +-
 include/linux/cred.h | 31 +++++++++++++++----------------
 kernel/cred.c        |  6 +++---
 3 files changed, 19 insertions(+), 20 deletions(-)

[NOTE: no comment on the patch context yet, just process comments below]

FWIW, I really haven't commented on these last couple of cred patches
mostly because I've been assuming someone else would emerge from the
shadows as the cred maintainer, or at least someone who felt strongly
enough about these changes would merge them.  Unfortunately, I worry
that isn't really going to happen and I'd hate for some of the cred
improvements to die on the lists.

If no one starts grabbing your cred patches I can start taking cred
patches as part of the LSM tree, I've done it a couple of times in the
past and Linus didn't seem to mind.

-- 
paul-moore.com