On Mon, 2020-03-09 at 13:22 -0400, Jeff Layton wrote: > On Mon, 2020-03-09 at 08:52 -0700, Linus Torvalds wrote: > > On Mon, Mar 9, 2020 at 7:36 AM Jeff Layton wrote: > > > On Sun, 2020-03-08 at 22:03 +0800, kernel test robot wrote: > > > > FYI, we noticed a -96.6% regression of will-it-scale.per_process_ops due to commit: > > > > > > This is not completely unexpected as we're banging on the global > > > blocked_lock_lock now for every unlock. This test just thrashes file > > > locks and unlocks without doing anything in between, so the workload > > > looks pretty artificial [1]. > > > > > > It would be nice to avoid the global lock in this codepath, but it > > > doesn't look simple to do. I'll keep thinking about it, but for now I'm > > > inclined to ignore this result unless we see a problem in more realistic > > > workloads. > > > > That is a _huge_ regression, though. > > > > What about something like the attached? Wouldn't that work? And make > > the code actually match the old comment about wow "fl_blocker" being > > NULL being special. > > > > The old code seemed to not know about things like memory ordering either. > > > > Patch is entirely untested, but aims to have that "smp_store_release() > > means I'm done and not going to touch it any more", making that > > smp_load_acquire() test hopefully be valid as per the comment.. > > Yeah, something along those lines maybe. I don't think we can use > fl_blocker that way though, as the wait_event_interruptible is waiting > on it to go to NULL, and the wake_up happens before fl_blocker is > cleared. > > Maybe we need to mix in some sort of FL_BLOCK_ACTIVE flag and use that > instead of testing for !fl_blocker to see whether we can avoid the > blocked_lock_lock? > How about something like this instead? (untested other than for compilation) Basically, this just switches the waiters over to wait for fl_blocked_member to go empty. That still happens before the wakeup, so it should be ok to wait on that. I think we can also eliminate the lockless list_empty check in locks_delete_block, as the fl_blocker check should be sufficient now. -- Jeff Layton