On Wed, 2018-11-28 at 11:53 +1100, NeilBrown wrote:
> The kernel test robot reported two performance regressions
> caused by recent patches.
> Both appear to related to the global spinlock blocked_lock_lock
> being taken more often.
>
> This patch avoids taking that lock in the cases tested.
>
> Reported-by: kernel test robot <rong.a.c...@intel.com>
> Signed-off-by: NeilBrown <ne...@suse.com>
> ---
>
> Hi Jeff,
> you might like to merge these back into the patches that introduced
> the problem.
> Or you might like me to re-send the series with these merged in,
> in which case, please ask.
>
Thanks Neil,
This looks great. I'll go ahead and toss this patch on top of the pile
in linux-next for now.
Would you mind resending the series with this patch merged in? I took a
quick stab at squashing it into the earlier patch, but there is some
churn in this area.
Maybe you can also turn that Reported-by: into a Tested-by: in the
changelog afterward?
> And a BIG thank-you to the kernel-test-robot team!!
>
Absolutely! We love you guys!
> Thanks,
> NeilBrown
>
> fs/locks.c | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
>
> diff --git a/fs/locks.c b/fs/locks.c
> index f456cd3d9d50..67519a43e27a 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -444,6 +444,13 @@ static void locks_move_blocks(struct file_lock *new,
> struct file_lock *fl)
> {
> struct file_lock *f;
>
> + /*
> + * As ctx->flc_lock is held, new requests cannot be added to
> + * ->fl_blocked_requests, so we don't need a lock to check if it
> + * is empty.
> + */
> + if (list_empty(&fl->fl_blocked_requests))
> + return;
> spin_lock(&blocked_lock_lock);
> list_splice_init(&fl->fl_blocked_requests, &new->fl_blocked_requests);
> list_for_each_entry(f, &fl->fl_blocked_requests, fl_blocked_member)
> @@ -749,6 +756,20 @@ int locks_delete_block(struct file_lock *waiter)
> {
> int status = -ENOENT;
>
> + /*
> + * If fl_blocker is NULL, it won't be set again as this thread
> + * "owns" the lock and is the only one that might try to claim
> + * the lock. So it is safe to test fl_blocker locklessly.
> + * Also if fl_blocker is NULL, this waiter is not listed on
> + * fl_blocked_requests for some lock, so no other request can
> + * be added to the list of fl_blocked_requests for this
> + * request. So if fl_blocker is NULL, it is safe to
> + * locklessly check if fl_blocked_requests is empty. If both
> + * of these checks succeed, there is no need to take the lock.
> + */
> + if (waiter->fl_blocker == NULL &&
> + list_empty(&waiter->fl_blocked_requests))
> + return status;
> spin_lock(&blocked_lock_lock);
> if (waiter->fl_blocker)
> status = 0;
--
Jeff Layton <jlay...@redhat.com>
