fs/epoll: drop ovflist branch prediction

The ep->ovflist is a secondary ready-list to temporarily store events
that might occur when doing sproc without holding the ep->wq.lock.  This
accounts for every time we check for ready events and also send events
back to userspace; both callbacks, particularly the latter because of
copy_to_user, can account for a non-trivial time.

As such, the unlikely() check to see if the pointer is being used, seems
both misleading and sub-optimal.  In fact, we go to an awful lot of
trouble to sync both lists, and populating the ovflist is far from an
uncommon scenario.

For example, profiling a concurrent epoll_wait(2) benchmark, with
CONFIG_PROFILE_ANNOTATED_BRANCHES shows that for a two threads a 33%
incorrect rate was seen; and when incrementally increasing the number of
epoll instances (which is used, for example for multiple queuing load
balancing models), up to a 90% incorrect rate was seen.

Similarly, by deleting the prediction, 3% throughput boost was seen
across incremental threads.

Link: http://lkml.kernel.org/r/20181108051006.18751-4-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jason Baron <jbaron@akamai.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index aaf614e..fb7f05f 100644 (file)
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1153,7 +1153,7 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
         * semantics). All the events that happen during that period of time are
         * chained in ep->ovflist and requeued later on.
         */
-       if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
+       if (ep->ovflist != EP_UNACTIVE_PTR) {
                if (epi->next == EP_UNACTIVE_PTR) {
                        epi->next = ep->ovflist;
                        ep->ovflist = epi;