[PATCH 4.20 286/352] fs/epoll: drop ovflist branch prediction

From: Greg Kroah-Hartman
Date: Mon Feb 11 2019 - 09:34:43 EST

4.20-stable review patch. If anyone has any objections, please let me know.


[ Upstream commit 76699a67f3041ff4c7af6d6ee9be2bfbf1ffb671 ]

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@xxxxxxxxxxxx
Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Jason Baron <jbaron@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
fs/eventpoll.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 42bbe6824b4b..58f48ea0db23 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1154,7 +1154,7 @@ 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;