[lkml]   [2019]   [Feb]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
Subject[PATCH 4.14 162/205] fs/epoll: drop ovflist branch prediction
4.14-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.

Signed-off-by: Davidlohr Bueso <>
Reviewed-by: Andrew Morton <>
Cc: Al Viro <>
Cc: Jason Baron <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Sasha Levin <>
fs/eventpoll.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 2fabd19cdeea..c291bf61afb9 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1167,7 +1167,7 @@ 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;

 \ /
  Last update: 2019-02-11 16:02    [W:0.543 / U:1.424 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site