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Subject[PATCH 3/6] fs/epoll: drop ovflist branch prediction
Date
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 later 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 <dbueso@suse.de>
---
fs/eventpoll.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 101d46b81f64..347da3f4f5d3 100644
--- 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;
--
2.16.4
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