From: Davidlohr Bueso Date: Thu, 3 Jan 2019 23:27:09 +0000 (-0800) Subject: fs/epoll: drop ovflist branch prediction X-Git-Tag: v5.0-rc1~38^2~37 X-Git-Url: https://git.kernel.dk/?p=linux-2.6-block.git;a=commitdiff_plain;h=76699a67f3041ff4c7af6d6ee9be2bfbf1ffb671 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 Reviewed-by: Andrew Morton Cc: Al Viro Cc: Jason Baron Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/fs/eventpoll.c b/fs/eventpoll.c index aaf614ee08e4..fb7f05f4099d 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;