4.9-stable review patch. If anyone has any objections, please let me know.
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[ 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@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 Signed-off-by: Sasha Levin sashal@kernel.org --- fs/eventpoll.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 3cbc30413add..a9c0bf8782f5 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1040,7 +1040,7 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k * 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;