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Date: Mon, 18 Feb 2019 14:43:50 +0100 From: Greg Kroah-Hartman <gregkh@...uxfoundation.org> To: linux-kernel@...r.kernel.org Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, stable@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>, Andrew Morton <akpm@...ux-foundation.org>, Al Viro <viro@...iv.linux.org.uk>, Jason Baron <jbaron@...mai.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Sasha Levin <sashal@...nel.org> Subject: [PATCH 3.18 054/108] fs/epoll: drop ovflist branch prediction 3.18-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@stgolabs.net Signed-off-by: Davidlohr Bueso <dbueso@...e.de> Reviewed-by: Andrew Morton <akpm@...ux-foundation.org> Cc: Al Viro <viro@...iv.linux.org.uk> Cc: Jason Baron <jbaron@...mai.com> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org> Signed-off-by: Sasha Levin <sashal@...nel.org> --- fs/eventpoll.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 2b5285c40b40..8e50ef617d85 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1037,7 +1037,7 @@ 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; -- 2.19.1
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