From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2B06EC32789 for ; Thu, 8 Nov 2018 05:10:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F0EDE20857 for ; Thu, 8 Nov 2018 05:10:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F0EDE20857 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=stgolabs.net Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728985AbeKHOoI (ORCPT ); Thu, 8 Nov 2018 09:44:08 -0500 Received: from smtp2.provo.novell.com ([137.65.250.81]:52650 "EHLO smtp2.provo.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726145AbeKHOoH (ORCPT ); Thu, 8 Nov 2018 09:44:07 -0500 Received: from linux-r8p5.suse.de (prv-ext-foundry1int.gns.novell.com [137.65.251.240]) by smtp2.provo.novell.com with ESMTP (TLS encrypted); Wed, 07 Nov 2018 22:10:19 -0700 From: Davidlohr Bueso To: akpm@linux-foundation.org Cc: jbaron@akamai.com, viro@zeniv.linux.org.uk, dave@stgolabs.net, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Davidlohr Bueso Subject: [PATCH 3/6] fs/epoll: drop ovflist branch prediction Date: Wed, 7 Nov 2018 21:10:03 -0800 Message-Id: <20181108051006.18751-4-dave@stgolabs.net> X-Mailer: git-send-email 2.16.4 In-Reply-To: <20181108051006.18751-1-dave@stgolabs.net> References: <20181108051006.18751-1-dave@stgolabs.net> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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