From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753435AbdJaO7X (ORCPT ); Tue, 31 Oct 2017 10:59:23 -0400 Received: from mx2.suse.de ([195.135.220.15]:56334 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751366AbdJaO7V (ORCPT ); Tue, 31 Oct 2017 10:59:21 -0400 Date: Tue, 31 Oct 2017 07:58:21 -0700 From: Davidlohr Bueso To: Jason Baron Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, Alexander Viro , Salman Qazi , Hou Tao Subject: Re: [PATCH] epoll: remove ep_call_nested() from ep_eventpoll_poll() Message-ID: <20171031145821.ieflhlvs3la25vdw@linux-n805> References: <1509430214-5599-1-git-send-email-jbaron@akamai.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <1509430214-5599-1-git-send-email-jbaron@akamai.com> User-Agent: NeoMutt/20170421 (1.8.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 31 Oct 2017, Jason Baron wrote: >The use of ep_call_nested() in ep_eventpoll_poll(), which is the .poll >routine for an epoll fd, is used to prevent excessively deep epoll >nesting, and to prevent circular paths. However, we are already preventing >these conditions during EPOLL_CTL_ADD. In terms of too deep epoll chains, >we do in fact allow deep nesting of the epoll fds themselves (deeper >than EP_MAX_NESTS), however we don't allow more than EP_MAX_NESTS when >an epoll file descriptor is actually connected to a wakeup source. Thus, >we do not require the use of ep_call_nested(), since ep_eventpoll_poll(), >which is called via ep_scan_ready_list() only continues nesting if there >are events available. Since ep_call_nested() is implemented using a global >lock, applications that make use of nested epoll can see large performance >improvements with this change. Improvements are quite obscene actually, such as for the following epoll_wait() benchmark with 2 level nesting on a 80 core IvyBridge: ncpus vanilla dirty delta 1 2447092 3028315 +23.75% 4 231265 2986954 +1191.57% 8 121631 2898796 +2283.27% 16 59749 2902056 +4757.07% 32 26837 2326314 +8568.30% 64 12926 1341281 +10276.61% (http://linux-scalability.org/epoll/epoll-test.c) Thanks, Davidlohr