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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=no 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 B5540C433E9 for ; Fri, 12 Mar 2021 20:30:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 87A7F64F87 for ; Fri, 12 Mar 2021 20:30:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234918AbhCLU3t (ORCPT ); Fri, 12 Mar 2021 15:29:49 -0500 Received: from out02.mta.xmission.com ([166.70.13.232]:52704 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234673AbhCLU3l (ORCPT ); Fri, 12 Mar 2021 15:29:41 -0500 Received: from in02.mta.xmission.com ([166.70.13.52]) by out02.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1lKoPk-0000qd-Qy; Fri, 12 Mar 2021 13:29:40 -0700 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=fess.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1lKoPj-006Zm2-Ts; Fri, 12 Mar 2021 13:29:40 -0700 From: ebiederm@xmission.com (Eric W. Biederman) To: Jim Newsome Cc: Andrew Morton , Oleg Nesterov , Christian Brauner , linux-kernel@vger.kernel.org References: <20210312173855.24843-1-jnewsome@torproject.org> Date: Fri, 12 Mar 2021 14:29:46 -0600 In-Reply-To: <20210312173855.24843-1-jnewsome@torproject.org> (Jim Newsome's message of "Fri, 12 Mar 2021 11:38:55 -0600") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1lKoPj-006Zm2-Ts;;;mid=;;;hst=in02.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/qwjM8dl0Gb1QwnotGtvTTQxD9aoOPQ9s= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH v5] do_wait: make PIDTYPE_PID case O(1) instead of O(n) X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Jim Newsome writes: > do_wait is an internal function used to implement waitpid, waitid, > wait4, etc. To handle the general case, it does an O(n) linear scan of > the thread group's children and tracees. > > This patch adds a special-case when waiting on a pid to skip these scans > and instead do an O(1) lookup. This improves performance when waiting on > a pid from a thread group with many children and/or tracees. I am going to kibitz just a little bit more. When I looked at this a second time it became apparent that using pid_task twice should actually be faster as it removes a dependent load caused by thread_group_leader, and replaces it by accessing two adjacent pointers in the same cache line. I know the algorithmic improvement is the main advantage, but removing 60ns or so for a dependent load can't hurt. Plus I think using the two pid types really makes it clear that one is always a process and the other is always potentially a thread. /* * Optimization for waiting on PIDTYPE_PID. No need to iterate through child * and tracee lists to find the target task. */ static int do_wait_pid(struct wait_opts *wo) { bool ptrace; struct task_struct *target; int retval; ptrace = false; target = pid_task(wo->wo_pid, PIDTYPE_TGID); if (target && is_effectively_child(wo, ptrace, target)) { retval = wait_consider_task(wo, ptrace, target); if (retval) return retval; } ptrace = true; target = pid_task(wo->wo_pid, PIDTYPE_PID); if (target && target->ptrace && is_effectively_child(wo, ptrace, target)) { retval = wait_consider_task(wo, ptrace, target); if (retval) return retval; } return 0; } Since the probably needs to be respun to include the improved description can we look at my micro performance improvement? Eric