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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 51CC4C433DF for ; Fri, 21 Aug 2020 01:42:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2E097208E4 for ; Fri, 21 Aug 2020 01:42:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726965AbgHUBmf (ORCPT ); Thu, 20 Aug 2020 21:42:35 -0400 Received: from mx2.suse.de ([195.135.220.15]:33604 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726819AbgHUBme (ORCPT ); Thu, 20 Aug 2020 21:42:34 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id DCE4CB1E7; Fri, 21 Aug 2020 01:43:00 +0000 (UTC) Date: Thu, 20 Aug 2020 18:27:50 -0700 From: Davidlohr Bueso To: Qian Cai Cc: akpm@linux-foundation.org, catalin.marinas@arm.com, oleg@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Davidlohr Bueso Subject: Re: [PATCH] mm/kmemleak: rely on rcu for task stack scanning Message-ID: <20200821012750.qxiklfhuaryajvhn@linux-p48b> Mail-Followup-To: Qian Cai , akpm@linux-foundation.org, catalin.marinas@arm.com, oleg@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Davidlohr Bueso References: <20200820203902.11308-1-dave@stgolabs.net> <20200821002554.GB4622@lca.pw> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20200821002554.GB4622@lca.pw> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 20 Aug 2020, Qian Cai wrote: >On Thu, Aug 20, 2020 at 01:39:02PM -0700, Davidlohr Bueso wrote: >> kmemleak_scan() currently relies on the big tasklist_lock >> hammer to stabilize iterating through the tasklist. Instead, >> this patch proposes simply using rcu along with the rcu-safe >> for_each_process_thread flavor (without changing scan semantics), >> which doesn't make use of next_thread/p->thread_group and thus >> cannot race with exit. Furthermore, any races with fork() >> and not seeing the new child should be benign as it's not >> running yet and can also be detected by the next scan. > >It is not entirely clear to me what problem the patch is trying to solve. If >this is about performance, we will probably need some number. So in this case avoiding the tasklist_lock could prove beneficial for performance considering the scan operation is done periodically. I have seen improvements of 30%-ish when doing similar replacements on very pathological microbenchmarks (ie stressing get/setpriority(2)). However my main motivation is that it's one less user of the global lock, something that Linus has long time wanted to see gone eventually (if ever) even if the traditional fairness issues has been dealt with now with qrwlocks. Of course this is a very long ways ahead. This patch also kills another user of the deprecated tsk->thread_group. Thanks, Davidlohr