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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 9D485FA372C for ; Fri, 8 Nov 2019 11:48:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 74CFB222CE for ; Fri, 8 Nov 2019 11:48:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2391725AbfKHLsT (ORCPT ); Fri, 8 Nov 2019 06:48:19 -0500 Received: from foss.arm.com ([217.140.110.172]:41468 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389983AbfKHLry (ORCPT ); Fri, 8 Nov 2019 06:47:54 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 00B0531B; Fri, 8 Nov 2019 03:47:54 -0800 (PST) Received: from [10.0.2.15] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 604333F719; Fri, 8 Nov 2019 03:47:51 -0800 (PST) Subject: Re: NULL pointer dereference in pick_next_task_fair To: Quentin Perret , Peter Zijlstra Cc: Kirill Tkhai , linux-kernel@vger.kernel.org, aaron.lwe@gmail.com, mingo@kernel.org, pauld@redhat.com, jdesfossez@digitalocean.com, naravamudan@digitalocean.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, juri.lelli@redhat.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, kernel-team@android.com, john.stultz@linaro.org References: <20191028174603.GA246917@google.com> <20191106120525.GX4131@hirez.programming.kicks-ass.net> <33643a5b-1b83-8605-2347-acd1aea04f93@virtuozzo.com> <20191106165437.GX4114@hirez.programming.kicks-ass.net> <20191106172737.GM5671@hirez.programming.kicks-ass.net> <831c2cd4-40a4-31b2-c0aa-b5f579e770d6@virtuozzo.com> <20191107132628.GZ4114@hirez.programming.kicks-ass.net> <20191107153848.GA31774@google.com> <20191107184356.GF4114@hirez.programming.kicks-ass.net> <20191107192907.GA30258@worktop.programming.kicks-ass.net> <20191108110212.GA204618@google.com> From: Valentin Schneider Message-ID: <07d30588-22e6-e098-b591-29c7cd3c8054@arm.com> Date: Fri, 8 Nov 2019 11:47:44 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20191108110212.GA204618@google.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/11/2019 11:02, Quentin Perret wrote: >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index a14487462b6c..6b983214e00f 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -6746,10 +6746,18 @@ done: __maybe_unused; >> return NULL; >> } >> >> +static int balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) >> +{ >> + if (rq->cfs.nr_running) >> + return 1; >> + >> + return newidle_balance(rq, rf) != 0; > > And you can ignore the RETRY_TASK case here under the assumption that > we must have tried to pull from RT/DL before ending up here ? > I think we can ignore RETRY_TASK because this happens before the picking loop, so we'll observe any new DL/RT task that got enqueued while newidle released the lock. This also means we can safely break the balance loop in pick_next_task() when we get RETRY_TASK, because we've got something to pick (some new RT/DL task). This wants a comment though, methinks. Other than that I agree with Quentin, it's a much cleaner approach and I quite like it. > Thanks, > Quentin >