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=-3.7 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 C867CC433DB for ; Wed, 23 Dec 2020 12:03:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 88A9E223E8 for ; Wed, 23 Dec 2020 12:03:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728439AbgLWMC6 (ORCPT ); Wed, 23 Dec 2020 07:02:58 -0500 Received: from foss.arm.com ([217.140.110.172]:49412 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728307AbgLWMC5 (ORCPT ); Wed, 23 Dec 2020 07:02:57 -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 CE243101E; Wed, 23 Dec 2020 04:02:11 -0800 (PST) Received: from e107158-lin (unknown [10.1.194.78]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5FC9E3F6CF; Wed, 23 Dec 2020 04:02:10 -0800 (PST) Date: Wed, 23 Dec 2020 12:02:07 +0000 From: Qais Yousef To: Steven Rostedt Cc: Thomas Gleixner , LKML , Peter Zijlstra , Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira Subject: Re: sched: Reenable interrupts in do sched_yield() Message-ID: <20201223120207.csiynjfmzavveesf@e107158-lin> References: <87r1pt7y5c.fsf@nanos.tec.linutronix.de> <20201020113830.378b4a4c@gandalf.local.home> <87o8kw93n4.fsf@nanos.tec.linutronix.de> <20201020160732.5f8fc24e@oasis.local.home> <87h7qo6ntx.fsf@nanos.tec.linutronix.de> <20201021100714.5ba25a96@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20201021100714.5ba25a96@gandalf.local.home> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/21/20 10:07, Steven Rostedt wrote: > On Wed, 21 Oct 2020 09:27:22 +0200 > Thomas Gleixner wrote: > > > On Tue, Oct 20 2020 at 16:07, Steven Rostedt wrote: > > > On Tue, 20 Oct 2020 20:02:55 +0200 > > > Thomas Gleixner wrote: > > > What I wrote wasn't exactly what I meant. What I meant to have: > > > > > > /* > > > * Since we are going to call schedule() anyways, there's > > > * no need to do the preemption check when the rq_lock is released. > > > */ > > > > > > That is, to document why we have the preempt_disable() before the unlock: > > > > which is pretty obvious, but I let Peter decide on that. > > To us maybe, but I like to have comments that explain why things are done to > average people. ;-) > > If I go to another kernel developer outside the core kernel, would they know > why there's a preempt_disable() there? > > > preempt_disable(); > rq_unlock_irq(rq, &rf); > sched_preempt_enable_no_resched(); > > schedule(); > > > Not everyone knows that the rq_unlock_irq() would trigger a schedule if an > interrupt happened as soon as irqs were enabled again and need_resched was > set. Sorry a bit late to the party. Personally, what actually is tripping me off is that rq_unlock_irq() will end up calling preempt_enable(), and then we do sched_preempt_enable_no_resched(). Was there an earlier preempt_disable() called up in the chain that I couldn't figure out that's why it's okay to do the 2? Otherwise I see we have imbalanced preempt_disable/enable. preempt_disable() rq_unlock_irq() __raw_spin_unlock_irq() local_irq_enable() preempt_enable() // first preempt_count_dec() sched_preempt_enable_no_resched() // second preempt_count_dec() Thanks -- Qais Yousef