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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, 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 38113C3F2D1 for ; Fri, 6 Mar 2020 01:40:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 055F920870 for ; Fri, 6 Mar 2020 01:40:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1583458830; bh=vAaXrFvvBzKC59uTVSh9ANdsT23g+5eypfwNYZ3kHAo=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=rrKOVKhFAMF7gVnFbKOj/GvhCeievTrIjzGJq/4c9bSXK6PNMgCWbyivQlgwYjNpL LUu7ItweYGRZxVgO+fXVZoVXkJ4Onind9lvSAkWgOmgq0eq8fd1Tg/VWuyWLudDVn6 jDlyW8oU/68nU9GglZ8jWYoa/F4xdwMyVkvfHpCE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726769AbgCFBk2 (ORCPT ); Thu, 5 Mar 2020 20:40:28 -0500 Received: from mail.kernel.org ([198.145.29.99]:39214 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726162AbgCFBk2 (ORCPT ); Thu, 5 Mar 2020 20:40:28 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 6208F2073B; Fri, 6 Mar 2020 01:40:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1583458827; bh=vAaXrFvvBzKC59uTVSh9ANdsT23g+5eypfwNYZ3kHAo=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=yhCBsvj0ROX4kmrkD2N2GOh7aZBfUK7c8brZu0wycjM3bsMV9JOwaq0yWEZEB7dXT G8Yjr2SAcIF/f2taE0xhWc3DZGdATLHEL2lKn8nFTkrqFeFFDrnFaPqetXpVhlCrq+ zWQqBt9NwKgJsef9+lYfKQW//d56ml3Y2f84G44U= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 38F203522806; Thu, 5 Mar 2020 17:40:27 -0800 (PST) Date: Thu, 5 Mar 2020 17:40:27 -0800 From: "Paul E. McKenney" To: Steven Rostedt Cc: Peter Zijlstra , mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, bsegall@google.com, mgorman@suse.de, linux-kernel@vger.kernel.org Subject: Re: Pinning down a blocked task to extract diagnostics Message-ID: <20200306014027.GA11942@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200305005049.GA21120@paulmck-ThinkPad-P72> <20200305080755.GS2596@hirez.programming.kicks-ass.net> <20200305081337.GA2619@hirez.programming.kicks-ass.net> <20200305142245.GB2935@paulmck-ThinkPad-P72> <20200305092845.4296c35e@gandalf.local.home> <20200305153638.GC2935@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200305153638.GC2935@paulmck-ThinkPad-P72> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 05, 2020 at 07:36:38AM -0800, Paul E. McKenney wrote: > On Thu, Mar 05, 2020 at 09:28:45AM -0500, Steven Rostedt wrote: > > On Thu, 5 Mar 2020 06:22:45 -0800 > > "Paul E. McKenney" wrote: > > > > > On Thu, Mar 05, 2020 at 09:13:37AM +0100, Peter Zijlstra wrote: [ . . . ] > > > How about if I add something like this, located right by try_to_wake_up()? > > > > > > bool try_to_keep_sleeping(struct task_struct *t) > > > { > > > raw_spin_lock_irq(&t->pi_lock); > > > switch (t->state) { > > > case TASK_RUNNING: > > > case TASK_WAKING: > > > raw_spin_unlock_irq(&t->pi_lock); > > > return false; > > > > > > default: > > > if (t->on_rq) { > > > > Somehow I think there still needs to be a read barrier before the test to > > on_rq. > > This is nowhere near a fastpath, so if there is uncertainty it gets > the smp_rmb(). Or an smp_load_acquire() on t->state. > > > > raw_spin_unlock_irq(&t->pi_lock); > > > return false; > > > } > > > > > > /* OK to extract consistent diagnostic information. */ > > > return true; > > > } > > > /* NOTREACHED */ > > > } > > > > > > Then a use might look like this: > > > > > > if (try_to_keep_sleeping(t)) > > > /* Extract consistent diagnostic information. */ > > > raw_spin_unlock_irq(&t->pi_lock); > > > > Perhaps we should have a allow_awake(t) to match it? > > > > allow_awake(t); > > > > Where we have: > > > > static inline allow_awake(struct task_struct *t) > > { > > raw_spin_unlock_irq(&t->pi_lock); > > } > > Makes sense to me! So how about like this? Thanx, Paul ------------------------------------------------------------------------ commit e2821ae6c6a6adaabc89ccd9babf4375a78e0626 Author: Paul E. McKenney Date: Thu Mar 5 16:53:58 2020 -0800 sched/core: Add functions to prevent sleepers from awakening In some cases, it is necessary to examine a consistent version of a sleeping process's state, in other words, it is necessary to keep that process in sleeping state. This commit therefore provides a try_to_keep_sleeping() function that acquires ->pi_lock to prevent wakeups from proceeding, returning true if the function is still asleep, and otherwise releasing ->pi_lock and returning false. This commit also provides an allow_awake() function (as suggested by by Steven Rostedt) that reverses the effect of a successful call to try_to_keep_sleeping(), allowing the process to once again be awakened. Signed-off-by: Paul E. McKenney [ paulmck: Apply feedback from Peter Zijlstra and Steven Rostedt. ] Cc: Ingo Molnar Cc: Peter Zijlstra Cc: Juri Lelli Cc: Vincent Guittot Cc: Dietmar Eggemann Cc: Steven Rostedt Cc: Ben Segall Cc: Mel Gorman diff --git a/include/linux/wait.h b/include/linux/wait.h index 3283c8d..aefea4a 100644 --- a/include/linux/wait.h +++ b/include/linux/wait.h @@ -1148,4 +1148,7 @@ int autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, i (wait)->flags = 0; \ } while (0) +bool try_to_keep_sleeping(struct task_struct *p); +void allow_awake(struct task_struct *p); + #endif /* _LINUX_WAIT_H */ diff --git a/kernel/sched/core.c b/kernel/sched/core.c index fc1dfc0..b665ff7 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2654,6 +2654,48 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) } /** + * try_to_keep_sleeping - Attempt to force task to remain off runqueues + * @p: The process to remain asleep. + * + * Acquires the process's ->pi_lock and checks state. If the process + * is still blocked, returns @true and leave ->pi_lock held, otherwise + * releases ->pi_locked and returns @false. + */ +bool try_to_keep_sleeping(struct task_struct *p) +{ + lockdep_assert_irqs_enabled(); + raw_spin_lock_irq(&p->pi_lock); + switch (p->state) { + case TASK_RUNNING: + case TASK_WAKING: + raw_spin_unlock_irq(&p->pi_lock); + return false; + + default: + smp_rmb(); /* See comments in try_to_wake_up(). */ + if (p->on_rq) { + raw_spin_unlock_irq(&p->pi_lock); + return false; + } + return true; /* Process is now stuck in blocked state. */ + } + /* NOTREACHED */ +} + +/** + * allow_awake - Allow a kept-sleeping process to awaken + * @p: Process to be allowed to awaken. + * + * Given that @p was passed to an earlier call to try_to_keep_sleeping + * that returned @true, hence preventing @p from waking up, allow @p + * to once again be awakened. + */ +void allow_awake(struct task_struct *p) +{ + raw_spin_unlock_irq(&p->pi_lock); +} + +/** * wake_up_process - Wake up a specific process * @p: The process to be woken up. *