From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757590AbZBEOfh (ORCPT ); Thu, 5 Feb 2009 09:35:37 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757225AbZBEOfJ (ORCPT ); Thu, 5 Feb 2009 09:35:09 -0500 Received: from mx2.mail.elte.hu ([157.181.151.9]:56335 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753180AbZBEOfH (ORCPT ); Thu, 5 Feb 2009 09:35:07 -0500 Date: Thu, 5 Feb 2009 15:34:53 +0100 From: Ingo Molnar To: Mandeep Singh Baines Cc: =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker , Peter Zijlstra , linux-kernel@vger.kernel.org, rientjes@google.com, mbligh@google.com, thockin@google.com, Andrew Morton Subject: Re: [PATCH 2/2 v4] softlockup: check all tasks in hung_task Message-ID: <20090205143453.GG28443@elte.hu> References: <20090204194339.GB22608@elte.hu> <20090205043548.GA18933@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20090205043548.GA18933@google.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Mandeep Singh Baines wrote: > The batch_count controls the preemptibility of hung_task. While it might > not make sense to expose the value to user-space, we may want to use a > different value for the PREEMPT config (not sure what the specific values > should be): > > #if defined(CONFIG_PREEMPT) && !defined(CONFIG_PREEMPT_RCU) > static const int check_count_batching = 256; > #else > static const int check_count_batching = 2048; > #endif That really is not worth it - and such #ifdefs always look ugly and split testing as well. I've applied your patch to tip:core/softlockup, thanks Mandeep! Ingo -------------> >>From 9952001914a7230ab88212a1e2d0e80208bdb907 Mon Sep 17 00:00:00 2001 From: Mandeep Singh Baines Date: Wed, 4 Feb 2009 20:35:48 -0800 Subject: [PATCH] softlockup: check all tasks in hung_task MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Impact: extend the scope of hung-task checks Changed the default value of hung_task_check_count to PID_MAX_LIMIT. hung_task_batch_count added to put an upper bound on the critical section. Every hung_task_batch_count checks, the rcu lock is never held for a too long time. Keeping the critical section small minimizes time preemption is disabled and keeps rcu grace periods small. To prevent following a stale pointer, get_task_struct is called on g and t. To verify that g and t have not been unhashed while outside the critical section, the task states are checked. The design was proposed by Frédéric Weisbecker. Signed-off-by: Mandeep Singh Baines Suggested-by: Frédéric Weisbecker Signed-off-by: Ingo Molnar --- kernel/hung_task.c | 39 +++++++++++++++++++++++++++++++++++++-- 1 files changed, 37 insertions(+), 2 deletions(-) diff --git a/kernel/hung_task.c b/kernel/hung_task.c index ba8ccd4..d32d293 100644 --- a/kernel/hung_task.c +++ b/kernel/hung_task.c @@ -17,9 +17,18 @@ #include /* - * Have a reasonable limit on the number of tasks checked: + * The number of tasks checked: */ -unsigned long __read_mostly sysctl_hung_task_check_count = 1024; +unsigned long __read_mostly sysctl_hung_task_check_count = PID_MAX_LIMIT; + +/* + * Limit number of tasks checked in a batch. + * + * This value controls the preemptibility of khungtaskd since preemption + * is disabled during the critical section. It also controls the size of + * the RCU grace period. So it needs to be upper-bound. + */ +static const int hung_task_batching = 1024; /* * Zero means infinite timeout - no checking done: @@ -109,6 +118,24 @@ static void check_hung_task(struct task_struct *t, unsigned long now, panic("hung_task: blocked tasks"); } + /* + * To avoid extending the RCU grace period for an unbounded amount of time, + * periodically exit the critical section and enter a new one. + * + * For preemptible RCU it is sufficient to call rcu_read_unlock in order + * exit the grace period. For classic RCU, a reschedule is required. + */ +static void rcu_lock_break(struct task_struct *g, struct task_struct *t) +{ + get_task_struct(g); + get_task_struct(t); + rcu_read_unlock(); + cond_resched(); + rcu_read_lock(); + put_task_struct(t); + put_task_struct(g); +} + /* * Check whether a TASK_UNINTERRUPTIBLE does not get woken up for * a really long time (120 seconds). If that happens, print out @@ -116,6 +143,7 @@ static void check_hung_task(struct task_struct *t, unsigned long now, */ static void check_hung_uninterruptible_tasks(unsigned long timeout) { + int batch_count = hung_task_batching; int max_count = sysctl_hung_task_check_count; unsigned long now = get_timestamp(); struct task_struct *g, *t; @@ -131,6 +159,13 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout) do_each_thread(g, t) { if (!--max_count) goto unlock; + if (!--batch_count) { + batch_count = hung_task_batching; + rcu_lock_break(g, t); + /* Exit if t or g was unhashed during refresh. */ + if (t->state == TASK_DEAD || g->state == TASK_DEAD) + goto unlock; + } /* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */ if (t->state == TASK_UNINTERRUPTIBLE) check_hung_task(t, now, timeout);