From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761690AbZD2Wln (ORCPT ); Wed, 29 Apr 2009 18:41:43 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759723AbZD2WV5 (ORCPT ); Wed, 29 Apr 2009 18:21:57 -0400 Received: from kroah.org ([198.145.64.141]:40771 "EHLO coco.kroah.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760322AbZD2WVw (ORCPT ); Wed, 29 Apr 2009 18:21:52 -0400 X-Mailbox-Line: From gregkh@mini.kroah.org Wed Apr 29 15:09:30 2009 Message-Id: <20090429220930.695005091@mini.kroah.org> User-Agent: quilt/0.48-1 Date: Wed, 29 Apr 2009 15:07:51 -0700 From: Greg KH To: linux-kernel@vger.kernel.org, stable@kernel.org Cc: Justin Forbes , Zwane Mwaikambo , "Theodore Ts'o" , Randy Dunlap , Dave Jones , Chuck Wolber , Chris Wedgwood , Michael Krufky , Chuck Ebbert , Domenico Andreoli , Willy Tarreau , Rodrigo Rubira Branco , Jake Edge , Eugene Teo , torvalds@linux-foundation.org, akpm@linux-foundation.org, alan@lxorguk.ukuu.org.uk, Miklos Szeredi Subject: [patch 52/58] fix ptrace slowness References: <20090429220659.339950874@mini.kroah.org> Content-Disposition: inline; filename=fix-ptrace-slowness.patch In-Reply-To: <20090429221657.GA11765@kroah.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2.6.27-stable review patch. If anyone has any objections, please let us know. ------------------ From: Miklos Szeredi commit 53da1d9456fe7f87a920a78fdbdcf1225d197cb7 upstream. This patch fixes bug #12208: Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=12208 Subject : uml is very slow on 2.6.28 host This turned out to be not a scheduler regression, but an already existing problem in ptrace being triggered by subtle scheduler changes. The problem is this: - task A is ptracing task B - task B stops on a trace event - task A is woken up and preempts task B - task A calls ptrace on task B, which does ptrace_check_attach() - this calls wait_task_inactive(), which sees that task B is still on the runq - task A goes to sleep for a jiffy - ... Since UML does lots of the above sequences, those jiffies quickly add up to make it slow as hell. This patch solves this by not rescheduling in read_unlock() after ptrace_stop() has woken up the tracer. Thanks to Oleg Nesterov and Ingo Molnar for the feedback. Signed-off-by: Miklos Szeredi CC: stable@kernel.org Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- kernel/signal.c | 8 ++++++++ 1 file changed, 8 insertions(+) --- a/kernel/signal.c +++ b/kernel/signal.c @@ -1549,7 +1549,15 @@ static void ptrace_stop(int exit_code, i read_lock(&tasklist_lock); if (may_ptrace_stop()) { do_notify_parent_cldstop(current, CLD_TRAPPED); + /* + * Don't want to allow preemption here, because + * sys_ptrace() needs this task to be inactive. + * + * XXX: implement read_unlock_no_resched(). + */ + preempt_disable(); read_unlock(&tasklist_lock); + preempt_enable_no_resched(); schedule(); } else { /*