Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758170AbZCWPHt (ORCPT ); Mon, 23 Mar 2009 11:07:49 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757874AbZCWPHi (ORCPT ); Mon, 23 Mar 2009 11:07:38 -0400 Received: from fxip-0047f.externet.hu ([88.209.222.127]:43428 "EHLO pomaz-ex.szeredi.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757085AbZCWPHh (ORCPT ); Mon, 23 Mar 2009 11:07:37 -0400 To: linux-kernel@vger.kernel.org CC: mingo@elte.hu, oleg@redhat.com, peterz@infradead.org, roland@redhat.com, efault@gmx.de, rjw@sisk.pl, jdike@addtoit.com, user-mode-linux-devel@lists.sourceforge.net, torvalds@linux-foundation.org, akpm@linux-foundation.org, stable@kernel.org Subject: [patch] fix ptrace slowness Message-Id: From: Miklos Szeredi Date: Mon, 23 Mar 2009 16:07:24 +0100 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2119 Lines: 67 This one incorporates comments from Oleg and Ingo. Please apply to 2.6.29 and 2.6.2[78]-stable trees. Thanks, Miklos ---- From: Miklos Szeredi 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 --- kernel/signal.c | 8 ++++++++ 1 file changed, 8 insertions(+) Index: linux.git/kernel/signal.c =================================================================== --- linux.git.orig/kernel/signal.c 2009-03-20 09:41:04.000000000 +0100 +++ linux.git/kernel/signal.c 2009-03-23 15:40:57.000000000 +0100 @@ -1575,7 +1575,15 @@ 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 { /* -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/