From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763162AbXGFOzo (ORCPT ); Fri, 6 Jul 2007 10:55:44 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1762270AbXGFOzf (ORCPT ); Fri, 6 Jul 2007 10:55:35 -0400 Received: from mail.screens.ru ([213.234.233.54]:55284 "EHLO mail.screens.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754743AbXGFOzf (ORCPT ); Fri, 6 Jul 2007 10:55:35 -0400 Date: Fri, 6 Jul 2007 18:56:34 +0400 From: Oleg Nesterov To: Mathieu Desnoyers Cc: Ingo Molnar , Steven Rostedt , linux-kernel@vger.kernel.org Subject: Re: [RFC] Thread Migration Preemption Message-ID: <20070706145634.GA198@tv-sign.ru> References: <20070706060257.GA188@tv-sign.ru> <20070706142339.GA32754@Krystal> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070706142339.GA32754@Krystal> User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On 07/06, Mathieu Desnoyers wrote: > > * Oleg Nesterov (oleg@tv-sign.ru) wrote: > > Mathieu Desnoyers wrote: > > > > > > This patch adds the ability to protect critical sections from migration to > > > another CPU without disabling preemption. > > > > > > Typical use: > > > > > > migration_disable(); > > > local_inc(&__get_cpu_var(&my_local_t_var)); > > > migration_enable(); > > > > > > --- linux-2.6-lttng.orig/kernel/sched.c 2007-07-05 16:28:15.000000000 -0400 > > > +++ linux-2.6-lttng/kernel/sched.c 2007-07-05 16:53:24.000000000 -0400 > > > @@ -1996,6 +1996,7 @@ > > > * 1) running (obviously), or > > > * 2) cannot be migrated to this CPU due to cpus_allowed, or > > > * 3) are cache-hot on their current CPU. > > > + * 4) migration preemption is non 0 for this non running task. > > > */ > > > if (!cpu_isset(this_cpu, p->cpus_allowed)) > > > return 0; > > > @@ -2003,6 +2004,8 @@ > > > > > > if (task_running(rq, p)) > > > return 0; > > > + if (task_thread_info(p)->migration_count) > > > + return 0; > > > > Hi Oleg, > > > Question: This means that the task could be preempted, but can't sleep, yes? > > > > No exactly. This means that the task can be preempted, and can therefore > sleep, but we do not allow this sleeping task to be migrated to a > different CPU. > > > Because try_to_wake_up() can change ->cpu. Shouldn't might_sleep() check > > ->migration_count then? Or we can change try_to_wake_up(). > > > > If we look at try_to_wake_up, it seems to take the new_cpu from > task_cpu(), which comes from the thread info ->cpu, updated by > set_task_cpu(). The *_move_tasks seems to be responsible for calling > can_migrate_task() to see if, in a deferred way, the migration_thread > can use set_task_cpu() to change the task's cpu. Hmm. Perhaps I missed something, perhaps you... Or we both :) try_to_wake_up() can choose new_cpu which differs from initial ->cpu. Then it does set_task_cpu() which just sets ->cpu, it doesn't check can_migrate_task(). This means that the task could be waken on another CPU, even if it sleeps under migration_disable(). Btw, I think you should also fix debug_smp_processor_id(), it won't like migration_disable() + __get_cpu_var(). Oleg.