From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932879Ab2AEVpU (ORCPT ); Thu, 5 Jan 2012 16:45:20 -0500 Received: from merlin.infradead.org ([205.233.59.134]:49627 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758290Ab2AEVpT convert rfc822-to-8bit (ORCPT ); Thu, 5 Jan 2012 16:45:19 -0500 Message-ID: <1325799904.3508.12.camel@twins> Subject: Re: [PATCH] sched_rt: the task in irq context can be migrated during context switching From: Peter Zijlstra To: Steven Rostedt Cc: Chanho Min , linux-kernel@vger.kernel.org, Ingo Molnar , chanho.min@lge.com Date: Thu, 05 Jan 2012 22:45:04 +0100 In-Reply-To: <1325787342.12696.59.camel@gandalf.stny.rr.com> References: <1325786132.3508.1.camel@twins> <1325787342.12696.59.camel@gandalf.stny.rr.com> Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT X-Mailer: Evolution 3.2.1- Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2012-01-05 at 13:15 -0500, Steven Rostedt wrote: > On Thu, 2012-01-05 at 18:55 +0100, Peter Zijlstra wrote: > > > So the problem is quite real, as already said we don't need to worry > > about the future, but we might want to fix this in previous kernels. > > What I'm not entirely sure of is the proposed solution, Steven don't we > > get in trouble by simply bailing out on the push? > > It shouldn't break anything. We shouldn't be pushing tasks that are > running on a rq anyway. Its not running, but its in the middle of getting scheduled out. > I don't see any harm here. As this scenario can > only happen if we get an interrupt after letting go of the rq lock and > before doing the switch_to(). The schedule_tail() calls > post_schedule_rt() which does the push again, and will push task A at > that time. Right, so the post_schedule() hook will try again. > That said, I'm not sure this patch is enough. I'm worried about a pull > happening. As task A is running, we could possible possibly pick it on > another CPU to do a pull. > > Hmm, looking at the code, the pull already does a task_running() test, > so I guess we should be fine. Yeah, I'm not sure all those task_running() things make sense though, when !->on_rq && ->on_cpu we should busy wait for tasks, not skip them. Then again, with this WANT_INTERRUPTS_ON_CTXSW the busy wait crap is tricky. Luckily its going the way of the Dodo very soon.