Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754221AbXJIDEu (ORCPT ); Mon, 8 Oct 2007 23:04:50 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752311AbXJIDEm (ORCPT ); Mon, 8 Oct 2007 23:04:42 -0400 Received: from ms-smtp-03.nyroc.rr.com ([24.24.2.57]:50385 "EHLO ms-smtp-03.nyroc.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751704AbXJIDEm (ORCPT ); Mon, 8 Oct 2007 23:04:42 -0400 Date: Mon, 8 Oct 2007 23:04:12 -0400 From: Steven Rostedt To: Mike Kravetz Cc: Ingo Molnar , Linux Kernel Mailing List Subject: Re: -rt more realtime scheduling issues Message-ID: <20071009030412.GB12915@goodmis.org> References: <20071006021548.GE4587@monkey.ibm.com> <20071008184523.GA29656@monkey.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20071008184523.GA29656@monkey.ibm.com> User-Agent: Mutt/1.5.16 (2007-06-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2392 Lines: 64 On Mon, Oct 08, 2007 at 11:45:23AM -0700, Mike Kravetz wrote: > On Fri, Oct 05, 2007 at 07:15:48PM -0700, Mike Kravetz wrote: > > After applying the fix to try_to_wake_up() I was still seeing some large > > latencies for realtime tasks. > > I've been looking for places in the code where reschedule IPIs should > be sent in the case of 'overload' to redistribute RealTime tasks based > on priority. However, an even more basic question to ask might be: Are > the use of reschedule IPIs reliable enough for this purpose. In the > code, there is the following comment: > > /* > * this function sends a 'reschedule' IPI to another CPU. > * it goes straight through and wastes no time serializing > * anything. Worst case is that we lose a reschedule ... > */ > > After a quick read of the code, it does appear that reschedule's can > be lost if the the IPI is sent at just the right time in schedule > processing. Can someone confirm this is actually the case? > > The issue I see is that the 'rt_overload' mechanism depends on reschedule > IPIs for RealTime scheduling semantics. If this is not a reliable > mechanism then this can lead to breakdowns in RealTime scheduling semantics. > > Are these accurate statements? I'll start working on a reliable delivery > mechanism for RealTime scheduling. But, I just want to make sure that > is really necessary. For i386 I don't think so. Seems that the interrupt handler will set the current task to "need_resched" and on exit of the interrupt handler, the schedule should take place. I don't see the race (that doesn't mean there is one). For x86_64 though, I don't think that we schedule. All the reschedule vector does is return with a comment: /* * Reschedule call back. Nothing to do, * all the work is done automatically when * we return from the interrupt. */ asmlinkage void smp_reschedule_interrupt(void) { ack_APIC_irq(); } I'm thinking that this was the case for i386 a while back, and we fixed it for RT. /me does a quick search... http://lkml.org/lkml/2005/5/13/174 Yep! This is a bug in x86_64. I'll fix this up tomorrow and send out a patch. -- Steve - 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/