From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755449AbZEKPFf (ORCPT ); Mon, 11 May 2009 11:05:35 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753456AbZEKPFY (ORCPT ); Mon, 11 May 2009 11:05:24 -0400 Received: from mx2.redhat.com ([66.187.237.31]:54132 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753340AbZEKPFX (ORCPT ); Mon, 11 May 2009 11:05:23 -0400 Message-ID: <4A083E9D.2040003@redhat.com> Date: Mon, 11 May 2009 18:05:01 +0300 From: Avi Kivity User-Agent: Thunderbird 2.0.0.21 (X11/20090320) MIME-Version: 1.0 To: Peter Zijlstra CC: Ingo Molnar , Mark Langsdorf , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH][KVM][retry 1] Add support for Pause Filtering to AMD SVM References: <200905050909.58583.mark.langsdorf@amd.com> <20090507135522.GJ4059@amd.com> <200905071000.14038.mark.langsdorf@amd.com> <4A02FECC.6060609@redhat.com> <20090511141503.GC6175@elte.hu> <4A083539.407@redhat.com> <1242052937.11251.275.camel@twins> In-Reply-To: <1242052937.11251.275.camel@twins> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Peter Zijlstra wrote: > On Mon, 2009-05-11 at 17:24 +0300, Avi Kivity wrote: > > >>> I.e. this is a somewhat poor solution as far as scheduling goes. But >>> i'm wondering what the CPU side does. Can REP-NOP really take >>> thousands of cycles? If yes, under what circumstances? >>> >>> >> The guest is running rep-nop in a loop while trying to acquire a >> spinlock. The hardware detects this (most likely, repeated rep-nop with >> the same rip) and exits. We can program the loop count; obviously if >> we're spinning for only a short while it's better to keep spinning while >> hoping the lock will be released soon. >> >> The idea is to detect that the guest is not making forward progress and >> yield. If I could tell the scheduler, you may charge me a couple of >> milliseconds, I promise not to sue, that would be ideal. Other tasks >> can become eligible, hopefully the task holding the spinlock, and by the >> time we're scheduled back the long running task will have finished and >> released the lock. >> >> For newer Linux as a guest we're better off paravirtualizing this, so we >> can tell the host which vcpu holds the lock; in this case kvm will want >> to say, take a couple milliseconds off my account and transfer it to >> this task (so called directed yield). However there's no reason to >> paravirtualize all cpu_relax() calls. >> > > So we're now officially giving up on (soft) realtime virtualization? > > Wouldn't realtime guests be in a realtime scheduling class? That ought to ignore this time_yield() (or however it is eventually named). -- error compiling committee.c: too many arguments to function