From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750968AbbEBEGZ (ORCPT ); Sat, 2 May 2015 00:06:25 -0400 Received: from mail-wi0-f178.google.com ([209.85.212.178]:34783 "EHLO mail-wi0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750785AbbEBEGX (ORCPT ); Sat, 2 May 2015 00:06:23 -0400 Message-ID: <1430539581.3137.61.camel@gmail.com> Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry From: Mike Galbraith To: Rik van Riel Cc: Ingo Molnar , Andy Lutomirski , "linux-kernel@vger.kernel.org" , X86 ML , williams@redhat.com, Andrew Lutomirski , fweisbec@redhat.com, Peter Zijlstra , Heiko Carstens , Thomas Gleixner , Ingo Molnar , Paolo Bonzini Date: Sat, 02 May 2015 06:06:21 +0200 In-Reply-To: <5543C05E.9040209@redhat.com> References: <1430429035-25563-1-git-send-email-riel@redhat.com> <1430429035-25563-4-git-send-email-riel@redhat.com> <20150501064044.GA18957@gmail.com> <554399D1.6010405@redhat.com> <20150501155912.GA451@gmail.com> <20150501162109.GA1091@gmail.com> <5543A94B.3020108@redhat.com> <20150501163431.GB1327@gmail.com> <5543C05E.9040209@redhat.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.11 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2015-05-01 at 14:05 -0400, Rik van Riel wrote: > On 05/01/2015 12:34 PM, Ingo Molnar wrote: > > > > * Rik van Riel wrote: > > > >>> I can understand people running hard-RT workloads not wanting to > >>> see the overhead of a timer tick or a scheduler tick with variable > >>> (and occasionally heavy) work done in IRQ context, but the jitter > >>> caused by a single trivial IPI with constant work should be very, > >>> very low and constant. > >> > >> Not if the realtime workload is running inside a KVM guest. > > > > I don't buy this: > > > >> At that point an IPI, either on the host or in the guest, involves a > >> full VMEXIT & VMENTER cycle. > > > > So a full VMEXIT/VMENTER costs how much, 2000 cycles? That's around 1 > > usec on recent hardware, and I bet it will get better with time. > > > > I'm not aware of any hard-RT workload that cannot take 1 usec > > latencies. > > Now think about doing this kind of IPI from inside a guest, > to another VCPU on the same guest. > > Now you are looking at VMEXIT/VMENTER on the first VCPU, > plus the cost of the IPI on the host, plus the cost of > the emulation layer, plus VMEXIT/VMENTER on the second > VCPU to trigger the IPI work, and possibly a second > VMEXIT/VMENTER for IPI completion. > > I suspect it would be better to do RCU callback offload > in some other way. I don't get it. How the heck do people manage to talk about realtime in virtual boxen, and not at least crack a smile. Real, virtual, real, virtual... what's wrong with this picture? Why is virtual realtime not an oxymoron? I did that for grins once, and it was either really funny, or really sad, not sure which... but it did not look really really useful. -Mike