From mboxrd@z Thu Jan 1 00:00:00 1970 From: Radim =?utf-8?B?S3LEjW3DocWZ?= Subject: Re: vtime accounting Date: Tue, 14 Mar 2017 20:32:02 +0100 Message-ID: <20170314193201.GC5432@potion> References: <20170308105700.GA109453@lvm> <20170313162259.GE18298@potion> <20170314082601.GC1277@cbox> <20170314165858.GA5435@potion> <1d205be1-50ab-59cf-9908-dbbe8c939309@redhat.com> <20170314184133.GG1277@cbox> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: Paolo Bonzini , Christoffer Dall , kvm@vger.kernel.org, Marc Zyngier , Rik van Riel To: Christoffer Dall Return-path: Received: from mx1.redhat.com ([209.132.183.28]:43642 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752471AbdCNTcG (ORCPT ); Tue, 14 Mar 2017 15:32:06 -0400 Content-Disposition: inline In-Reply-To: <20170314184133.GG1277@cbox> Sender: kvm-owner@vger.kernel.org List-ID: 2017-03-14 19:41+0100, Christoffer Dall: > On Tue, Mar 14, 2017 at 06:09:45PM +0100, Paolo Bonzini wrote: >> On 14/03/2017 17:58, Radim Krčmář wrote: >> >> I assume there's a good reason why we call guest_enter() and >> >> guest_exit() in the hot path on every KVM architecture? >> > I consider myself biased when it comes to jiffies, so no judgement. :) >> > >> > From what I see, the mode switch is used only for statistics. >> >> vtime is only for statistics, but guest_enter/exit are important because >> they enter an RCU extended quiescent state. This means that (physical) >> CPUs running a guest are effectively "off" from the point of view of the >> RCU accounting machinery. Not having to perform any RCU work is very >> good for jitter. Ah, good point. > So would it be worth considering factoring out vtime accounting from > guest_enter/exit, such that we could do the vtime accounting from vcpu > load/put and mark the RCU extended quiescent state in the run loop? RCU is the reason why guest_exit() needs disabled interrupts, so if we split them, we could do rcu_virt_note_context_switch() before enabling interrupts, and guest_exit() right after. > Disclaimer: I haven't completely convinced myself that vtime accounting > from load/put works as it should. For example, when servicing a VM from > KVM, should we really be accounting this as kernel time, or as guest > time? I think we do the former now, but if the latter is the right > thing, would changing the behavior constitute an ABI change to > userspace? Not considering that option would be best. :)