From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH 6/6] kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest Date: Tue, 10 Feb 2015 11:59:09 -0800 Message-ID: <54DA630D.6020601@amacapital.net> References: <1423579310-24555-1-git-send-email-riel@redhat.com> <1423579310-24555-7-git-send-email-riel@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Cc: will.deacon@arm.com, linux-kernel@vger.kernel.org, Catalin.Marinas@arm.com, fweisbec@gmail.com, kvm@vger.kernel.org, mtosatti@redhat.com, borntraeger@de.ibm.com, mingo@kernel.org, oleg@redhat.com, lcapitulino@redhat.com, pbonzini@redhat.com To: riel@redhat.com, paulmck@linux.vnet.ibm.com Return-path: In-Reply-To: <1423579310-24555-7-git-send-email-riel@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 02/10/2015 06:41 AM, riel@redhat.com wrote: > From: Rik van Riel > > The host kernel is not doing anything while the CPU is executing > a KVM guest VCPU, so it can be marked as being in an extended > quiescent state, identical to that used when running user space > code. > > The only exception to that rule is when the host handles an > interrupt, which is already handled by the irq code, which > calls rcu_irq_enter and rcu_irq_exit. > > The guest_enter and guest_exit functions already switch vtime > accounting independent of context tracking. Leave those calls > where they are, instead of moving them into the context tracking > code. > > Signed-off-by: Rik van Riel > --- > include/linux/context_tracking.h | 6 ++++++ > include/linux/context_tracking_state.h | 1 + > include/linux/kvm_host.h | 3 ++- > 3 files changed, 9 insertions(+), 1 deletion(-) > > diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h > index 954253283709..b65fd1420e53 100644 > --- a/include/linux/context_tracking.h > +++ b/include/linux/context_tracking.h > @@ -80,10 +80,16 @@ static inline void guest_enter(void) > vtime_guest_enter(current); > else > current->flags |= PF_VCPU; > + > + if (context_tracking_is_enabled()) > + context_tracking_enter(IN_GUEST); Why the if statement? Also, have you checked how much this hurts guest lightweight entry/exit latency? Context tracking is shockingly expensive for reasons I don't fully understand, but hopefully most of it is the vtime stuff. (Context tracking is *so* expensive that I almost think we should set the performance taint flag if we enable it, assuming that flag ended up getting merged. Also, we should make context tracking faster.) --Andy