From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH 6/6] kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest Date: Tue, 10 Feb 2015 12:14:02 -0800 Message-ID: <20150210201402.GU4166@linux.vnet.ibm.com> References: <1423579310-24555-1-git-send-email-riel@redhat.com> <1423579310-24555-7-git-send-email-riel@redhat.com> <54DA630D.6020601@amacapital.net> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: riel@redhat.com, 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: Andy Lutomirski Return-path: Content-Disposition: inline In-Reply-To: <54DA630D.6020601@amacapital.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Tue, Feb 10, 2015 at 11:59:09AM -0800, Andy Lutomirski wrote: > 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.) It turns out that context_tracking_is_enabled() is a static inline that uses a static_key, so the overhead should be minimal on platforms having a full implementation of static keys. Thanx, Paul