From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rik van Riel Subject: Re: [PATCH 6/6] kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest Date: Tue, 10 Feb 2015 15:13:22 -0500 Message-ID: <54DA6662.7060601@redhat.com> References: <1423579310-24555-1-git-send-email-riel@redhat.com> <1423579310-24555-7-git-send-email-riel@redhat.com> <54DA630D.6020601@amacapital.net> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 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: Andy Lutomirski , paulmck@linux.vnet.ibm.com Return-path: In-Reply-To: <54DA630D.6020601@amacapital.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 02/10/2015 02:59 PM, 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. Guest_enter and guest_exit already do the vtime stuff today. This patch series adds the rcu stuff, and modifies context_tracking_enter & context_tracking_exit to not do the vtime stuff twice. > (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.) I am all for making it faster :)