From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rik van Riel Subject: Re: [PATCH 4/4] kvm,rcu: use RCU extended quiescent state when running KVM guest Date: Thu, 05 Feb 2015 11:52:37 -0500 Message-ID: <54D39FD5.2040402@redhat.com> References: <1423154134-17391-1-git-send-email-riel@redhat.com> <1423154134-17391-5-git-send-email-riel@redhat.com> <54D39DF0.3020109@de.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, mtosatti@redhat.com, mingo@kernel.org, ak@linux.intel.com, oleg@redhat.com, masami.hiramatsu.pt@hitachi.com, fweisbec@gmail.com, paulmck@linux.vnet.ibm.com, lcapitulino@redhat.com, pbonzini@redhat.com To: Christian Borntraeger , kvm@vger.kernel.org Return-path: In-Reply-To: <54D39DF0.3020109@de.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 02/05/2015 11:44 AM, Christian Borntraeger wrote: > Am 05.02.2015 um 17:35 schrieb riel@redhat.com: >> 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, so 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 | 8 +++++++- >> include/linux/context_tracking_state.h | 1 + >> 2 files changed, 8 insertions(+), 1 deletion(-) >> >> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h >> index bd9f000fc98d..a5d3bb44b897 100644 >> --- a/include/linux/context_tracking.h >> +++ b/include/linux/context_tracking.h >> @@ -43,7 +43,7 @@ static inline enum ctx_state exception_enter(void) >> static inline void exception_exit(enum ctx_state prev_ctx) >> { >> if (context_tracking_is_enabled()) { >> - if (prev_ctx == IN_USER) >> + if (prev_ctx == IN_USER || prev_ctx == IN_GUEST) >> context_tracking_user_enter(prev_ctx); >> } >> } >> @@ -78,6 +78,9 @@ static inline void guest_enter(void) >> vtime_guest_enter(current); >> else >> current->flags |= PF_VCPU; >> + >> + if (context_tracking_is_enabled()) >> + context_tracking_user_enter(IN_GUEST); >> } > > > Couldnt we make > rcu_virt_note_context_switch(smp_processor_id()); > conditional in include/linux/kvm_host.h (kvm_guest_enter) > > e.g. something like > if (!context_tracking_is_enabled()) > rcu_virt_note_context_switch(smp_processor_id()); Possibly. I considered the same, but I do not know whether or not just rcu_user_enter / rcu_user_exit is enough. I could certainly try it out and see whether anything explodes, but I am not convinced that is careful enough when it comes to handling RCU code... Paul? :)