Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762665AbcLSLmr (ORCPT ); Mon, 19 Dec 2016 06:42:47 -0500 Received: from mx1.redhat.com ([209.132.183.28]:38648 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755164AbcLSLmp (ORCPT ); Mon, 19 Dec 2016 06:42:45 -0500 Date: Mon, 19 Dec 2016 12:42:41 +0100 From: Andrea Arcangeli To: Pan Xinhui Cc: linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, qemu-devel@nongnu.org, Paolo Bonzini , rkrcmar@redhat.com, "Dr. David Alan Gilbert" Subject: Re: [PATCH v7 08/11] x86, kvm/x86.c: support vcpu preempted check Message-ID: <20161219114241.GD4927@redhat.com> References: <1478077718-37424-1-git-send-email-xinhui.pan@linux.vnet.ibm.com> <1478077718-37424-9-git-send-email-xinhui.pan@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1478077718-37424-9-git-send-email-xinhui.pan@linux.vnet.ibm.com> User-Agent: Mutt/1.7.2 (2016-11-26) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Mon, 19 Dec 2016 11:42:45 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5495 Lines: 151 Hello, On Wed, Nov 02, 2016 at 05:08:35AM -0400, Pan Xinhui wrote: > Support the vcpu_is_preempted() functionality under KVM. This will > enhance lock performance on overcommitted hosts (more runnable vcpus > than physical cpus in the system) as doing busy waits for preempted > vcpus will hurt system performance far worse than early yielding. > > Use one field of struct kvm_steal_time ::preempted to indicate that if > one vcpu is running or not. > > Signed-off-by: Pan Xinhui > Acked-by: Paolo Bonzini > --- > arch/x86/include/uapi/asm/kvm_para.h | 4 +++- > arch/x86/kvm/x86.c | 16 ++++++++++++++++ > 2 files changed, 19 insertions(+), 1 deletion(-) > [..] > +static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu) > +{ > + if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED)) > + return; > + > + vcpu->arch.st.steal.preempted = 1; > + > + kvm_write_guest_offset_cached(vcpu->kvm, &vcpu->arch.st.stime, > + &vcpu->arch.st.steal.preempted, > + offsetof(struct kvm_steal_time, preempted), > + sizeof(vcpu->arch.st.steal.preempted)); > +} > + > void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) > { > + kvm_steal_time_set_preempted(vcpu); > kvm_x86_ops->vcpu_put(vcpu); > kvm_put_guest_fpu(vcpu); > vcpu->arch.last_host_tsc = rdtsc(); You can't call kvm_steal_time_set_preempted in atomic context (neither in sched_out notifier nor in vcpu_put() after preempt_disable)). __copy_to_user in kvm_write_guest_offset_cached schedules and locks up the host. kvm->srcu (or kvm->slots_lock) is also not taken and kvm_write_guest_offset_cached needs to call kvm_memslots which requires it. This I think is why postcopy live migration locks up with current upstream, and it doesn't seem related to userfaultfd at all (initially I suspected the vmf conversion but it wasn't that) and in theory it can happen with heavy swapping or page migration too. Just the page is written so frequently it's unlikely to be swapped out. The page being written so frequently also means it's very likely found as re-dirtied when postcopy starts and that pretty much guarantees an userfault will trigger a scheduling event in kvm_steal_time_set_preempted in destination. There are opposite probabilities of reproducing this with swapping vs postcopy live migration. For now I applied the below two patches, but this just will skip the write and only prevent the host instability as nobody checks the retval of __copy_to_user (what happens to guest after the write is skipped is not as clear and should be investigated, but at least the host will survive and not all guests will care about this flag being updated). For this to be fully safe the preempted information should be just an hint and not fundamental for correct functionality of the guest pv spinlock code. This bug was introduced in commit 0b9f6c4615c993d2b552e0d2bd1ade49b56e5beb in v4.9-rc7. >From 458897fd44aa9b91459a006caa4051a7d1628a23 Mon Sep 17 00:00:00 2001 From: Andrea Arcangeli Date: Sat, 17 Dec 2016 18:43:52 +0100 Subject: [PATCH 1/2] kvm: fix schedule in atomic in kvm_steal_time_set_preempted() kvm_steal_time_set_preempted() isn't disabling the pagefaults before calling __copy_to_user and the kernel debug notices. Signed-off-by: Andrea Arcangeli --- arch/x86/kvm/x86.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 1f0d238..2dabaeb 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -2844,7 +2844,17 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu) void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) { + /* + * Disable page faults because we're in atomic context here. + * kvm_write_guest_offset_cached() would call might_fault() + * that relies on pagefault_disable() to tell if there's a + * bug. NOTE: the write to guest memory may not go through if + * during postcopy live migration or if there's heavy guest + * paging. + */ + pagefault_disable(); kvm_steal_time_set_preempted(vcpu); + pagefault_enable(); kvm_x86_ops->vcpu_put(vcpu); kvm_put_guest_fpu(vcpu); vcpu->arch.last_host_tsc = rdtsc(); >From 2845eba22ac74c5e313e3b590f9dac33e1b3cfef Mon Sep 17 00:00:00 2001 From: Andrea Arcangeli Date: Sat, 17 Dec 2016 19:13:32 +0100 Subject: [PATCH 2/2] kvm: take srcu lock around kvm_steal_time_set_preempted() kvm_memslots() will be called by kvm_write_guest_offset_cached() so take the srcu lock. Signed-off-by: Andrea Arcangeli --- arch/x86/kvm/x86.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 2dabaeb..02e6ab4 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -2844,6 +2844,7 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu) void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) { + int idx; /* * Disable page faults because we're in atomic context here. * kvm_write_guest_offset_cached() would call might_fault() @@ -2853,7 +2854,13 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) * paging. */ pagefault_disable(); + /* + * kvm_memslots() will be called by + * kvm_write_guest_offset_cached() so take the srcu lock. + */ + idx = srcu_read_lock(&vcpu->kvm->srcu); kvm_steal_time_set_preempted(vcpu); + srcu_read_unlock(&vcpu->kvm->srcu, idx); pagefault_enable(); kvm_x86_ops->vcpu_put(vcpu); kvm_put_guest_fpu(vcpu);