From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935250AbeE2ObJ (ORCPT ); Tue, 29 May 2018 10:31:09 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:33088 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S934857AbeE2ObF (ORCPT ); Tue, 29 May 2018 10:31:05 -0400 Date: Tue, 29 May 2018 16:31:02 +0200 From: Radim =?utf-8?B?S3LEjW3DocWZ?= To: Wanpeng Li Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Paolo Bonzini Subject: Re: [PATCH] KVM: VMX: Optimize tscdeadline timer latency Message-ID: <20180529143102.GC7819@flask> References: <1527576797-5738-1-git-send-email-wanpengli@tencent.com> <20180529142324.GA7819@flask> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20180529142324.GA7819@flask> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2018-05-29 16:23+0200, Radim Krčmář: > 2018-05-29 14:53+0800, Wanpeng Li: > > From: Wanpeng Li > > > > 'Commit d0659d946be0 ("KVM: x86: add option to advance tscdeadline > > hrtimer expiration")' advances the tscdeadline (the timer is emulated > > by hrtimer) expiration in order that the latency which is incurred > > by hypervisor (apic_timer_fn -> vmentry) can be avoided. This patch > > adds the advance tscdeadline expiration support to which the tscdeadline > > timer is emulated by VMX preemption timer to reduce the hypervisor > > lantency (handle_preemption_timer -> vmentry). clockevents infrastruture > > can program minimum delay if hrtimer feeds a expiration in the past, > > we set delta_tsc to 1(which will be converted to 0 before vmentry) > > which can lead to an immediately vmexit when delta_tsc is not bigger > > than advance ns. > > > > This patch can reduce ~63% latency (~4450 cycles to ~1660 cycles on > > a haswell desktop) for kvm-unit-tests/tscdeadline_latency when testing > > busy waits. > > > > Cc: Paolo Bonzini > > Cc: Radim Krčmář > > Signed-off-by: Wanpeng Li > > --- > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c > > @@ -12444,6 +12444,12 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc) > > tscl = rdtsc(); > > guest_tscl = kvm_read_l1_tsc(vcpu, tscl); > > delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl; > > + lapic_timer_advance_cycles = nsec_to_cycles(vcpu, lapic_timer_advance_ns); > > + if (delta_tsc > lapic_timer_advance_cycles) > > + delta_tsc -= lapic_timer_advance_cycles; > > + else > > + delta_tsc = 1; > > Why don't we just "return 1" to say that the timer has expired? This case might be rare, so setting delta_tsc = 0 would be safer.