From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965387AbeE2RI5 (ORCPT ); Tue, 29 May 2018 13:08:57 -0400 Received: from mail-wm0-f67.google.com ([74.125.82.67]:36981 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S936821AbeE2RIw (ORCPT ); Tue, 29 May 2018 13:08:52 -0400 X-Google-Smtp-Source: ADUXVKK4Qy5mcE57p7z4nxkZiacVMAlphTHg1u9o4m2+uSnJ+8j5X+xzenQzVyJGpMquSpeKqtlh9g== Subject: Re: [PATCH] KVM: VMX: Optimize tscdeadline timer latency To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Wanpeng Li Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org References: <1527576797-5738-1-git-send-email-wanpengli@tencent.com> <20180529142324.GA7819@flask> <20180529143102.GC7819@flask> From: Paolo Bonzini Openpgp: preference=signencrypt Autocrypt: addr=pbonzini@redhat.com; keydata= xsEhBFRCcBIBDqDGsz4K0zZun3jh+U6Z9wNGLKQ0kSFyjN38gMqU1SfP+TUNQepFHb/Gc0E2 CxXPkIBTvYY+ZPkoTh5xF9oS1jqI8iRLzouzF8yXs3QjQIZ2SfuCxSVwlV65jotcjD2FTN04 hVopm9llFijNZpVIOGUTqzM4U55sdsCcZUluWM6x4HSOdw5F5Utxfp1wOjD/v92Lrax0hjiX DResHSt48q+8FrZzY+AUbkUS+Jm34qjswdrgsC5uxeVcLkBgWLmov2kMaMROT0YmFY6A3m1S P/kXmHDXxhe23gKb3dgwxUTpENDBGcfEzrzilWueOeUWiOcWuFOed/C3SyijBx3Av/lbCsHU Vx6pMycNTdzU1BuAroB+Y3mNEuW56Yd44jlInzG2UOwt9XjjdKkJZ1g0P9dwptwLEgTEd3Fo UdhAQyRXGYO8oROiuh+RZ1lXp6AQ4ZjoyH8WLfTLf5g1EKCTc4C1sy1vQSdzIRu3rBIjAvnC tGZADei1IExLqB3uzXKzZ1BZ+Z8hnt2og9hb7H0y8diYfEk2w3R7wEr+Ehk5NQsT2MPI2QBd wEv1/Aj1DgUHZAHzG1QN9S8wNWQ6K9DqHZTBnI1hUlkp22zCSHK/6FwUCuYp1zcAEQEAAc0f UGFvbG8gQm9uemluaSA8Ym9uemluaUBnbnUub3JnPsLBTQQTAQIAIwUCVEJ7AwIbAwcLCQgH AwIBBhUIAgkKCwQWAgMBAh4BAheAAAoJEH4VEAzNNmmxNcwOniaZVLsuy1lW/ntYCA0Caz0i sHpmecK8aWlvL9wpQCk4GlOX9L1emyYXZPmzIYB0IRqmSzAlZxi+A2qm9XOxs5gJ2xqMEXX5 FMtUH3kpkWWJeLqe7z0EoQdUI4EG988uv/tdZyqjUn2XJE+K01x7r3MkUSFz/HZKZiCvYuze VlS0NTYdUt5jBXualvAwNKfxEkrxeHjxgdFHjYWhjflahY7TNRmuqPM/Lx7wAuyoDjlYNE40 Z+Kun4/KjMbjgpcF4Nf3PJQR8qXI6p3so2qsSn91tY7DFSJO6v2HwFJkC2jU95wxfNmTEUZc znXahYbVOwCDJRuPrE5GKFd/XJU9u5hNtr/uYipHij01WXal2cce1S5mn1/HuM1yo1u8xdHy IupCd57EWI948e8BlhpujUCU2tzOb2iYS0kpmJ9/oLVZrOcSZCcCl2P0AaCAsj59z2kwQS9D du0WxUs8waso0Qq6tDEHo8yLCOJDzSz4oojTtWe4zsulVnWV+wu70AioemAT8S6JOtlu60C5 dHgQUD1Tp+ReXpDKXmjbASJx4otvW0qah3o6JaqO79tbDqIvncu3tewwp6c85uZd48JnIOh3 utBAu684nJakbbvZUGikJfxd887ATQRUQnHuAQgAx4dxXO6/Zun0eVYOnr5GRl76+2UrAAem Vv9Yfn2PbDIbxXqLff7oyVJIkw4WdhQIIvvtu5zH24iYjmdfbg8iWpP7NqxUQRUZJEWbx2CR wkMHtOmzQiQ2tSLjKh/cHeyFH68xjeLcinR7jXMrHQK+UCEw6jqi1oeZzGvfmxarUmS0uRuf fAb589AJW50kkQK9VD/9QC2FJISSUDnRC0PawGSZDXhmvITJMdD4TjYrePYhSY4uuIV02v02 8TVAaYbIhxvDY0hUQE4r8ZbGRLn52bEzaIPgl1p/adKfeOUeMReg/CkyzQpmyB1TSk8lDMxQ zCYHXAzwnGi8WU9iuE1P0wARAQABwsEzBBgBAgAJBQJUQnHuAhsMAAoJEH4VEAzNNmmxp1EO oJy0uZggJm7gZKeJ7iUpeX4eqUtqelUw6gU2daz2hE/jsxsTbC/w5piHmk1H1VWDKEM4bQBT uiJ0bfo55SWsUNN+c9hhIX+Y8LEe22izK3w7mRpvGcg+/ZRG4DEMHLP6JVsv5GMpoYwYOmHn plOzCXHvmdlW0i6SrMsBDl9rw4AtIa6bRwWLim1lQ6EM3PWifPrWSUPrPcw4OLSwFk0CPqC4 HYv/7ZnASVkR5EERFF3+6iaaVi5OgBd81F1TCvCX2BEyIDRZLJNvX3TOd5FEN+lIrl26xecz 876SvcOb5SL5SKg9/rCBufdPSjojkGFWGziHiFaYhbuI2E+NfWLJtd+ZvWAAV+O0d8vFFSvr iy9enJ8kxJwhC0ECbSKFY+W1eTIhMD3aeAKY90drozWEyHhENf4l/V+Ja5vOnW+gCDQkGt2Y 1lJAPPSIqZKvHzGShdh8DduC0U3xYkfbGAUvbxeepjgzp0uEnBXfPTy09JGpgWbg0w91GyfT /ujKaGd4vxG2Ei+MMNDmS1SMx7wu0evvQ5kT9NPzyq8R2GIhVSiAd2jioGuTjX6AZCFv3ToO 53DliFMkVTecLptsXaesuUHgL9dKIfvpm+rNXRn9wAwGjk0X/A== Message-ID: Date: Tue, 29 May 2018 19:08:48 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20180529143102.GC7819@flask> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 29/05/2018 16:31, Radim Krčmář wrote: > 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. Queued with this change. Indeed this case matches vmx_arm_hv_timer so it's preferrable. Paolo