From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C7690E82CAE for ; Wed, 27 Sep 2023 16:10:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231442AbjI0QKe (ORCPT ); Wed, 27 Sep 2023 12:10:34 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57812 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229527AbjI0QKc (ORCPT ); Wed, 27 Sep 2023 12:10:32 -0400 Received: from mail-pg1-x54a.google.com (mail-pg1-x54a.google.com [IPv6:2607:f8b0:4864:20::54a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CBC5E196 for ; Wed, 27 Sep 2023 09:10:29 -0700 (PDT) Received: by mail-pg1-x54a.google.com with SMTP id 41be03b00d2f7-56c306471ccso10778419a12.3 for ; Wed, 27 Sep 2023 09:10:29 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1695831029; x=1696435829; darn=vger.kernel.org; h=content-transfer-encoding:cc:to:from:subject:message-id:references :mime-version:in-reply-to:date:from:to:cc:subject:date:message-id :reply-to; bh=VjR/OUxnEHg50NT9HcYxI5Bi85H8/C7qbzkbq7YZT9o=; b=lPvEZIbBP40aOV3jk3EixgnxYab3aqg9ETiazkHqc6+FcNytxaD2m++K6dcWRRmoyk BXqwXQ9zsaDNCBafDnidF8xS/569MDtkZXgiVoM7Xb+QdpKEvwi1wOGJbD0li4DcXB8i DfvxndOsvK9PlHl4ztMAF5EH69Bv9jPZ/pGhG/TtY+62/yJlDSJJO3q0H4aNOzZ+U1O4 eNOyOAqrYSOzIuFAI20+IcBm1RMLr7V0DcmRX/+Op4iON6jhAu/f5Wi4p9T70HPFfeYf W08lNRPkZEPJcmOFeWE45Vy/zzkbKgfDqJgwCicqnE2NT73T4nvp/78IpZFQCpDetr6j EtMQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1695831029; x=1696435829; h=content-transfer-encoding:cc:to:from:subject:message-id:references :mime-version:in-reply-to:date:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to; bh=VjR/OUxnEHg50NT9HcYxI5Bi85H8/C7qbzkbq7YZT9o=; b=Kzqivlgul0GGu7JpEexsxCWaaP79SOsE8F5c0FMfZYEZtIT7qoOJ+Dbxs3FtQF3KT6 hIbDdfSzFt5Wn4HsJimrYLOFvBofD8KcB4lbraF8NQIIBUSNQ+jy+7690NSfFv3et7z0 ejAFtMNlA6Y50lhLligE+cx1yCPMntJX/GMq/4yeOhIFxk4afB07TWHmq1VDd3xz/R6U Wa7vg4QeNnGT5W+mbop8y2IrhCjidT2I0f0LwpJPeziIAi2So0cQaNZ1K8dIPVlAXsA9 cnI2fPlo9aaUhT14bwXXnxB/gpF2pLBeW/JAJte9ZC2bSMOenH4uRXNDSsXS5qJGkhqj Pilg== X-Gm-Message-State: AOJu0YwjxVie7SjEy7zlw5gg3e3+om7QXhR4oK/c7x1g3TQw/6bp30zh Wn8qAu2lXp0qnvGfAbQ99z88Gp2SgrE= X-Google-Smtp-Source: AGHT+IEiZO/2KVpTzKd45Y4lPezPcxhhNBzB1Qhy2IdwNVbLPwbxI9nWCzs94jYGwnXDZXo78Of8yCNff1s= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a17:903:1ca:b0:1bc:5182:1de2 with SMTP id e10-20020a17090301ca00b001bc51821de2mr35706plh.1.1695831029192; Wed, 27 Sep 2023 09:10:29 -0700 (PDT) Date: Wed, 27 Sep 2023 09:10:27 -0700 In-Reply-To: <2c79115e-e16d-49cc-8f5b-2363d7910269@zytor.com> Mime-Version: 1.0 References: <20230927040939.342643-1-mizhang@google.com> <2c79115e-e16d-49cc-8f5b-2363d7910269@zytor.com> Message-ID: Subject: Re: [PATCH] KVM: x86: Move kvm_check_request(KVM_REQ_NMI) after kvm_check_request(KVM_REQ_NMI) From: Sean Christopherson To: Xin Li Cc: Mingwei Zhang , Paolo Bonzini , "H. Peter Anvin" , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Jim Mattson , Like Xu , Kan Liang , Dapeng1 Mi Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Tue, Sep 26, 2023, Xin Li wrote: > On 9/26/2023 9:15 PM, Mingwei Zhang wrote: > > ah, typo in the subject: The 2nd KVM_REQ_NMI should be KVM_REQ_PMI. > > Sorry about that. > >=20 > > On Tue, Sep 26, 2023 at 9:09=E2=80=AFPM Mingwei Zhang wrote: > > >=20 > > > Move kvm_check_request(KVM_REQ_NMI) after kvm_check_request(KVM_REQ_N= MI). >=20 > Please remove it, no need to repeat the subject. Heh, from Documentation/process/maintainer-kvm-x86.rst: Changelog ~~~~~~~~~ Most importantly, write changelogs using imperative mood and avoid pronou= ns. See :ref:`describe_changes` for more information, with one amendment: lea= d with a short blurb on the actual changes, and then follow up with the context = and background. Note! This order directly conflicts with the tip tree's pre= ferred approach! Please follow the tip tree's preferred style when sending patc= hes that primarily target arch/x86 code that is _NOT_ KVM code. That said, I do prefer that the changelog intro isn't just a copy+paste of = the shortlog, and the shortlog and changelog should use conversational language= instead of describing the literal code movement. > > > When vPMU is active use, processing each KVM_REQ_PMI will generate a This is not guaranteed. > > > KVM_REQ_NMI. Existing control flow after KVM_REQ_PMI finished will fa= il the > > > guest enter, jump to kvm_x86_cancel_injection(), and re-enter > > > vcpu_enter_guest(), this wasted lot of cycles and increase the overhe= ad for > > > vPMU as well as the virtualization. As above, use conversational language, the changelog isn't meant to be a pl= ay-by-play. E.g. KVM: x86: Service NMI requests *after* PMI requests in VM-Enter path Move the handling of NMI requests after PMI requests in the VM-Enter path so that KVM doesn't need to cancel and redo VM-Enter in the likely scenario that the vCPU has configured its LVPTC entry to generate an NMI. Because APIC emulation "injects" NMIs via KVM_REQ_NMI, handling PMI requests after NMI requests means KVM won't detect the pending NMI reques= t until the final check for outstanding requests. Detecting requests at th= e final stage is costly as KVM has already loaded guest state, potentially queued events for injection, disabled IRQs, dropped SRCU, etc., most of which needs to be unwound. > Optimization is after correctness, so please explain if this is correct > first! Not first. Leading with an in-depth description of KVM requests and NMI ha= ndling is not going to help understand *why* this change is being first. But I do= agree that this should provide an analysis of why it's ok to swap the order, spec= ificially why it's architecturally ok if KVM drops an NMI due to the swapped ordering= , e.g. if the PMI is coincident with two other NMIs (or one other NMI and NMIs are= blocked). > > > So move the code snippet of kvm_check_request(KVM_REQ_NMI) to make KV= M > > > runloop more efficient with vPMU. > > >=20 > > > To evaluate the effectiveness of this change, we launch a 8-vcpu QEMU= VM on Avoid pronouns. There's no need for all the "fluff", just state the setup,= the test, and the results. Really getting into the nits, but the whole "8-vcpu QEMU VM" versus "the setup of using single core, single thread" is confusing IMO. If there= were potential performance downsides and/or tradeoffs, then getting the gory det= ails might be necessary, but that's not the case here, and if it were really nec= essary to drill down that deep, then I would want to better quantify the impact, e= .g. in terms latency. E.g. on Intel SPR running SPEC2017 benchmark and Intel vtune in the guest= , handling PMI requests before NMI requests reduces the number of canceled runs by ~1500 per second, per vCPU (counted by probing calls to vmx_cancel_injection()). > > > an Intel SPR CPU. In the VM, we run perf with all 48 events Intel vtu= ne > > > uses. In addition, we use SPEC2017 benchmark programs as the workload= with > > > the setup of using single core, single thread. > > >=20 > > > At the host level, we probe the invocations to vmx_cancel_injection()= with > > > the following command: > > >=20 > > > $ perf probe -a vmx_cancel_injection > > > $ perf stat -a -e probe:vmx_cancel_injection -I 10000 # per 10 s= econds > > >=20 > > > The following is the result that we collected at beginning of the spe= c2017 > > > benchmark run (so mostly for 500.perlbench_r in spec2017). Kindly for= give > > > the incompleteness. > > >=20 > > > On kernel without the change: > > > 10.010018010 14254 probe:vmx_cancel_injection > > > 20.037646388 15207 probe:vmx_cancel_injection > > > 30.078739816 15261 probe:vmx_cancel_injection > > > 40.114033258 15085 probe:vmx_cancel_injection > > > 50.149297460 15112 probe:vmx_cancel_injection > > > 60.185103088 15104 probe:vmx_cancel_injection > > >=20 > > > On kernel with the change: > > > 10.003595390 40 probe:vmx_cancel_injection > > > 20.017855682 31 probe:vmx_cancel_injection > > > 30.028355883 34 probe:vmx_cancel_injection > > > 40.038686298 31 probe:vmx_cancel_injection > > > 50.048795162 20 probe:vmx_cancel_injection > > > 60.069057747 19 probe:vmx_cancel_injection > > >=20 > > > From the above, it is clear that we save 1500 invocations per vcpu p= er > > > second to vmx_cancel_injection() for workloads like perlbench. Nit, this really should have: Suggested-by: Sean Christopherson I personally don't care about the attribution, but (a) others often do care= and (b) the added context is helpful. E.g. for bad/questionable suggestsions/i= deas, knowing that person X was also involved helps direct and/or curate question= s/comments accordingly.