linux-kernel - RE: [PATCH v1 01/11] perf/x86/core: Support KVM to assign a dedicated counter for guest PEBS

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Message-ID: <82D7661F83C1A047AF7DC287873BF1E1738B3097@SHSMSX104.ccr.corp.intel.com>
Date:   Tue, 7 Apr 2020 12:34:22 +0000
From:   "Kang, Luwei" <luwei.kang@...el.com>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "bp@...en8.de" <bp@...en8.de>, "hpa@...or.com" <hpa@...or.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "pawan.kumar.gupta@...ux.intel.com" 
        <pawan.kumar.gupta@...ux.intel.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "like.xu@...ux.intel.com" <like.xu@...ux.intel.com>,
        "Wang, Wei W" <wei.w.wang@...el.com>
Subject: RE: [PATCH v1 01/11] perf/x86/core: Support KVM to assign a
 dedicated counter for guest PEBS

> > On 3/9/2020 11:05 AM, Peter Zijlstra wrote:
> >>> In the new proposal, KVM user is treated the same as other host
> >>> events with event constraint. The scheduler is free to choose
> >>> whether or not to assign a counter for it.
> >> That's what it does, I understand that. I'm saying that that is
> >> creating artificial contention.
> >>
> >>
> >> Why is this needed anyway? Can't we force the guest to flush and then
> >> move it over to a new counter?
> >
> 
> Current perf scheduling is pure software behavior. KVM only traps the MSR
> access. It’s impossible for KVM to impact the guest’s scheduling with current
> implementation.
> 
> To address the concern regarding to 'artificial contention', we have two
> proposals.
> Could you please take a look, and share your thoughts?
> 
> Proposal 1:
> Reject the guest request, if host has to use the counter which occupied by
> guest. At the meantime, host prints a warning.
> I still think the contention should rarely happen in practical.
> Personally, I prefer this proposal.
> 
> 
> Proposal 2:
> Add HW advisor for the scheduler in guest.
> Starts from Architectural Perfmon Version 4, IA32_PERF_GLOBAL_INUSE MSR
> is introduced. It provides an “InUse” bit for each programmable
> performance counter and fixed counter in the processor.
> 
> In perf, the scheduler will read the MSR and mask the “in used”
> counters. I think we can use X86_FEATURE_HYPERVISOR to limit the check
> in guest. For non-virtualization usage and host, nothing changed for
> scheduler.
> 
> But there is still a problem for this proposal. Host may request a
> counter later, which has been used by guest.
> We can only do multiplexing or grab the counter just like proposal 1.

Hi Peter,
    What is your opinion?

Thanks,
Luwei Kang