linux-kernel - Re: [kvm-unit-tests Patch v3 00/11] pmu test bugs fix and improvements

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Date: Wed, 24 Jan 2024 16:18:32 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Jim Mattson <jmattson@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Zhenyu Wang <zhenyuw@...ux.intel.com>, Zhang Xiong
 <xiong.y.zhang@...el.com>, Mingwei Zhang <mizhang@...gle.com>,
 Like Xu <like.xu.linux@...il.com>, Jinrong Liang <cloudliang@...cent.com>,
 Dapeng Mi <dapeng1.mi@...el.com>
Subject: Re: [kvm-unit-tests Patch v3 00/11] pmu test bugs fix and
 improvements

Kindly ping ...

On 1/3/2024 11:13 AM, Dapeng Mi wrote:
> When running pmu test on Sapphire Rapids, we found sometimes pmu test
> reports the following failures.
>
> 1. FAIL: Intel: all counters
> 2. FAIL: Intel: core cycles-0
> 3. FAIL: Intel: llc misses-4
>
> Further investigation shows these failures are all false alarms rather
> than real vPMU issues.
>
> The failure 1 is caused by a bug in check_counters_many() which defines
> a cnt[] array with length 10. On Sapphire Rapids KVM supports 8 GP
> counters and 3 fixed counters, obviously the total counter number (11)
> of Sapphire Rapids exceed current cnt[] length 10, it would cause a out
> of memory access and lead to the "all counters" false alarm. Patch
> 02~03 would fix this issue.
>
> The failure 2 is caused by pipeline and cache warm-up latency.
> Currently "core cycles" is the first executed event. When the measured
> loop() program is executed at the first time, cache hierarchy and pipeline
> are needed to warm up. All these warm-up work consumes so much cycles
> that it exceeds the predefined upper boundary and cause the failure.
> Patch 04 fixes this issue.
>
> The failure 3 is caused by 0 llc misses count. It's possible and
> reasonable that there is no llc misses happened for such simple loop()
> asm blob especially along with larger and larger LLC size on new
> processors. Patch 09 would fix this issue by introducing clflush
> instruction to force LLC miss.
>
> Besides above bug fixes, this patch series also includes several
> optimizations.
>
> One important optimization (patch 07~08) is to move
> GLOBAL_CTRL enabling/disabling into the loop asm blob, so the precise
> count for instructions and branches events can be measured and the
> verification can be done against the precise count instead of the rough
> count range. This improves the verification accuracy.
>
> Another important optimization (patch 10~11) is to leverage IBPB command
> to force to trigger a branch miss, so the lower boundary of branch miss
> event can be set to 1 instead of the ambiguous 0. This eliminates the
> ambiguity brought from 0.
>
> All these changes are tested on Intel Sapphire Rapids server platform
> and the pmu test passes. Since I have no AMD platforms on my hand, these
> changes are not verified on AMD platforms yet. If someone can help to
> verify these changes on AMD platforms, it's welcome and appreciated.
>
> Changes:
>    v2 -> v3:
>          fix "core cycles" failure,
>          introduce precise verification for instructions/branches,
>          leverage IBPB command to optimize branch misses verification,
>          drop v2 introduced slots event verification
>    v1 -> v2:
>          introduce clflush to optimize llc misses verification
>          introduce rdrand to optimize branch misses verification
>
> History:
>    v2: https://lore.kernel.org/lkml/20231031092921.2885109-1-dapeng1.mi@linux.intel.com/
>    v1: https://lore.kernel.org/lkml/20231024075748.1675382-1-dapeng1.mi@linux.intel.com/
>
> Dapeng Mi (10):
>    x86: pmu: Enlarge cnt[] length to 64 in check_counters_many()
>    x86: pmu: Add asserts to warn inconsistent fixed events and counters
>    x86: pmu: Switch instructions and core cycles events sequence
>    x86: pmu: Refine fixed_events[] names
>    x86: pmu: Remove blank line and redundant space
>    x86: pmu: Enable and disable PMCs in loop() asm blob
>    x86: pmu: Improve instruction and branches events verification
>    x86: pmu: Improve LLC misses event verification
>    x86: pmu: Add IBPB indirect jump asm blob
>    x86: pmu: Improve branch misses event verification
>
> Xiong Zhang (1):
>    x86: pmu: Remove duplicate code in pmu_init()
>
>   lib/x86/pmu.c |   5 --
>   x86/pmu.c     | 201 ++++++++++++++++++++++++++++++++++++++++++--------
>   2 files changed, 171 insertions(+), 35 deletions(-)
>