acrn-hypervisor/doc/tutorials/realtime_performance_tuning...

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.. _rt_performance_tuning:
ACRN Real-Time (RT) Performance Analysis
########################################
The document describes the methods to collect trace/data for ACRN real-time VM (RTVM)
real-time performance analysis. Two parts are included:
- Method to trace ``vmexit`` occurrences for analysis.
- Method to collect Performance Monitoring Counters information for tuning based on Performance Monitoring Unit, or PMU.
vmexit Analysis for ACRN RT Performance
***************************************
``vmexit`` are triggered in response to certain instructions and events and are
a key source of performance degradation in virtual machines. During the runtime
of a hard RTVM of ACRN, the following impacts real-time deterministic latency:
- CPUID
- TSC_Adjust read/write
- TSC write
- APICID/LDR read
- ICR write
Generally, we don't want to see any ``vmexit`` occur during the critical section of the RT task.
The methodology of ``vmexit`` analysis is very simple. First, we clearly
identify the **critical section** of the RT task. The critical section is
the duration of time where we do not want to see any ``vmexit`` occur.
Different RT tasks use different critical sections. This document uses
the cyclictest benchmark as an example of how to do ``vmexit`` analysis.
The Critical Sections
=====================
Here is example pseudocode of a cyclictest implementation.
.. code-block:: none
while (!shutdown) {
...
clock_nanosleep(&next)
clock_gettime(&now)
latency = calcdiff(now, next)
...
next += interval
}
Time point ``now`` is the actual point at which the cyclictest app is woken up
and scheduled. Time point ``next`` is the expected point at which we want
the cyclictest to be awakened and scheduled. Here we can get the latency by
``now - next``. We don't want to see a ``vmexit`` in between ``next`` and ``now``.
So, we define the starting point of the critical section as ``next`` and
the ending point as ``now``.
Log and Trace Data Collection
=============================
#. Add time stamps (in TSC) at ``next`` and ``now``.
#. Capture the log with the above time stamps in the RTVM.
#. Capture the ``acrntrace`` log in the Service VM at the same time.
Offline Analysis
================
#. Convert the raw trace data to human readable format.
#. Merge the logs in the RTVM and the ACRN hypervisor trace based on time stamps (in TSC).
#. Check to see if any ``vmexit`` occurred within the critical sections. The pattern is as follows:
.. figure:: images/vm_exits_log.png
:align: center
:name: vm_exits_log
Collecting Performance Monitoring Counters Data
***********************************************
Enable Performance Monitoring Unit (PMU) Support in VM
======================================================
By default, the ACRN hypervisor doesn't expose the PMU-related CPUID and
MSRs to the guest VM. In order to use Performance Monitoring Counters (PMCs)
in the guest VM, modify the ACRN hypervisor code in order to expose the
capability to the RTVM.
Note that Precise Event Based Sampling (PEBS) is not yet enabled in the VM.
#. Expose the CPUID leaf 0xA as below:
.. code-block:: none
--- a/hypervisor/arch/x86/guest/vcpuid.c
+++ b/hypervisor/arch/x86/guest/vcpuid.c
@@ -345,7 +345,7 @@ int32_t set_vcpuid_entries(struct acrn_vm *vm)
break;
/* These features are disabled */
/* PMU is not supported */
- case 0x0aU:
+ //case 0x0aU:
/* Intel RDT */
case 0x0fU:
case 0x10U:
#. Expose the PMU-related MSRs to the VM as below:
.. code-block:: none
--- a/hypervisor/arch/x86/guest/vmsr.c
+++ b/hypervisor/arch/x86/guest/vmsr.c
@@ -337,6 +337,41 @@ void init_msr_emulation(struct acrn_vcpu *vcpu)
/* don't need to intercept rdmsr for these MSRs */
enable_msr_interception(msr_bitmap, MSR_IA32_TIME_STAMP_COUNTER, INTERCEPT_WRITE);
+
+ /* Passthru PMU related MSRs to guest */
+ enable_msr_interception(msr_bitmap, MSR_IA32_FIXED_CTR_CTL, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PERF_GLOBAL_CTRL, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PERF_GLOBAL_STATUS, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PERF_GLOBAL_OVF_CTRL, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PERF_GLOBAL_STATUS_SET, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PERF_GLOBAL_INUSE, INTERCEPT_DISABLE);
+
+ enable_msr_interception(msr_bitmap, MSR_IA32_FIXED_CTR0, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_FIXED_CTR1, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_FIXED_CTR2, INTERCEPT_DISABLE);
+
+ enable_msr_interception(msr_bitmap, MSR_IA32_PMC0, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PMC1, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PMC2, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PMC3, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PMC4, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PMC5, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PMC6, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PMC7, INTERCEPT_DISABLE);
+
+ enable_msr_interception(msr_bitmap, MSR_IA32_A_PMC0, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_A_PMC1, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_A_PMC2, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_A_PMC3, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_A_PMC4, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_A_PMC5, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_A_PMC6, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_A_PMC7, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PERFEVTSEL0, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PERFEVTSEL1, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PERFEVTSEL2, INTERCEPT_DISABLE);
+ enable_msr_interception(msr_bitmap, MSR_IA32_PERFEVTSEL3, INTERCEPT_DISABLE);
+
/* Setup MSR bitmap - Intel SDM Vol3 24.6.9 */
value64 = hva2hpa(vcpu->arch.msr_bitmap);
exec_vmwrite64(VMX_MSR_BITMAP_FULL, value64);
Perf/PMU Tools in Performance Analysis
======================================
After exposing PMU-related CPUID/MSRs to the VM, performance analysis tools
such as ``perf`` and ``PMU`` can be used inside the VM to locate
the bottleneck of the application.
``Perf`` is a profiler tool for Linux 2.6+ based systems that abstracts away
CPU hardware differences in Linux performance measurements and presents a
simple command-line interface. Perf is based on the ``perf_events`` interface
exported by recent versions of the Linux kernel.
``PMU tools`` is a collection of tools for profile collection and
performance analysis on Intel CPUs on top of Linux Perf. Refer to the
following links for perf usage:
- https://perf.wiki.kernel.org/index.php/Main_Page
- https://perf.wiki.kernel.org/index.php/Tutorial
Refer to https://github.com/andikleen/pmu-tools for PMU usage.
Top-Down Microarchitecture Analysis Method (TMAM)
==================================================
The top-down microarchitecture analysis method (TMAM), based on top-down
characterization methodology, aims to provide an insight into whether you
have made wise choices with your algorithms and data structures. See the
Intel |reg| 64 and IA-32 `Architectures Optimization Reference Manual
<http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-optimization-manual.pdf>`_,
Appendix B.1 for more details on TMAM. Refer to this `technical paper
<https://fd.io/docs/whitepapers/performance_analysis_sw_data_planes_dec21_2017.pdf>`_
that adopts TMAM for systematic performance benchmarking and analysis
of compute-native Network Function data planes that are executed on
commercial-off-the-shelf (COTS) servers using available open-source
measurement tools.
Example: Using Perf to analyze TMAM level 1 on CPU core 1:
.. code-block:: console
perf stat --topdown -C 1 taskset -c 1 dd if=/dev/zero of=/dev/null count=10
10+0 records in
10+0 records out
5120 bytes (5.1 kB, 5.0 KiB) copied, 0.00336348 s, 1.5 MB/s
Performance counter stats for 'CPU(s) 1':
retiring bad speculation frontend bound backend bound
S0-C1 1 10.6% 1.5% 3.9% 84.0%
0.006737123 seconds time elapsed