The TopDown Microarchitecture Analysis (TMA) Method is a structured
analysis methodology to identify critical performance bottlenecks in
out-of-order processors. The details about topdown metrics support on
Intel processors can be found in section "Performance Metrics" of Intel's
SDM Volume 3[1]. Kernel enabling code has also been merged, see
patchset[2] to learn more about the feature.
The TMA method is quite powerful and efficient to help developers to
identify the performance bottleneck in the program. The TMA method has
been integrated into multiple performance analysis tools, such as perf,
Vtune. Developers can leverage TMA method to analyze their program's
performance bottleneck easily with these tools and improve their program's
performance. TMA method is becoming the most widely used performance
analysis method on x86 platform. Currently the TMA method has been
supported fairly well on Native, but it's still not supported in Guest
environment. Since the environment difference between Host and Guest,
even same program may show different performance bottleneck between Guest
and Host. Obviously, the most straightforward and best method to
profiling Guest performance bottleneck is to run the TMA method in Guest
directly. So supporting topdown perf metrics in Guest becomes a real and
important requirement and we hope this patchset can mitigate this gap.
Like Xu posted a patch series to support guest Topdown[3], the patchset
creates a group of topdown metric events in KVM by binding to fixed counter
3 to obtain hardware values and the guest value of PERF_METRICS MSR is
assembled based on the count of grouped metric events.
This patchset improves Like's proposal, it leverages mature vPMU PMC
emulation framework and current perf topdown metric handling logic to
support guest topdown metrics feature.
In current perf logic, an events group is required to handle the topdown
metrics profiling, and the events group couples a slots event which
acts events group leader and multiple metric events. To coordinate with
the perf topdown metrics handing logic and reduce the code changes in
KVM, we choose to follow current mature vPMU PMC emulation framework. The
only difference is that we need to create a events group for fixed
counter 3 and manipulate FIXED_CTR3 and PERF_METRICS MSRS together
instead of a single event and only manipulating FIXED_CTR3 MSR.
When guest writes PERF_METRICS MSR at first, KVM would create an event
group which couples a slots event and a virtual metrics event. In this
event group, slots event claims the fixed counter 3 HW resource and acts
as group leader which is required by perf system. The virtual metrics
event claims the PERF_METRICS MSR. This event group is just like the perf
metrics events group on host and is scheduled by host perf system.
In this proposal, the count of slots event is calculated and emulated
on host and returned to guest just like other normal counters, but there
is a difference for the metrics event processing. KVM doesn't calculate
the real count of topdown metrics, it just stores the raw data of
PERF_METRICS MSR and directly returnthe stored raw data to guest. Thus,
guest can get the real HW PERF_METRICS data and guarantee the calculation
accuracy of topdown metrics.
Comparing with Like's patchset, this proposal brings two benefits.
1. Reduce the created perf events number
Like's patchset needs to create 4 (Ice Lake) or 8 (Sapphire Rapids)
metric events, whereas this patchset only needs to create 1 metric
event.
2. Increase the accuracy of metric calculation
Like's patchset needs to do twice metric count conversion. The first
conversion happens on perf system, perf topdown metrics handling
logic reads the metric percentage from PERF_METRICS MSR and times with
elapsed slots count and obtains the metric count. The second conversion
happens on KVM, KVM needs to convert the metric count back to metric
percentage by using metric count divide elapsed slots again and then
assembles the 4 or 8 metric percentage values to the virtual
PERF_METRICS MSR and return to Guest at last. Considering each metric
percentage in PERF_METRICS MSR is represented with only 8 bits, the
twice conversions (once multiplication and once division) definitely
cause accuracy loss in theory. Since this patchset directly returns
the raw data of PERF_METRICS MSR to guest, it won't have any accuracy
loss.
The patchset is rebased on latest kvm-x86/next branch and it is tested
on both Host and Guest (SPR Platform) with below perf commands. The 'foo'
is a backend-bound benchmark. We can see the output of perf commands are
quite close between host and guest.
1. perf stat ./foo
Host outputs:
Performance counter stats for '/home/sdp/work/foo/foo':
26,525.25 msec task-clock # 1.000 CPUs utilized
9 context-switches # 0.339 /sec
2 cpu-migrations # 0.075 /sec
51 page-faults # 1.923 /sec
125,330,033,745 cycles # 4.725 GHz
238,172,965,287 instructions # 1.90 insn per cycle
44,904,300,430 branches # 1.693 G/sec
69,299,003 branch-misses # 0.15% of all branches
751,979,445,222 TOPDOWN.SLOTS # 59.2 % tma_backend_bound
# 38.0 % tma_retiring
# 0.8 % tma_bad_speculation
# 1.9 % tma_frontend_bound
286,047,083,084 topdown-retiring
14,744,695,003 topdown-fe-bound
445,289,789,131 topdown-be-bound
5,897,878,000 topdown-bad-spec
138,674,397 INT_MISC.UOP_DROPPING # 5.228 M/sec
26.528600835 seconds time elapsed
26.527849000 seconds user
0.000000000 seconds sys
Guest outputs:
Performance counter stats for '/home/pnp/foo/foo':
29,051.43 msec task-clock # 1.000 CPUs utilized
10 context-switches # 0.344 /sec
0 cpu-migrations # 0.000 /sec
51 page-faults # 1.756 /sec
125,337,801,996 cycles # 4.314 GHz
238,139,676,030 instructions # 1.90 insn per cycle
44,897,906,380 branches # 1.545 G/sec
69,402,326 branch-misses # 0.15% of all branches
752,022,710,490 TOPDOWN.SLOTS # 58.4 % tma_backend_bound
# 37.6 % tma_retiring
# 1.2 % tma_bad_speculation
# 2.7 % tma_frontend_bound
283,114,432,184 topdown-retiring
20,643,760,680 topdown-fe-bound
439,417,191,619 topdown-be-bound
8,847,326,005 topdown-bad-spec
138,873,309 INT_MISC.UOP_DROPPING # 4.780 M/sec
29.058833449 seconds time elapsed
29.048761000 seconds user
0.004003000 seconds sys
2. perf stat -e slots ./foo
Host outputs:
Performance counter stats for '/home/sdp/work/foo/foo':
713,292,346,950 slots
25.472861484 seconds time elapsed
25.470978000 seconds user
0.000000000 seconds sys
Guest outputs:
Performance counter stats for '/home/pnp/foo/foo':
713,286,331,824 slots
25.264007882 seconds time elapsed
25.259790000 seconds user
0.004002000 seconds sys
3.
echo 0 > /proc/sys/kernel/nmi_watchdog
echo 25 > /proc/sys/kernel/perf_cpu_time_max_percent
echo 100000 > /proc/sys/kernel/perf_event_max_sample_rate
echo 0 > /proc/sys/kernel/perf_cpu_time_max_percent
perf record -e slots ./foo && perf report
Host outputs:
# Total Lost Samples: 0
#
# Samples: 109K of event 'slots'
# Event count (approx.): 715723903347
#
# Overhead Command Shared Object Symbol
# ........ ....... ................ ..................................
#
74.83% foo libc.so.6 [.] __random
7.22% foo foo [.] qux
7.07% foo libc.so.6 [.] __random_r
5.40% foo foo [.] main
1.82% foo foo [.] bar
1.75% foo foo [.] random@plt
1.73% foo foo [.] foo
0.02% foo [kernel.vmlinux] [k] arch_asym_cpu_priority
Guest outputs:
# Total Lost Samples: 0
#
# Samples: 7K of event 'slots'
# Event count (approx.): 24532005986
#
# Overhead Command Shared Object Symbol
# ........ ....... ................ ....................
#
75.21% foo libc.so.6 [.] __random
7.19% foo libc.so.6 [.] __random_r
7.12% foo foo [.] qux
5.21% foo foo [.] main
1.90% foo foo [.] foo
1.81% foo foo [.] bar
1.56% foo foo [.] random@plt
0.00% perf-ex [kernel.vmlinux] [k] native_write_msr
To support the guest topdown metrics feature, we have to do several
fundamental changes for perf system and vPMU code, we tried to avoid
these changes AMAP, but it seems it's inevitable. If you have any idea,
please suggest.
The fundamental changes:
1. perf/core: Add function perf_event_create_group_kernel_counters()
Add a new API to create group events from kernel space
2. perf/core: Add new function perf_event_topdown_metrics()
Add a new API to update topdown metrics values
3. perf/x86/intel: Handle KVM virtual metrics event in perf system
Add virtual metrics event processing logic in topdown metrics
processing code
4. KVM: x86/pmu: Extend pmc_reprogram_counter() to create group events
Extend pmc_reprogram_counter() to be capable to create group events
instead of just single event
References:
[1]: Intel 64 and IA-32 Architectures Software Developer Manual
Combined Volumes: 1, 2A, 2B, 2C, 2D, 3A, 3B, 3C, 3D, and 4
https://cdrdv2.intel.com/v1/dl/getContent/671200
[2]: perf/x86/intel: Support TopDown metrics on Ice Lake
https://lwn.net/ml/linux-kernel/20191203141212.7704-1-kan.liang@xxxxxxxxxxxxxxx/
[3]: KVM: x86/pmu: Enable Fixed Counter3 and Topdown Perf Metrics
https://lwn.net/ml/linux-kernel/20221212125844.41157-1-likexu@xxxxxxxxxxx/
Dapeng Mi (13):
KVM: x86/pmu: Add Intel CPUID-hinted TopDown slots event
KVM: x86/pmu: Support PMU fixed counter 3
perf/core: Add function perf_event_group_leader_check()
perf/core: Add function perf_event_move_group()
perf/core: Add function perf_event_create_group_kernel_counters()
perf/x86: Fix typos and inconsistent indents in perf_event header
perf/x86: Add constraint for guest perf metrics event
perf/core: Add new function perf_event_topdown_metrics()
perf/x86/intel: Handle KVM virtual metrics event in perf system
KVM: x86/pmu: Extend pmc_reprogram_counter() to create group events
KVM: x86/pmu: Support topdown perf metrics feature
KVM: x86/pmu: Handle PERF_METRICS overflow
KVM: x86/pmu: Expose Topdown in MSR_IA32_PERF_CAPABILITIES
arch/x86/events/intel/core.c | 72 +++++--
arch/x86/events/perf_event.h | 10 +-
arch/x86/include/asm/kvm_host.h | 19 +-
arch/x86/include/asm/perf_event.h | 21 +-
arch/x86/kvm/pmu.c | 142 +++++++++++--
arch/x86/kvm/pmu.h | 50 ++++-
arch/x86/kvm/svm/pmu.c | 2 +
arch/x86/kvm/vmx/capabilities.h | 1 +
arch/x86/kvm/vmx/pmu_intel.c | 67 ++++++
arch/x86/kvm/vmx/vmx.c | 2 +
arch/x86/kvm/vmx/vmx.h | 5 +
arch/x86/kvm/x86.c | 5 +-
include/linux/perf_event.h | 19 ++
kernel/events/core.c | 326 ++++++++++++++++++++----------
14 files changed, 590 insertions(+), 151 deletions(-)
base-commit: 240f736891887939571854bd6d734b6c9291f22e
--
2.34.1
