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@xxxxxxxxxxxxxxx/
v1: https://lore.kernel.org/lkml/20231024075748.1675382-1-dapeng1.mi@xxxxxxxxxxxxxxx/
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(-)
--
2.34.1
