[kvm-unit-tests,v7,08/18] x86: pmu: Fix cycles event validation failure

Commit Message

Sean Christopherson Feb. 15, 2025, 1:36 a.m. UTC

From: Dapeng Mi <[e-mail]>

When running pmu test on SPR, sometimes the following failure is
reported.

PMU version:         2
GP counters:         8
GP counter width:    48
Mask length:         8
Fixed counters:      3
Fixed counter width: 48
1000000 <= 55109398 <= 50000000
FAIL: Intel: core cycles-0
1000000 <= 18279571 <= 50000000
PASS: Intel: core cycles-1
1000000 <= 12238092 <= 50000000
PASS: Intel: core cycles-2
1000000 <= 7981727 <= 50000000
PASS: Intel: core cycles-3
1000000 <= 6984711 <= 50000000
PASS: Intel: core cycles-4
1000000 <= 6773673 <= 50000000
PASS: Intel: core cycles-5
1000000 <= 6697842 <= 50000000
PASS: Intel: core cycles-6
1000000 <= 6747947 <= 50000000
PASS: Intel: core cycles-7

The count of the "core cycles" on first counter would exceed the upper
boundary and leads to a failure, and then the "core cycles" count would
drop gradually and reach a stable state.

That looks reasonable. The "core cycles" event is defined as the 1st
event in xxx_gp_events[] array and it is always verified at first.
when the program loop() is executed at the first time it needs to warm
up the pipeline and cache, such as it has to wait for cache is filled.
All these warm-up work leads to a quite large core cycles count which
may exceeds the verification range.

To avoid the false positive of cycles event caused by warm-up,
explicitly introduce a warm-up state before really starting
verification.

Signed-off-by: Dapeng Mi <[e-mail]>
[sean: use a for loop and an more obviously arbitrary number]
Signed-off-by: Sean Christopherson <[e-mail]>
---
 x86/pmu.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

Patch

diff --git a/x86/pmu.c b/x86/pmu.c
index 4353d1da..e672b540 100644
--- a/x86/pmu.c
+++ b/x86/pmu.c
@@ -603,11 +603,27 @@  static void check_tsx_cycles(void)
 	report_prefix_pop();
 }
 
+static void warm_up(void)
+{
+	int i;
+
+	/*
+	 * Since cycles event is always run as the first event, there would be
+	 * a warm-up state to warm up the cache, it leads to the measured cycles
+	 * value may exceed the pre-defined cycles upper boundary and cause
+	 * false positive. To avoid this, introduce an warm-up state before
+	 * the real verification.
+	 */
+	for (i = 0; i < 10; i++)
+		loop();
+}
+
 static void check_counters(void)
 {
 	if (is_fep_available())
 		check_emulated_instr();
 
+	warm_up();
 	check_gp_counters();
 	check_fixed_counters();
 	check_rdpmc();