Re: [Patch v5 12/18] x86: pmu: Improve instruction and branches events verification

Sandipan Das
2024-07-04T08:02:49Z

On 7/3/2024 3:27 PM, Dapeng Mi wrote:
> If HW supports GLOBAL_CTRL MSR, enabling and disabling PMCs are moved in
> __precise_count_loop(). Thus, instructions and branches events can be
> verified against a precise count instead of a rough range.
>
> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> ---
> x86/pmu.c | 31 +++++++++++++++++++++++++++++++
> 1 file changed, 31 insertions(+)
>
> diff --git a/x86/pmu.c b/x86/pmu.c
> index d005e376..ffb7b4a4 100644
> --- a/x86/pmu.c
> +++ b/x86/pmu.c
> @@ -19,6 +19,11 @@
> #define EXPECTED_INSTR 17
> #define EXPECTED_BRNCH 5
>
> +
> +/* Enable GLOBAL_CTRL + disable GLOBAL_CTRL instructions */
> +#define EXTRA_INSTRNS (3 + 3)
> +#define LOOP_INSTRNS (N * 10 + EXTRA_INSTRNS)
> +#define LOOP_BRANCHES (N)
> #define LOOP_ASM(_wrmsr) \
> _wrmsr "\n\t" \
> "mov %%ecx, %%edi; mov %%ebx, %%ecx;\n\t" \
> @@ -122,6 +127,24 @@ static inline void loop(u64 cntrs)
> __precise_loop(cntrs);
> }
>
> +static void adjust_events_range(struct pmu_event *gp_events,
> + int instruction_idx, int branch_idx)
> +{
> + /*
> + * If HW supports GLOBAL_CTRL MSR, enabling and disabling PMCs are
> + * moved in __precise_loop(). Thus, instructions and branches events
> + * can be verified against a precise count instead of a rough range.
> + */
> + if (this_cpu_has_perf_global_ctrl()) {

This causes some intermittent failures on AMD processors using PerfMonV2
due to variance in counts. This probably has to do with the way instructions
leading to a VM-Entry or VM-Exit are accounted when counting retired
instructions and branches. Adding the following change makes all the tests
pass again.

diff --git a/x86/pmu.c b/x86/pmu.c
index 0658a1c1..09a34a3f 100644
--- a/x86/pmu.c
+++ b/x86/pmu.c
@@ -222,7 +222,7 @@ static void adjust_events_range(struct pmu_event *gp_events,
* moved in __precise_loop(). Thus, instructions and branches events
* can be verified against a precise count instead of a rough range.
*/
- if (this_cpu_has_perf_global_ctrl()) {
+ if (pmu.is_intel && this_cpu_has_perf_global_ctrl()) {
/* instructions event */
gp_events[instruction_idx].min = LOOP_INSTRNS;
gp_events[instruction_idx].max = LOOP_INSTRNS;

> + /* instructions event */
> + gp_events[instruction_idx].min = LOOP_INSTRNS;
> + gp_events[instruction_idx].max = LOOP_INSTRNS;
> + /* branches event */
> + gp_events[branch_idx].min = LOOP_BRANCHES;
> + gp_events[branch_idx].max = LOOP_BRANCHES;
> + }
> +}
> +
> volatile uint64_t irq_received;
>
> static void cnt_overflow(isr_regs_t *regs)
> @@ -823,6 +846,9 @@ static void check_invalid_rdpmc_gp(void)
>
> int main(int ac, char **av)
> {
> + int instruction_idx;
> + int branch_idx;
> +
> setup_vm();
> handle_irq(PMI_VECTOR, cnt_overflow);
> buf = malloc(N*64);
> @@ -836,13 +862,18 @@ int main(int ac, char **av)
> }
> gp_events = (struct pmu_event *)intel_gp_events;
> gp_events_size = sizeof(intel_gp_events)/sizeof(intel_gp_events[0]);
> + instruction_idx = INTEL_INSTRUCTIONS_IDX;
> + branch_idx = INTEL_BRANCHES_IDX;
> report_prefix_push("Intel");
> set_ref_cycle_expectations();
> } else {
> gp_events_size = sizeof(amd_gp_events)/sizeof(amd_gp_events[0]);
> gp_events = (struct pmu_event *)amd_gp_events;
> + instruction_idx = AMD_INSTRUCTIONS_IDX;
> + branch_idx = AMD_BRANCHES_IDX;
> report_prefix_push("AMD");
> }
> + adjust_events_range(gp_events, instruction_idx, branch_idx);
>
> printf("PMU version: %d\n", pmu.version);
> printf("GP counters: %d\n", pmu.nr_gp_counters);