Re: [kvm-unit-tests Patch v3 04/11] x86: pmu: Switch instructions and core cycles events sequence

[Mingwei Zhang]

On Wed, Jan 03, 2024, Dapeng Mi wrote:
> 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.
> 
> The event "instructions" instead of "core cycles" is a good choice as
> the warm-up event since it would always return a fixed count. Thus
> switch instructions and core cycles events sequence in the
> xxx_gp_events[] array.

The observation is great. However, it is hard to agree that we fix the
problem by switching the order. Maybe directly tweaking the N from 50 to
a larger value makes more sense.

Thanks.
-Mingwei
> 
> Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
> ---
>  x86/pmu.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/x86/pmu.c b/x86/pmu.c
> index a42fff8d8b36..67ebfbe55b49 100644
> --- a/x86/pmu.c
> +++ b/x86/pmu.c
> @@ -31,16 +31,16 @@ struct pmu_event {
>  	int min;
>  	int max;
>  } intel_gp_events[] = {
> -	{"core cycles", 0x003c, 1*N, 50*N},
>  	{"instructions", 0x00c0, 10*N, 10.2*N},
> +	{"core cycles", 0x003c, 1*N, 50*N},
>  	{"ref cycles", 0x013c, 1*N, 30*N},
>  	{"llc references", 0x4f2e, 1, 2*N},
>  	{"llc misses", 0x412e, 1, 1*N},
>  	{"branches", 0x00c4, 1*N, 1.1*N},
>  	{"branch misses", 0x00c5, 0, 0.1*N},
>  }, amd_gp_events[] = {
> -	{"core cycles", 0x0076, 1*N, 50*N},
>  	{"instructions", 0x00c0, 10*N, 10.2*N},
> +	{"core cycles", 0x0076, 1*N, 50*N},
>  	{"branches", 0x00c2, 1*N, 1.1*N},
>  	{"branch misses", 0x00c3, 0, 0.1*N},
>  }, fixed_events[] = {
> @@ -307,7 +307,7 @@ static void check_counter_overflow(void)
>  	int i;
>  	pmu_counter_t cnt = {
>  		.ctr = MSR_GP_COUNTERx(0),
> -		.config = EVNTSEL_OS | EVNTSEL_USR | gp_events[1].unit_sel /* instructions */,
> +		.config = EVNTSEL_OS | EVNTSEL_USR | gp_events[0].unit_sel /* instructions */,
>  	};
>  	overflow_preset = measure_for_overflow(&cnt);
>  
> @@ -365,11 +365,11 @@ static void check_gp_counter_cmask(void)
>  {
>  	pmu_counter_t cnt = {
>  		.ctr = MSR_GP_COUNTERx(0),
> -		.config = EVNTSEL_OS | EVNTSEL_USR | gp_events[1].unit_sel /* instructions */,
> +		.config = EVNTSEL_OS | EVNTSEL_USR | gp_events[0].unit_sel /* instructions */,
>  	};
>  	cnt.config |= (0x2 << EVNTSEL_CMASK_SHIFT);
>  	measure_one(&cnt);
> -	report(cnt.count < gp_events[1].min, "cmask");
> +	report(cnt.count < gp_events[0].min, "cmask");
>  }
>  
>  static void do_rdpmc_fast(void *ptr)
> @@ -446,7 +446,7 @@ static void check_running_counter_wrmsr(void)
>  	uint64_t count;
>  	pmu_counter_t evt = {
>  		.ctr = MSR_GP_COUNTERx(0),
> -		.config = EVNTSEL_OS | EVNTSEL_USR | gp_events[1].unit_sel,
> +		.config = EVNTSEL_OS | EVNTSEL_USR | gp_events[0].unit_sel,
>  	};
>  
>  	report_prefix_push("running counter wrmsr");
> @@ -455,7 +455,7 @@ static void check_running_counter_wrmsr(void)
>  	loop();
>  	wrmsr(MSR_GP_COUNTERx(0), 0);
>  	stop_event(&evt);
> -	report(evt.count < gp_events[1].min, "cntr");
> +	report(evt.count < gp_events[0].min, "cntr");
>  
>  	/* clear status before overflow test */
>  	if (this_cpu_has_perf_global_status())
> @@ -493,7 +493,7 @@ static void check_emulated_instr(void)
>  	pmu_counter_t instr_cnt = {
>  		.ctr = MSR_GP_COUNTERx(1),
>  		/* instructions */
> -		.config = EVNTSEL_OS | EVNTSEL_USR | gp_events[1].unit_sel,
> +		.config = EVNTSEL_OS | EVNTSEL_USR | gp_events[0].unit_sel,
>  	};
>  	report_prefix_push("emulated instruction");
>  
> -- 
> 2.34.1
>