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

[Mingwei Zhang]

On Wed, Mar 27, 2024, Mi, Dapeng wrote:
> 
> On 3/27/2024 1:36 PM, Mingwei Zhang wrote:
> > 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
> 
> yeah, a larger upper boundary can fix the fault as well, but the question is
> how large it would be enough. For different CPU model, the needed cycles
> could be different for warming up. So we may have to set a quite large upper
> boundary but a large boundary would decrease credibility of this validation.
> Not sure which one is better. Any inputs from other ones?
> 

Just checked with an expert from our side, so "core cycles" (0x003c)
is affected the current CPU state/frequency, ie., its counting value
could vary largely. In that sense, "warming" up seems reasonable.
However, switching the order would be a terrible idea for maintanence
since people will forget it and the problem will come back.

>From another perspective, "warming" up seems just a best effort. Nobody
knows how warm is really warm. Besides, some systems might turn off some
C-State and may set a cap on max turbo frequency. All of these will
directly affect the warm-up process and the counting result of 0x003c.

So, while adding a warm-up blob is reasonable, tweaking the heuristics
seems to be same for me. Regarding the value, I think I will rely on
your experiments and observation.

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
> > >