linux-kernel - Re: [PATCH v2] perf: honoring the architectural performance monitoring version

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Date:	Fri, 5 Jun 2015 17:42:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Imre Palik <imrep.amz@...il.com>
Cc:	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, "Palik, Imre" <imrep@...zon.de>,
	Anthony Liguori <aliguori@...zon.com>
Subject: Re: [PATCH v2] perf: honoring the architectural performance
 monitoring version

On Fri, Jun 05, 2015 at 05:28:45PM +0200, Imre Palik wrote:
> From: "Palik, Imre" <imrep@...zon.de>
> 
> Architectural performance monitoring version 1 doesn't support fixed
> counters.  Currently, even if a hypervisor advertises support for
> architectural performance monitoring version 1, perf may still tries to use
> the fixed counters, as the constraints are set up based on the CPU model.
> 
> This patch ensures that perf honors the architectural performance
> monitoring version returned by CPUID, and it only uses the fixed counters
> for version two and above.
> 
> Some of the ideas in this patch are coming from Peter Zijlstra.
> 
> Signed-off-by: Imre Palik <imrep@...zon.de>
> Cc: Anthony Liguori <aliguori@...zon.com>
> ---
>  arch/x86/kernel/cpu/perf_event_intel.c |    7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
> index 3998131..bde66aa 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel.c
> @@ -1870,7 +1870,7 @@ x86_get_event_constraints(struct cpu_hw_events *cpuc, int idx,
>  		for_each_event_constraint(c, x86_pmu.event_constraints) {
>  			if ((event->hw.config & c->cmask) == c->code) {
>  				event->hw.flags |= c->flags;
> -				return c;
> +				return  c->idxmsk64 ? c : NULL;

One too many spaces there :-) Returning c as found, even with empty
idxmsk is fine.

Also, I think this is broken, I think we hard assume
x86_get_event_constraints() returns a valid constraint, see for example:

	x86_schedule_event():

		c = x86_pmu.get_event_constraints()
			= intel_get_event_constraints()
				 = __intel_get_event_constraints()
					 = x86_get_event_constraints();

		cpuc->event_constraint[i] = c;

		...

		c = cpuc->event_constraint[i];

		if (!test_bit(hwc->idx, c->idxmask)) <-- *boom*


> @@ -3341,9 +3341,12 @@ __init int intel_pmu_init(void)
>  		for_each_event_constraint(c, x86_pmu.event_constraints) {
>  			if (c->cmask != FIXED_EVENT_FLAGS
>  			    || c->idxmsk64 == INTEL_PMC_MSK_FIXED_REF_CYCLES) {
> +				c->idxmsk64 &=
> +					~(~0UL << (INTEL_PMC_IDX_FIXED + x86_pmu.num_counters_fixed));

If you change idxmsk64 you also need to update weight.

>  				continue;
>  			}
> -
> +			c->idxmsk64 &=
> +				~(~0UL << (INTEL_PMC_IDX_FIXED + x86_pmu.num_counters_fixed));
>  			c->idxmsk64 |= (1ULL << x86_pmu.num_counters) - 1;
>  			c->weight += x86_pmu.num_counters;

And since we're now not unconditionally adding num_counters bits, that
weight update is broken.

For both sites, something like:

		c->weight = hweight64(c->idxmsk64);

Will recompute the weight.

Thanks!
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