Re: [PATCH] perf/x86: fix event counter update issue

From: Andi Kleen <ak@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>,
	"Liang, Kan" <kan.liang@intel.com>,
	"mingo@redhat.com" <mingo@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	"Odzioba, Lukasz" <lukasz.odzioba@intel.com>
Subject: Re: [PATCH] perf/x86: fix event counter update issue
Date: Tue, 29 Nov 2016 10:37:03 -0800	[thread overview]
Message-ID: <20161129183703.GC8388@tassilo.jf.intel.com> (raw)
In-Reply-To: <20161129173055.GP3092@twins.programming.kicks-ass.net>

On Tue, Nov 29, 2016 at 06:30:55PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 29, 2016 at 09:20:10AM -0800, Stephane Eranian wrote:
> > Max period is limited by the number of bits the kernel can write to an MSR.
> > Used to be 31, now it is 47 for core PMU as per patch pointed to by Kan.
> 
> No, I think it sets it to 48 now, which is the problem. It should be 1
> bit less than the total width.
> 
> So something like so.

That looks good.  Kan can you test it?

-Andi
> 
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index a74a2dbc0180..cb8522290e6a 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -4034,7 +4034,7 @@ __init int intel_pmu_init(void)
>  
>  	/* Support full width counters using alternative MSR range */
>  	if (x86_pmu.intel_cap.full_width_write) {
> -		x86_pmu.max_period = x86_pmu.cntval_mask;
> +		x86_pmu.max_period = x86_pmu.cntval_mask >> 1;
>  		x86_pmu.perfctr = MSR_IA32_PMC0;
>  		pr_cont("full-width counters, ");
>  	}