linux-kernel - Re: [RFC/PATCH] perf x86: Add off-core event constraints for Sandy/IvyBridge micro architecture

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Date:	Mon, 28 Jan 2013 18:49:55 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC/PATCH] perf x86: Add off-core event constraints for
 Sandy/IvyBridge micro architecture

On Sun, Jan 27, 2013 at 6:33 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> hi,
> I was looking at the offcore stuff and it looks like we might
> be missing some constraints for offcore response events on
> Sandy/IvyBridge.
>
> The table 18.8.5 (Off-core Response Performance Monitoring)
> in Intel SDM states PMC0 for 0xb7 and PMC3 for 0xbb, but
> there's no other explanation or related description.
>
> I can't say/ack if the counters looks bad or right with or
> without the patch so far.. so just curious ;-)
>
Those are artificial constraints which should not be there.
Remember that offcore_rsp uses an extra MSR which has
to be shared by all the counters on the PMU. So a way to
handle the sharing of that extra MSR is to impose an
artificial constraint on the event itself. If it can only run
on one counter, then you get the management of the
extra MSR for free, i.e., only one event gets it.

In perf_events, we use a more sophisticated dynamic scheme
which does not use this artificial constraint. We can measure
the event multiple times and share the extra MSR if possible
(same value). Why multiple times you might ask? For instance,
with different priv levels.


Hope this helps.

> ---
> The Intel SDM (18.8.5 Off-core Response Performance Monitoring)
> states the off-core events MSR_OFFCORE_RSP_0/MSR_OFFCORE_RSP_03
> to be defined only for PMC0/PMC3 respectively.
>
> Adding related constraints.
>
> Signed-off-by: Jiri Olsa <jolsa@...hat.com>
> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Corey Ashford <cjashfor@...ux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Stephane Eranian <eranian@...gle.com>
> ---
>  arch/x86/kernel/cpu/perf_event_intel.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
> index 5dc54fc..d1f240f 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel.c
> @@ -105,6 +105,8 @@ static struct event_constraint intel_snb_event_constraints[] __read_mostly =
>         INTEL_EVENT_CONSTRAINT(0x48, 0x4), /* L1D_PEND_MISS.PENDING */
>         INTEL_UEVENT_CONSTRAINT(0x01c0, 0x2), /* INST_RETIRED.PREC_DIST */
>         INTEL_EVENT_CONSTRAINT(0xcd, 0x8), /* MEM_TRANS_RETIRED.LOAD_LATENCY */
> +       INTEL_EVENT_CONSTRAINT(0xb7, 0x1), /* MSR_OFFCORE_RSP_0 - PMC0 only*/
> +       INTEL_EVENT_CONSTRAINT(0xbb, 0x8), /* MSR_OFFCORE_RSP_1 - PMC3 only*/
>         EVENT_CONSTRAINT_END
>  };
>
> --
> 1.7.11.7
>
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