linux-kernel - RE: [PATCH v1 2/2] perf/x86: add cycles:pp alias for Intel Atom

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Message-ID: <37D7C6CF3E00A74B8858931C1DB2F077019A6A26@SHSMSX103.ccr.corp.intel.com>
Date:	Wed, 2 Dec 2015 19:37:16 +0000
From:	"Liang, Kan" <kan.liang@...el.com>
To:	Stephane Eranian <eranian@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	"acme@...hat.com" <acme@...hat.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: RE: [PATCH v1 2/2] perf/x86: add cycles:pp alias for Intel Atom



> -----Original Message-----
> From: Stephane Eranian [mailto:eranian@...gle.com]
> Sent: Wednesday, December 02, 2015 1:28 PM
> To: linux-kernel@...r.kernel.org
> Cc: acme@...hat.com; peterz@...radead.org; mingo@...e.hu;
> ak@...ux.intel.com; Liang, Kan
> Subject: [PATCH v1 2/2] perf/x86: add cycles:pp alias for Intel Atom
> 
> This patch updates the PEBS support for Intel Atom to provide an alias for
> the cycles:pp event used by perf record/top by default nowadays.
> 
> On Atom only INST_RETIRED:ANY supports PEBS, so we use this event
> instead with a large cmask to count cycles.
> 
> Signed-off-by: Stephane Eranian <eranian@...gle.com>
> ---
>  arch/x86/kernel/cpu/perf_event_intel.c    | 30
> ++++++++++++++++++++++++++++++
>  arch/x86/kernel/cpu/perf_event_intel_ds.c |  2 ++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event_intel.c
> b/arch/x86/kernel/cpu/perf_event_intel.c
> index 61f2577..7ff1e30 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel.c
> @@ -2475,6 +2475,35 @@ static void intel_pebs_aliases_snb(struct
> perf_event *event)
>  	}
>  }
> 
> +static void intel_pebs_aliases_atom(struct perf_event *event) {
> +	if ((event->hw.config & X86_RAW_EVENT_MASK) == 0x003c) {
> +		/*
> +		 * Use an alternative encoding for
> CPU_CLK_UNHALTED.THREAD_P
> +		 * (0x003c) so that we can use it with PEBS.
> +		 *
> +		 * The regular CPU_CLK_UNHALTED.THREAD_P event
> (0x003c) isn't
> +		 * PEBS capable. However we can use UOPS_RETIRED.ALL
> +		 * (0x01c2), which is a PEBS capable event, to get the same

The comment isn't consistent with the code.

> +		 * count.
> +		 *
> +		 * INST_RETIRED.ANY counts the number of cycles that
> retires
> +		 * CNTMASK instructions. By setting CNTMASK to a value
> (16)
> +		 * larger than the maximum number of instructions that
> can be
> +		 * retired per cycle (4) and then inverting the condition, we
> +		 * count all cycles that retire 16 or less instructions, which
> +		 * is every cycle.
> +		 *
> +		 * Thereby we gain a PEBS capable cycle counter.
> +		 */
> +		u64 alt_config =
> X86_CONFIG(.event=0xc0, .umask=0x00, .inv=1,
> +.cmask=16);
> +
> +		alt_config |= (event->hw.config &
> ~X86_RAW_EVENT_MASK);
> +		event->hw.config = alt_config;
> +	}
> +}
> +
> +
>  static unsigned long intel_pmu_free_running_flags(struct perf_event
> *event)  {
>  	unsigned long flags = x86_pmu.free_running_flags; @@ -3332,6
> +3361,7 @@ __init int intel_pmu_init(void)
> 
>  		x86_pmu.event_constraints =
> intel_gen_event_constraints;
>  		x86_pmu.pebs_constraints =
> intel_atom_pebs_event_constraints;
> +		x86_pmu.pebs_aliases = intel_pebs_aliases_atom;

intel_pebs_aliases_atom looks the same as intel_pebs_aliases_core2.
Why not reuse the existing intel_pebs_aliases_core2?

Thanks,
Kan
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