linux-kernel - Re: [PATCH V3 01/13] perf/core: Add new branch sample type for LBR TOS

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Date:   Thu, 24 Oct 2019 15:41:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     acme@...nel.org, mingo@...nel.org, linux-kernel@...r.kernel.org,
        jolsa@...nel.org, namhyung@...nel.org, vitaly.slobodskoy@...el.com,
        pavel.gerasimov@...el.com, ak@...ux.intel.com, eranian@...gle.com,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH V3 01/13] perf/core: Add new branch sample type for LBR
 TOS

On Tue, Oct 22, 2019 at 10:11:24AM -0700, kan.liang@...ux.intel.com wrote:
> From: Kan Liang <kan.liang@...ux.intel.com>
> 
> In LBR call stack mode, the depth of reconstructed LBR call stack limits
> to the number of LBR registers. With LBR Top-of-Stack (TOS) information,
> perf tool may stitch the stacks of two samples. The reconstructed LBR
> call stack can break the HW limitation.
> 
> Add a new branch sample type to retrieve LBR TOS.
> 
> Only when the new branch sample type is set, the TOS information is
> dumped into the PERF_SAMPLE_BRANCH_STACK output.
> Perf tool should check the attr.branch_sample_type, and apply the
> corresponding format for PERF_SAMPLE_BRANCH_STACK samples.
> Otherwise, some user case may be broken. For example, users may parse a
> perf.data, which include the new branch sample type, with an old version
> perf tool (without the check). Users probably get incorrect information
> without any warning.
> 
> Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
> ---
>  include/linux/perf_event.h      |  2 ++
>  include/uapi/linux/perf_event.h | 10 +++++++++-
>  kernel/events/core.c            | 11 +++++++++++
>  3 files changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 61448c19a132..2b229ea1cc15 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -92,6 +92,7 @@ struct perf_raw_record {
>  /*
>   * branch stack layout:
>   *  nr: number of taken branches stored in entries[]
> + *  tos: Top-of-Stack (TOS) information. PMU specific data.
>   *
>   * Note that nr can vary from sample to sample
>   * branches (to, from) are stored from most recent
> @@ -100,6 +101,7 @@ struct perf_raw_record {
>   */
>  struct perf_branch_stack {
>  	__u64				nr;
> +	__u64				tos; /* PMU specific data */
>  	struct perf_branch_entry	entries[0];
>  };
>  
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index bb7b271397a6..b1f022190571 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -180,6 +180,8 @@ enum perf_branch_sample_type_shift {
>  
>  	PERF_SAMPLE_BRANCH_TYPE_SAVE_SHIFT	= 16, /* save branch type */
>  
> +	PERF_SAMPLE_BRANCH_LBR_TOS_SHIFT	= 17, /* save LBR TOS */

I think I prefer not having LBR here either, who knows what other
hardware can make use of that.

On that, you've completely failed to Cc the other architecture that
implement PERF_SAMPLE_BRANCH.

Aside from that I can live with this version.

> +
>  	PERF_SAMPLE_BRANCH_MAX_SHIFT		/* non-ABI */
>  };
>  
> @@ -207,6 +209,8 @@ enum perf_branch_sample_type {
>  	PERF_SAMPLE_BRANCH_TYPE_SAVE	=
>  		1U << PERF_SAMPLE_BRANCH_TYPE_SAVE_SHIFT,
>  
> +	PERF_SAMPLE_BRANCH_LBR_TOS	= 1U << PERF_SAMPLE_BRANCH_LBR_TOS_SHIFT,
> +
>  	PERF_SAMPLE_BRANCH_MAX		= 1U << PERF_SAMPLE_BRANCH_MAX_SHIFT,
>  };
>  
> @@ -849,7 +853,11 @@ enum perf_event_type {
>  	 *	  char                  data[size];}&& PERF_SAMPLE_RAW
>  	 *
>  	 *	{ u64                   nr;
> -	 *        { u64 from, to, flags } lbr[nr];} && PERF_SAMPLE_BRANCH_STACK
> +	 *        { u64 from, to, flags } lbr[nr];
> +	 *
> +	 *        # only available if PERF_SAMPLE_BRANCH_LBR_TOS is set
> +	 *        u64			tos;
> +	 *      } && PERF_SAMPLE_BRANCH_STACK
>  	 *
>  	 * 	{ u64			abi; # enum perf_sample_regs_abi
>  	 * 	  u64			regs[weight(mask)]; } && PERF_SAMPLE_REGS_USER
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 9ec0b0bfddbd..18b0a7d2c67e 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6343,6 +6343,11 @@ static void perf_output_read(struct perf_output_handle *handle,
>  		perf_output_read_one(handle, event, enabled, running);
>  }
>  
> +static inline bool perf_sample_save_lbr_tos(struct perf_event *event)
> +{
> +	return event->attr.branch_sample_type & PERF_SAMPLE_BRANCH_LBR_TOS;
> +}
> +
>  void perf_output_sample(struct perf_output_handle *handle,
>  			struct perf_event_header *header,
>  			struct perf_sample_data *data,
> @@ -6432,6 +6437,8 @@ void perf_output_sample(struct perf_output_handle *handle,
>  
>  			perf_output_put(handle, data->br_stack->nr);
>  			perf_output_copy(handle, data->br_stack->entries, size);
> +			if (perf_sample_save_lbr_tos(event))
> +				perf_output_put(handle, data->br_stack->tos);
>  		} else {
>  			/*
>  			 * we always store at least the value of nr
> @@ -6619,7 +6626,11 @@ void perf_prepare_sample(struct perf_event_header *header,
>  		if (data->br_stack) {
>  			size += data->br_stack->nr
>  			      * sizeof(struct perf_branch_entry);
> +
> +			if (perf_sample_save_lbr_tos(event))
> +				size += sizeof(u64);
>  		}
> +
>  		header->size += size;
>  	}
>  
> -- 
> 2.17.1
>