linux-kernel - Re: [PATCH V2 01/13] perf/core: Add new branch sample type for LBR TOS

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Date:   Tue, 22 Oct 2019 11:39:26 +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
Subject: Re: [PATCH V2 01/13] perf/core: Add new branch sample type for LBR
 TOS

On Mon, Oct 21, 2019 at 01:03:02PM -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      |  4 ++++
>  include/uapi/linux/perf_event.h | 10 +++++++++-
>  kernel/events/core.c            | 10 ++++++++++
>  3 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 61448c19a132..0cebc8ec44fa 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -972,6 +972,10 @@ struct perf_sample_data {
>  	u64				stack_user_size;
>  
>  	u64				phys_addr;
> +
> +	/* PMU specific data */
> +	u64				lbr_tos;
> +
>  } ____cacheline_aligned;

Last time you put this in perf_branch_stack, that was a much better
place. Can't this work now?