Re: [RESEND PATCH V5 1/2] perf/core: Add new branch sample type for HW index of raw branch records

[Stephane Eranian]

On Thu, Jan 16, 2020 at 7:59 AM <kan.liang@xxxxxxxxxxxxxxx> wrote:
>
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
>
> The low level index of raw branch records is very useful for
> reconstructing the call stack. For example, in Intel LBR call stack mode,
> the depth of reconstructed LBR call stack limits to the number of LBR
> registers. With the HW index 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 the low level index of raw
> branch records. Only need to save the index for the most recent branch
> aka entries[0]. Others can be calculated by the max number of HW
> supported branch records later in perf tool.
>
You need to define what the low level index is about w.r.t. the branch_entries[]
abstraction. You need to say it is a value between -1 (unknown) and max depth
(which can be retrieved in /sys/devices/cpu/caps/branches). It returns the index
in the underlying hardware buffer of the most recently captured taken branch
which is always saved in branch_entries[0]. As such, it is not necessary to
process branches in order. It may be used in certain modes to stitch
together multiple
BRANCH_STACK records in call stack mode.

>
> If we don't know the order of raw branch records, the hw_idx should be
> -1ULL. This patch sets -1ULL for all architectures for now. It can be
> changed later if needed.
>
> Only when the new branch sample type is set, the HW index 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@xxxxxxxxxxxxxxx>
> ---
>  arch/powerpc/perf/core-book3s.c |  1 +
>  arch/x86/events/intel/lbr.c     |  3 +++
>  include/linux/perf_event.h      | 12 ++++++++++++
>  include/uapi/linux/perf_event.h | 10 +++++++++-
>  kernel/events/core.c            | 11 +++++++++++
>  5 files changed, 36 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
> index 48604625ab31..fe7de222229a 100644
> --- a/arch/powerpc/perf/core-book3s.c
> +++ b/arch/powerpc/perf/core-book3s.c
> @@ -524,6 +524,7 @@ static void power_pmu_bhrb_read(struct perf_event *event, struct cpu_hw_events *
>                 }
>         }
>         cpuhw->bhrb_stack.nr = u_index;
> +       cpuhw->bhrb_stack.hw_idx = -1ULL;
>         return;
>  }
>
> diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> index 534c76606049..7639e2097101 100644
> --- a/arch/x86/events/intel/lbr.c
> +++ b/arch/x86/events/intel/lbr.c
> @@ -585,6 +585,7 @@ static void intel_pmu_lbr_read_32(struct cpu_hw_events *cpuc)
>                 cpuc->lbr_entries[i].reserved   = 0;
>         }
>         cpuc->lbr_stack.nr = i;
> +       cpuc->lbr_stack.hw_idx = -1ULL;
>  }
>
>  /*
> @@ -680,6 +681,7 @@ static void intel_pmu_lbr_read_64(struct cpu_hw_events *cpuc)
>                 out++;
>         }
>         cpuc->lbr_stack.nr = out;
> +       cpuc->lbr_stack.hw_idx = -1ULL;
>  }
>
>  void intel_pmu_lbr_read(void)
> @@ -1120,6 +1122,7 @@ void intel_pmu_store_pebs_lbrs(struct pebs_lbr *lbr)
>         int i;
>
>         cpuc->lbr_stack.nr = x86_pmu.lbr_nr;
> +       cpuc->lbr_stack.hw_idx = -1ULL;
>         for (i = 0; i < x86_pmu.lbr_nr; i++) {
>                 u64 info = lbr->lbr[i].info;
>                 struct perf_branch_entry *e = &cpuc->lbr_entries[i];
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 011dcbdbccc2..554621d99864 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -93,14 +93,26 @@ struct perf_raw_record {
>  /*
>   * branch stack layout:
>   *  nr: number of taken branches stored in entries[]
> + *  hw_idx: The low level index of raw branch records
> + *          for the most recent branch.
> + *          -1ULL means invalid.
>   *
>   * Note that nr can vary from sample to sample
>   * branches (to, from) are stored from most recent
>   * to least recent, i.e., entries[0] contains the most
>   * recent branch.
> + * The entries[] is an abstraction of raw branch records,
> + * which may not be stored in age order in HW, e.g. Intel LBR.
> + * The hw_idx is to expose the low level index of raw
> + * branch record for the most recent branch aka entries[0].
> + * For the architectures whose raw branch records are
> + * already stored in age order, the hw_idx should be 0.
> + * If we don't know the order of raw branch records,
> + * the hw_idx should be -1ULL.
>   */
>  struct perf_branch_stack {
>         __u64                           nr;
> +       __u64                           hw_idx;
>         struct perf_branch_entry        entries[0];
>  };
>
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index bb7b271397a6..14110837b130 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_HW_INDEX_SHIFT       = 17, /* save low level index of raw branch records */
> +
>         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_HW_INDEX     = 1U << PERF_SAMPLE_BRANCH_HW_INDEX_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_HW_INDEX is set
> +        *        u64                   hw_idx;
> +        *      } && 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 cfd89b4a02d8..4be3ba12333f 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6391,6 +6391,11 @@ static void perf_output_read(struct perf_output_handle *handle,
>                 perf_output_read_one(handle, event, enabled, running);
>  }
>
> +static inline bool perf_sample_save_hw_index(struct perf_event *event)
> +{
> +       return event->attr.branch_sample_type & PERF_SAMPLE_BRANCH_HW_INDEX;
> +}
> +
>  void perf_output_sample(struct perf_output_handle *handle,
>                         struct perf_event_header *header,
>                         struct perf_sample_data *data,
> @@ -6480,6 +6485,8 @@ 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_hw_index(event))
> +                               perf_output_put(handle, data->br_stack->hw_idx);
>                 } else {
>                         /*
>                          * we always store at least the value of nr
> @@ -6667,7 +6674,11 @@ 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_hw_index(event))
> +                               size += sizeof(u64);
>                 }
> +
>                 header->size += size;
>         }
>
> --
> 2.17.1
>