Re: [PATCH 10/14] perf, core: simplify need branch stack check

[Stephane Eranian]

On Fri, Jan 3, 2014 at 6:48 AM, Yan, Zheng <zheng.z.yan@xxxxxxxxx> wrote:
> event->attr.branch_sample_type is non-zero no matter branch stack
> is enabled explicitly or is enabled implicitly. So we can use it
> toreplace intel_pmu_needs_lbr_smpl(). This avoids duplicating code
> that implicitly enables the LBR.
>
This patch does more than what you describe here.
Explain the simplifications.
Explain the difference between has_branch_stack() and needs_branch_stack().

Given the way you've implemented LBR_CALLSTACK (not exposed to users).
You reusing the attr->sample_branch_type to stash you CALLSTACK mode.
So you end up in a situation where you have sample_branch_stack != 0 but
(sample_format_type & PERF_SAMPLE_BRANCH) == 0. Yet, you need
to detect if branch stack is used, thus you need to use sample_branch_type.


> Signed-off-by: Yan, Zheng <zheng.z.yan@xxxxxxxxx>
> ---
>  arch/x86/kernel/cpu/perf_event_intel.c | 20 +++-----------------
>  include/linux/perf_event.h             |  5 +++++
>  kernel/events/core.c                   | 11 +++++++----
>  3 files changed, 15 insertions(+), 21 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
> index 84a1c09..722171c 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel.c
> @@ -1030,20 +1030,6 @@ static __initconst const u64 slm_hw_cache_event_ids
>   },
>  };
>
> -static inline bool intel_pmu_needs_lbr_smpl(struct perf_event *event)
> -{
> -       /* user explicitly requested branch sampling */
> -       if (has_branch_stack(event))
> -               return true;
> -
> -       /* implicit branch sampling to correct PEBS skid */
> -       if (x86_pmu.intel_cap.pebs_trap && event->attr.precise_ip > 1 &&
> -           x86_pmu.intel_cap.pebs_format < 2)
> -               return true;
> -
> -       return false;
> -}
> -
>  static void intel_pmu_disable_all(void)
>  {
>         struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
> @@ -1208,7 +1194,7 @@ static void intel_pmu_disable_event(struct perf_event *event)
>          * must disable before any actual event
>          * because any event may be combined with LBR
>          */
> -       if (intel_pmu_needs_lbr_smpl(event))
> +       if (needs_branch_stack(event))
>                 intel_pmu_lbr_disable(event);
>
>         if (unlikely(hwc->config_base == MSR_ARCH_PERFMON_FIXED_CTR_CTRL)) {
> @@ -1269,7 +1255,7 @@ static void intel_pmu_enable_event(struct perf_event *event)
>          * must enabled before any actual event
>          * because any event may be combined with LBR
>          */
> -       if (intel_pmu_needs_lbr_smpl(event))
> +       if (needs_branch_stack(event))
>                 intel_pmu_lbr_enable(event);
>
>         if (event->attr.exclude_host)
> @@ -1741,7 +1727,7 @@ static int intel_pmu_hw_config(struct perf_event *event)
>         if (event->attr.precise_ip && x86_pmu.pebs_aliases)
>                 x86_pmu.pebs_aliases(event);
>
> -       if (intel_pmu_needs_lbr_smpl(event)) {
> +       if (needs_branch_stack(event)) {
>                 ret = intel_pmu_setup_lbr_filter(event);
>                 if (ret)
>                         return ret;
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 147f9d3..0d88eb8 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -766,6 +766,11 @@ static inline bool has_branch_stack(struct perf_event *event)
>         return event->attr.sample_type & PERF_SAMPLE_BRANCH_STACK;
>  }
>
> +static inline bool needs_branch_stack(struct perf_event *event)
> +{
> +       return event->attr.branch_sample_type != 0;
> +}
> +
>  extern int perf_output_begin(struct perf_output_handle *handle,
>                              struct perf_event *event, unsigned int size);
>  extern void perf_output_end(struct perf_output_handle *handle);
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index d6d8dea..7dd4d58 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -1138,7 +1138,7 @@ list_add_event(struct perf_event *event, struct perf_event_context *ctx)
>         if (is_cgroup_event(event))
>                 ctx->nr_cgroups++;
>
> -       if (has_branch_stack(event))
> +       if (needs_branch_stack(event))
>                 ctx->nr_branch_stack++;
>
>         list_add_rcu(&event->event_entry, &ctx->event_list);
> @@ -1303,7 +1303,7 @@ list_del_event(struct perf_event *event, struct perf_event_context *ctx)
>                         cpuctx->cgrp = NULL;
>         }
>
> -       if (has_branch_stack(event))
> +       if (needs_branch_stack(event))
>                 ctx->nr_branch_stack--;
>
>         ctx->nr_events--;
> @@ -3202,7 +3202,7 @@ static void unaccount_event(struct perf_event *event)
>                 atomic_dec(&nr_freq_events);
>         if (is_cgroup_event(event))
>                 static_key_slow_dec_deferred(&perf_sched_events);
> -       if (has_branch_stack(event))
> +       if (needs_branch_stack(event))
>                 static_key_slow_dec_deferred(&perf_sched_events);
>
>         unaccount_event_cpu(event, event->cpu);
> @@ -6627,7 +6627,7 @@ static void account_event(struct perf_event *event)
>                 if (atomic_inc_return(&nr_freq_events) == 1)
>                         tick_nohz_full_kick_all();
>         }
> -       if (has_branch_stack(event))
> +       if (needs_branch_stack(event))
>                 static_key_slow_inc(&perf_sched_events.key);
>         if (is_cgroup_event(event))
>                 static_key_slow_inc(&perf_sched_events.key);
> @@ -6735,6 +6735,9 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
>         if (attr->inherit && (attr->read_format & PERF_FORMAT_GROUP))
>                 goto err_ns;
>
> +       if (!has_branch_stack(event))
> +               event->attr.branch_sample_type = 0;
> +
>         pmu = perf_init_event(event);
>         if (!pmu)
>                 goto err_ns;
> --
> 1.8.4.2
>
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