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Date:	Thu, 6 Feb 2014 16:46:47 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	"Yan, Zheng" <zheng.z.yan@...el.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH 12/14] perf, x86: use LBR call stack to get user callchain

On Fri, Jan 3, 2014 at 6:48 AM, Yan, Zheng <zheng.z.yan@...el.com> wrote:
> Haswell has a new feature that utilizes the existing Last Branch Record
> facility to record call chains. When the feature is enabled, function
> call will be collected as normal, but as return instructions are executed
> the last captured branch record is popped from the on-chip LBR registers.
> The LBR call stack facility can help perf to get call chains of progam
> without frame pointer.
>
> This patch makes x86's perf_callchain_user() failback to LBR callstack
> when there is no frame pointer in the user program.
>
> Signed-off-by: Yan, Zheng <zheng.z.yan@...el.com>
> ---
>  arch/x86/kernel/cpu/perf_event.c           | 33 ++++++++++++++++++++++++++----
>  arch/x86/kernel/cpu/perf_event_intel.c     | 11 +++++++++-
>  arch/x86/kernel/cpu/perf_event_intel_lbr.c |  2 ++
>  include/linux/perf_event.h                 |  1 +
>  4 files changed, 42 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index 49128e6..1509340 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -1965,12 +1965,28 @@ static unsigned long get_segment_base(unsigned int segment)
>         return get_desc_base(desc + idx);
>  }
>
> +static inline void
> +perf_callchain_lbr_callstack(struct perf_callchain_entry *entry,
> +                            struct perf_sample_data *data)
> +{
> +       struct perf_branch_stack *br_stack = data->br_stack;
> +
> +       if (br_stack && br_stack->user_callstack) {
> +               int i = 0;
> +               while (i < br_stack->nr && entry->nr < PERF_MAX_STACK_DEPTH) {
> +                       perf_callchain_store(entry, br_stack->entries[i].from);
> +                       i++;
> +               }
> +       }
> +}
> +
>  #ifdef CONFIG_COMPAT
>
>  #include <asm/compat.h>
>
>  static inline int
> -perf_callchain_user32(struct pt_regs *regs, struct perf_callchain_entry *entry)
> +perf_callchain_user32(struct perf_callchain_entry *entry,
> +                     struct pt_regs *regs, struct perf_sample_data *data)
>  {
>         /* 32-bit process in 64-bit kernel. */
>         unsigned long ss_base, cs_base;
> @@ -1999,11 +2015,16 @@ perf_callchain_user32(struct pt_regs *regs, struct perf_callchain_entry *entry)
>                 perf_callchain_store(entry, cs_base + frame.return_address);
>                 fp = compat_ptr(ss_base + frame.next_frame);
>         }
> +
> +       if (fp == compat_ptr(regs->bp))
> +               perf_callchain_lbr_callstack(entry, data);
> +
>         return 1;
>  }
>  #else
>  static inline int
> -perf_callchain_user32(struct pt_regs *regs, struct perf_callchain_entry *entry)
> +perf_callchain_user32(struct perf_callchain_entry *entry,
> +                     struct pt_regs *regs, struct perf_sample_data *data)
>  {
>      return 0;
>  }
> @@ -2033,12 +2054,12 @@ void perf_callchain_user(struct perf_callchain_entry *entry,
>         if (!current->mm)
>                 return;
>
> -       if (perf_callchain_user32(regs, entry))
> +       if (perf_callchain_user32(entry, regs, data))
>                 return;
>
>         while (entry->nr < PERF_MAX_STACK_DEPTH) {
>                 unsigned long bytes;
> -               frame.next_frame             = NULL;
> +               frame.next_frame = NULL;
>                 frame.return_address = 0;
>
>                 bytes = copy_from_user_nmi(&frame, fp, sizeof(frame));
> @@ -2051,6 +2072,10 @@ void perf_callchain_user(struct perf_callchain_entry *entry,
>                 perf_callchain_store(entry, frame.return_address);
>                 fp = frame.next_frame;
>         }
> +
> +       /* try LBR callstack if there is no frame pointer */
> +       if (fp == (void __user *)regs->bp)
> +               perf_callchain_lbr_callstack(entry, data);
>  }
>
>  /*
> diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
> index 722171c..8b7465c 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel.c
> @@ -1030,6 +1030,14 @@ static __initconst const u64 slm_hw_cache_event_ids
>   },
>  };
>
> +static inline bool intel_pmu_needs_lbr_callstack(struct perf_event *event)
> +{
> +       if ((event->attr.sample_type & PERF_SAMPLE_CALLCHAIN) &&
> +           (event->attr.branch_sample_type & PERF_SAMPLE_BRANCH_CALL_STACK))
> +               return true;
> +       return false;
> +}
> +
>  static void intel_pmu_disable_all(void)
>  {
>         struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
> @@ -1398,7 +1406,8 @@ again:
>
>                 perf_sample_data_init(&data, 0, event->hw.last_period);
>
> -               if (has_branch_stack(event))
> +               if (has_branch_stack(event) ||
> +                   (event->ctx->task && intel_pmu_needs_lbr_callstack(event)))

Isn't event->ctx->task redundant here. I thought you were already allowing
LBR_CALLSTACK only for per-process events. That should be checked during
setup, no need to do it for each interrupt.

Also it would be nicer to have:
        if (needs_lbr_stack(event))
                          data.br_stack = &cpuc->lbr_stack;

And you'd hide the two tests in that needs_lbr_stack() inline:
   has_branch_stack() and has_lbr_callstack().

That would be better for the eyes....


>
>                 if (perf_event_overflow(event, &data, regs))
> diff --git a/arch/x86/kernel/cpu/perf_event_intel_lbr.c b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> index 51e1842..08e3ba1 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> @@ -718,6 +718,8 @@ intel_pmu_lbr_filter(struct cpu_hw_events *cpuc)
>         int i, j, type;
>         bool compress = false;
>
> +       cpuc->lbr_stack.user_callstack = branch_user_callstack(br_sel);
> +
>         /* if sampling all branches, then nothing to filter */
>         if ((br_sel & X86_BR_ALL) == X86_BR_ALL)
>                 return;
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index c442276..d2f0488 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -74,6 +74,7 @@ struct perf_raw_record {
>   * recent branch.
>   */
>  struct perf_branch_stack {
> +       bool                            user_callstack;
>         __u64                           nr;
>         struct perf_branch_entry        entries[0];
>  };
> --
> 1.8.4.2
>
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