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    SubjectRe: [PATCH 12/14] perf, x86: use LBR call stack to get user callchain
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    On Fri, Jan 3, 2014 at 6:48 AM, Yan, Zheng <zheng.z.yan@intel.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@intel.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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