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    SubjectRe: [PATCH 06/12] perf: Return empty callchain instead of NULL
    On Sun, Jan 07, 2018 at 05:03:50PM +0100, Jiri Olsa wrote:
    > It simplifies the code a bit, because we dump the callchain
    > even if it's empty. With 'empty' callchain we can remove
    > all the NULL-checking code paths.

    Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

    >
    > Original-patch-from: Peter Zijlstra <peterz@infradead.org>
    > Link: http://lkml.kernel.org/n/tip-n4t869o2qc1b31v4wwvmxznb@git.kernel.org
    > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
    > ---
    > kernel/events/core.c | 30 ++++++++++++------------------
    > 1 file changed, 12 insertions(+), 18 deletions(-)
    >
    > diff --git a/kernel/events/core.c b/kernel/events/core.c
    > index 5fc1ded4b450..4e1a1bf8d867 100644
    > --- a/kernel/events/core.c
    > +++ b/kernel/events/core.c
    > @@ -5815,19 +5815,11 @@ void perf_output_sample(struct perf_output_handle *handle,
    > perf_output_read(handle, event);
    >
    > if (sample_type & PERF_SAMPLE_CALLCHAIN) {
    > - if (data->callchain) {
    > - int size = 1;
    > -
    > - if (data->callchain)
    > - size += data->callchain->nr;
    > -
    > - size *= sizeof(u64);
    > + int size = 1;
    >
    > - __output_copy(handle, data->callchain, size);
    > - } else {
    > - u64 nr = 0;
    > - perf_output_put(handle, nr);
    > - }
    > + size += data->callchain->nr;
    > + size *= sizeof(u64);
    > + __output_copy(handle, data->callchain, size);
    > }
    >
    > if (sample_type & PERF_SAMPLE_RAW) {
    > @@ -5980,6 +5972,8 @@ static u64 perf_virt_to_phys(u64 virt)
    > return phys_addr;
    > }
    >
    > +static struct perf_callchain_entry __empty_callchain = { .nr = 0, };
    > +
    > static struct perf_callchain_entry *
    > perf_callchain(struct perf_event *event, struct pt_regs *regs)
    > {
    > @@ -5988,12 +5982,14 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs)
    > /* Disallow cross-task user callchains. */
    > bool crosstask = event->ctx->task && event->ctx->task != current;
    > const u32 max_stack = event->attr.sample_max_stack;
    > + struct perf_callchain_entry *callchain;
    >
    > if (!kernel && !user)
    > - return NULL;
    > + return &__empty_callchain;
    >
    > - return get_perf_callchain(regs, 0, kernel, user,
    > - max_stack, crosstask, true);
    > + callchain = get_perf_callchain(regs, 0, kernel, user,
    > + max_stack, crosstask, true);
    > + return callchain ?: &__empty_callchain;
    > }
    >
    > void perf_prepare_sample(struct perf_event_header *header,
    > @@ -6018,9 +6014,7 @@ void perf_prepare_sample(struct perf_event_header *header,
    > int size = 1;
    >
    > data->callchain = perf_callchain(event, regs);
    > -
    > - if (data->callchain)
    > - size += data->callchain->nr;
    > + size += data->callchain->nr;
    >
    > header->size += size * sizeof(u64);
    > }
    > --
    > 2.13.6
    >

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