perf: Return empty callchain instead of NULL

It simplifies the code a bit, because we dump the callchain
Link: http://lkml.kernel.org/n/tip-uqp7qd6aif47g39glnbu95yl@git.kernel.org
even if it's empty. With 'empty' callchain we can remove
all the NULL-checking code paths.

Original-patch-from: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20180107160356.28203-7-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 5fc1ded..4e1a1bf 100644 (file)
--- 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);
        }