perf lock contention: Change stack_id type to s32

The bpf_get_stackid() helper returns a signed type to check whether it
failed to get a stacktrace or not.  But it saved the result in u32 and
checked if the value is negative.

      376         if (needs_callstack) {
      377                 pelem->stack_id = bpf_get_stackid(ctx, &stacks,
      378                                                   BPF_F_FAST_STACK_CMP | stack_skip);
  --> 379                 if (pelem->stack_id < 0)

  ./tools/perf/util/bpf_skel/lock_contention.bpf.c:379 contention_begin()
  warn: unsigned 'pelem->stack_id' is never less than zero.

Let's change the type to s32 instead.

Fixes: 6d499a6b3d90277d ("perf lock: Print the number of lost entries for BPF")
Reported-by: Dan Carpenter <[email protected]>
Signed-off-by: Namhyung Kim <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

2 files changed, 3 insertions, 2 deletions

diff --git a/tools/perf/util/bpf_skel/lock_data.h b/tools/perf/util/bpf_skel/lock_data.h
index 36af11faad03..de12892f992f 100644
--- a/tools/perf/util/bpf_skel/lock_data.h
+++ b/tools/perf/util/bpf_skel/lock_data.h
@@ -7,11 +7,11 @@ struct tstamp_data {
 	u64 timestamp;
 	u64 lock;
 	u32 flags;
-	u32 stack_id;
+	s32 stack_id;
 };
 
 struct contention_key {
-	u32 stack_id;
+	s32 stack_id;
 	u32 pid;
 	u64 lock_addr_or_cgroup;
 };
diff --git a/tools/perf/util/bpf_skel/vmlinux/vmlinux.h b/tools/perf/util/bpf_skel/vmlinux/vmlinux.h
index e9028235d771..d818e30c5457 100644
--- a/tools/perf/util/bpf_skel/vmlinux/vmlinux.h
+++ b/tools/perf/util/bpf_skel/vmlinux/vmlinux.h
@@ -15,6 +15,7 @@
 
 typedef __u8 u8;
 typedef __u32 u32;
+typedef __s32 s32;
 typedef __u64 u64;
 typedef __s64 s64;