[PATCH] perf script: Fix LBR skid dump problems in brstackinsn

[Andi Kleen]

From: Andi Kleen <ak@xxxxxxxxxxxxxxx>

This is a fix for another instance of the skid problem Milian
recently found [1]

The LBRs don't freeze at the exact same time as the PMI is triggered.
The perf script brstackinsn code that dumps LBR assembler
assumes that the last branch in the LBR leads to the sample point.
But with skid it's possible that the CPU executes one or more branches
before the sample, but which do not appear in the LBR.

What happens then is either that the sample point is before
the last LBR branch. In this case the dumper sees a negative
length and ignores it. Or it the sample point is long after
the last branch. Then the dumper sees a very long block and dumps
it upto its block limit (16k bytes), which is noise in the output.

On typical sample session this can happen regularly.

This patch tries to detect and handle the situation. On the last
block that is dumped by the LBR dumper we always stop on the first
branch. If the block length is negative just scan forward to the
first branch. Otherwise scan until a branch is found.

The PT decoder already has a function that uses the instruction
decoder to detect branches, so we can just reuse it here.

Then when a terminating branch is found print an indication
and stop dumping. This might miss a few instructions, but at least
shows no runaway blocks.

Cc: milian.wolff@xxxxxxxx
Cc: adrian.hunter@xxxxxxxxx
Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
 tools/perf/builtin-script.c                    | 18 +++++++++++++++++-
 tools/perf/util/dump-insn.c                    |  8 ++++++++
 tools/perf/util/dump-insn.h                    |  2 ++
 .../intel-pt-decoder/intel-pt-insn-decoder.c   |  8 ++++++++
 4 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/tools/perf/builtin-script.c b/tools/perf/builtin-script.c
index 4da5e32b9e03..11868bf39e66 100644
--- a/tools/perf/builtin-script.c
+++ b/tools/perf/builtin-script.c
@@ -1049,9 +1049,18 @@ static int perf_sample__fprintf_brstackinsn(struct perf_sample *sample,
 
 	/*
 	 * Print final block upto sample
+	 *
+	 * Due to pipeline delays the LBRs might be missing a branch
+	 * or two, which can result in very large or negative blocks
+	 * between final branch and sample. When this happens just
+	 * continue walking after the last TO until we hit a branch.
 	 */
 	start = br->entries[0].to;
 	end = sample->ip;
+	if (end < start) {
+		/* Missing jump. Scan 128 bytes for the next branch */
+		end = start + 128;
+	}
 	len = grab_bb(buffer, start, end, machine, thread, &x.is64bit, &x.cpumode, true);
 	printed += ip__fprintf_sym(start, thread, x.cpumode, x.cpu, &lastsym, attr, fp);
 	if (len <= 0) {
@@ -1060,7 +1069,6 @@ static int perf_sample__fprintf_brstackinsn(struct perf_sample *sample,
 			      machine, thread, &x.is64bit, &x.cpumode, false);
 		if (len <= 0)
 			goto out;
-
 		printed += fprintf(fp, "\t%016" PRIx64 "\t%s\n", sample->ip,
 			dump_insn(&x, sample->ip, buffer, len, NULL));
 		goto out;
@@ -1070,6 +1078,14 @@ static int perf_sample__fprintf_brstackinsn(struct perf_sample *sample,
 				   dump_insn(&x, start + off, buffer + off, len - off, &ilen));
 		if (ilen == 0)
 			break;
+		if (arch_is_branch(buffer + off, len - off, x.is64bit) &&
+			start + off != sample->ip) {
+			/*
+			 * Hit a missing branch. Just stop.
+			 */
+			printed += fprintf(fp, "\t... not reaching sample ...\n");
+			break;
+		}
 	}
 out:
 	return printed;
diff --git a/tools/perf/util/dump-insn.c b/tools/perf/util/dump-insn.c
index 10988d3de7ce..2bd8585db93c 100644
--- a/tools/perf/util/dump-insn.c
+++ b/tools/perf/util/dump-insn.c
@@ -13,3 +13,11 @@ const char *dump_insn(struct perf_insn *x __maybe_unused,
 		*lenp = 0;
 	return "?";
 }
+
+__weak
+int arch_is_branch(const unsigned char *buf __maybe_unused,
+		   size_t len __maybe_unused,
+		   int x86_64 __maybe_unused)
+{
+	return 0;
+}
diff --git a/tools/perf/util/dump-insn.h b/tools/perf/util/dump-insn.h
index 0e06280a8860..650125061530 100644
--- a/tools/perf/util/dump-insn.h
+++ b/tools/perf/util/dump-insn.h
@@ -20,4 +20,6 @@ struct perf_insn {
 
 const char *dump_insn(struct perf_insn *x, u64 ip,
 		      u8 *inbuf, int inlen, int *lenp);
+int arch_is_branch(const unsigned char *buf, size_t len, int x86_64);
+
 #endif
diff --git a/tools/perf/util/intel-pt-decoder/intel-pt-insn-decoder.c b/tools/perf/util/intel-pt-decoder/intel-pt-insn-decoder.c
index 54818828023b..1c0e289f01e6 100644
--- a/tools/perf/util/intel-pt-decoder/intel-pt-insn-decoder.c
+++ b/tools/perf/util/intel-pt-decoder/intel-pt-insn-decoder.c
@@ -180,6 +180,14 @@ int intel_pt_get_insn(const unsigned char *buf, size_t len, int x86_64,
 	return 0;
 }
 
+int arch_is_branch(const unsigned char *buf, size_t len, int x86_64)
+{
+	struct intel_pt_insn in;
+	if (intel_pt_get_insn(buf, len, x86_64, &in) < 0)
+		return -1;
+	return in.branch != INTEL_PT_BR_NO_BRANCH;
+}
+
 const char *dump_insn(struct perf_insn *x, uint64_t ip __maybe_unused,
 		      u8 *inbuf, int inlen, int *lenp)
 {
-- 
2.17.2