perf c2c: Add option to enable the LBR stitching approach

With the LBR stitching approach, the reconstructed LBR call stack can
break the HW limitation. However, it may reconstruct invalid call stacks
in some cases, e.g. exception handing such as setjmp/longjmp.  Also, it
may impact the processing time especially when the number of samples
with stitched LBRs are huge.

Add an option to enable the approach.
Signed-off-by: NKan Liang <kan.liang@linux.intel.com>
Reviewed-by: NAndi Kleen <ak@linux.intel.com>
Acked-by: NJiri Olsa <jolsa@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Pavel Gerasimov <pavel.gerasimov@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Vitaly Slobodskoy <vitaly.slobodskoy@intel.com>
Link: http://lore.kernel.org/lkml/20200319202517.23423-17-kan.liang@linux.intel.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-c2c.txt

@@ -111,6 +111,17 @@ REPORT OPTIONS
 --display::
 	Switch to HITM type (rmt, lcl) to display and sort on. Total HITMs as default.
 
+--stitch-lbr::
+	Show callgraph with stitched LBRs, which may have more complete
+	callgraph. The perf.data file must have been obtained using
+	perf c2c record --call-graph lbr.
+	Disabled by default. In common cases with call stack overflows,
+	it can recreate better call stacks than the default lbr call stack
+	output. But this approach is not full proof. There can be cases
+	where it creates incorrect call stacks from incorrect matches.
+	The known limitations include exception handing such as
+	setjmp/longjmp will have calls/returns not match.
+
 C2C RECORD
 ----------
 The perf c2c record command setup options related to HITM cacheline analysis

tools/perf/builtin-c2c.c

@@ -95,6 +95,7 @@ struct perf_c2c {
 	bool			 use_stdio;
 	bool			 stats_only;
 	bool			 symbol_full;
+	bool			 stitch_lbr;
 
 	/* HITM shared clines stats */
 	struct c2c_stats	hitm_stats;
@@ -273,6 +274,9 @@ static int process_sample_event(struct perf_tool *tool __maybe_unused,
 		return -1;
 	}
 
+	if (c2c.stitch_lbr)
+		al.thread->lbr_stitch_enable = true;
+
 	ret = sample__resolve_callchain(sample, &callchain_cursor, NULL,
 					evsel, &al, sysctl_perf_event_max_stack);
 	if (ret)
@@ -2601,6 +2605,12 @@ static int setup_callchain(struct evlist *evlist)
 		}
 	}
 
+	if (c2c.stitch_lbr && (mode != CALLCHAIN_LBR)) {
+		ui__warning("Can't find LBR callchain. Switch off --stitch-lbr.\n"
+			    "Please apply --call-graph lbr when recording.\n");
+		c2c.stitch_lbr = false;
+	}
+
 	callchain_param.record_mode = mode;
 	callchain_param.min_percent = 0;
 	return 0;
@@ -2752,6 +2762,8 @@ static int perf_c2c__report(int argc, const char **argv)
 	OPT_STRING('c', "coalesce", &coalesce, "coalesce fields",
 		   "coalesce fields: pid,tid,iaddr,dso"),
 	OPT_BOOLEAN('f', "force", &symbol_conf.force, "don't complain, do it"),
+	OPT_BOOLEAN(0, "stitch-lbr", &c2c.stitch_lbr,
+		    "Enable LBR callgraph stitching approach"),
 	OPT_PARENT(c2c_options),
 	OPT_END()
 	};