[PATCH 5/5] perf probe: Trace a magic number if variable is not found

[Masami Hiramatsu <mhiramat@kernel.org>]

Trace a magic number as immediate value if the target variable
is not found at some probe points which is based on one probe
event.

This feature is good for the case if you trace a source code
line with some local variables, which is compiled into several
instructions and some of the variables are optimized out on
some instructions. Even if so, with this feature, perf probe
trace a magic number instead of such disappeared variables and
fold those probes on one event.

E.g. without this patch,
# perf probe -D "pud_page_vaddr pud"
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
Failed to find 'pud' in this function.
p:probe/pud_page_vaddr_L0 _text+23480787 pud=%ax:x64
p:probe/pud_page_vaddr_L0 _text+23808289 pud=%bp:x64
p:probe/pud_page_vaddr_L0 _text+23558066 pud=%ax:x64
p:probe/pud_page_vaddr_L0 _text+327957 pud=%r8:x64
p:probe/pud_page_vaddr_L0 _text+348032 pud=%bx:x64
p:probe/pud_page_vaddr_L0 _text+23816654 pud=%bx:x64

With this patch,
# perf probe -D "pud_page_vaddr pud" | head -n 10
spurious_kernel_fault is blacklisted function, skip it.
vmalloc_fault is blacklisted function, skip it.
p:probe/pud_page_vaddr_L0 _text+23480787 pud=%ax:x64
p:probe/pud_page_vaddr_L0 _text+148635 pud=\deade12d:x64
p:probe/pud_page_vaddr_L0 _text+23808289 pud=%bp:x64
p:probe/pud_page_vaddr_L0 _text+315510 pud=\deade12d:x64
p:probe/pud_page_vaddr_L0 _text+23807045 pud=\deade12d:x64
p:probe/pud_page_vaddr_L0 _text+23557349 pud=%ax:x64
p:probe/pud_page_vaddr_L0 _text+313681 pud=%di:x64
p:probe/pud_page_vaddr_L0 _text+313599 pud=\deade12d:x64
p:probe/pud_page_vaddr_L0 _text+313477 pud=\deade12d:x64
p:probe/pud_page_vaddr_L0 _text+323667 pud=\deade12d:x64


Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
 tools/perf/util/probe-event.c  |    2 +
 tools/perf/util/probe-event.h  |    3 ++
 tools/perf/util/probe-finder.c |   62 +++++++++++++++++++++++++++++++++++++---
 tools/perf/util/probe-finder.h |    1 +
 4 files changed, 63 insertions(+), 5 deletions(-)

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index 23db6786c3ea..ceeb75849311 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -46,7 +46,7 @@
 #define PERFPROBE_GROUP "probe"
 
 bool probe_event_dry_run;	/* Dry run flag */
-struct probe_conf probe_conf;
+struct probe_conf probe_conf = { .magic_num = DEFAULT_PROBE_MAGIC_NUM };
 
 #define semantic_error(msg ...) pr_err("Semantic error :" msg)
 
diff --git a/tools/perf/util/probe-event.h b/tools/perf/util/probe-event.h
index 96a319cd2378..4f0eb3a20c36 100644
--- a/tools/perf/util/probe-event.h
+++ b/tools/perf/util/probe-event.h
@@ -16,10 +16,13 @@ struct probe_conf {
 	bool	no_inlines;
 	bool	cache;
 	int	max_probes;
+	unsigned long	magic_num;
 };
 extern struct probe_conf probe_conf;
 extern bool probe_event_dry_run;
 
+#define DEFAULT_PROBE_MAGIC_NUM	0xdeade12d	/* u32: 3735937325 */
+
 struct symbol;
 
 /* kprobe-tracer and uprobe-tracer tracing point */
diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
index 2e3a468c8350..eff6063bebe6 100644
--- a/tools/perf/util/probe-finder.c
+++ b/tools/perf/util/probe-finder.c
@@ -536,6 +536,14 @@ static int convert_variable_fields(Dwarf_Die *vr_die, const char *varname,
 		return 0;
 }
 
+static void print_var_not_found(const char *varname)
+{
+	pr_err("Failed to find the location of the '%s' variable at this address.\n"
+	       " Perhaps it has been optimized out.\n"
+	       " Use -V with the --range option to show '%s' location range.\n",
+		varname, varname);
+}
+
 /* Show a variables in kprobe event format */
 static int convert_variable(Dwarf_Die *vr_die, struct probe_finder *pf)
 {
@@ -547,11 +555,11 @@ static int convert_variable(Dwarf_Die *vr_die, struct probe_finder *pf)
 
 	ret = convert_variable_location(vr_die, pf->addr, pf->fb_ops,
 					&pf->sp_die, pf->machine, pf->tvar);
+	if (ret == -ENOENT && pf->skip_empty_arg)
+		/* This can be found in other place. skip it */
+		return 0;
 	if (ret == -ENOENT || ret == -EINVAL) {
-		pr_err("Failed to find the location of the '%s' variable at this address.\n"
-		       " Perhaps it has been optimized out.\n"
-		       " Use -V with the --range option to show '%s' location range.\n",
-		       pf->pvar->var, pf->pvar->var);
+		print_var_not_found(pf->pvar->var);
 	} else if (ret == -ENOTSUP)
 		pr_err("Sorry, we don't support this variable location yet.\n");
 	else if (ret == 0 && pf->pvar->field) {
@@ -598,6 +606,8 @@ static int find_variable(Dwarf_Die *sc_die, struct probe_finder *pf)
 		/* Search again in global variables */
 		if (!die_find_variable_at(&pf->cu_die, pf->pvar->var,
 						0, &vr_die)) {
+			if (pf->skip_empty_arg)
+				return 0;
 			pr_warning("Failed to find '%s' in this function.\n",
 				   pf->pvar->var);
 			ret = -ENOENT;
@@ -1348,6 +1358,44 @@ static int add_probe_trace_event(Dwarf_Die *sc_die, struct probe_finder *pf)
 	return ret;
 }
 
+static int fill_empty_trace_arg(struct perf_probe_event *pev,
+				struct probe_trace_event *tevs, int ntevs)
+{
+	char **valp;
+	char *type;
+	int i, j, ret;
+
+	for (i = 0; i < pev->nargs; i++) {
+		type = NULL;
+		for (j = 0; j < ntevs; j++) {
+			if (tevs[j].args[i].value) {
+				type = tevs[j].args[i].type;
+				break;
+			}
+		}
+		if (j == ntevs) {
+			print_var_not_found(pev->args[i].var);
+			return -ENOENT;
+		}
+		for (j = 0; j < ntevs; j++) {
+			valp = &tevs[j].args[i].value;
+			if (*valp)
+				continue;
+
+			ret = asprintf(valp, "\\%lx", probe_conf.magic_num);
+			if (ret < 0)
+				return -ENOMEM;
+			/* Note that type can be NULL */
+			if (type) {
+				tevs[j].args[i].type = strdup(type);
+				if (!tevs[j].args[i].type)
+					return -ENOMEM;
+			}
+		}
+	}
+	return 0;
+}
+
 /* Find probe_trace_events specified by perf_probe_event from debuginfo */
 int debuginfo__find_trace_events(struct debuginfo *dbg,
 				 struct perf_probe_event *pev,
@@ -1366,7 +1414,13 @@ int debuginfo__find_trace_events(struct debuginfo *dbg,
 	tf.tevs = *tevs;
 	tf.ntevs = 0;
 
+	if (pev->event && pev->nargs != 0 && immediate_value_is_supported())
+		tf.pf.skip_empty_arg = true;
+
 	ret = debuginfo__find_probes(dbg, &tf.pf);
+	if (ret >= 0 && tf.pf.skip_empty_arg)
+		ret = fill_empty_trace_arg(pev, tf.tevs, tf.ntevs);
+
 	if (ret < 0) {
 		for (i = 0; i < tf.ntevs; i++)
 			clear_probe_trace_event(&tf.tevs[i]);
diff --git a/tools/perf/util/probe-finder.h b/tools/perf/util/probe-finder.h
index 670c477bf8cf..11be10080613 100644
--- a/tools/perf/util/probe-finder.h
+++ b/tools/perf/util/probe-finder.h
@@ -87,6 +87,7 @@ struct probe_finder {
 	unsigned int		machine;	/* Target machine arch */
 	struct perf_probe_arg	*pvar;		/* Current target variable */
 	struct probe_trace_arg	*tvar;		/* Current result variable */
+	bool			skip_empty_arg;	/* Skip non-exist args */
 };
 
 struct trace_event_finder {