rtla/timerlat_bpf: Allow resuming tracing

Currently, rtla-timerlat BPF program uses a global variable stored in a
.bss section to store whether tracing has been stopped.

Move the information to a separate map, so that it is easily writable
from userspace, and add a function that clears the value, resuming
tracing after it has been stopped.

Cc: John Kacur <[email protected]>
Cc: Luis Goncalves <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Chang Yin <[email protected]>
Cc: Costa Shulyupin <[email protected]>
Cc: Crystal Wood <[email protected]>
Cc: Gabriele Monaco <[email protected]>
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Tomas Glozar <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: lenticularis39

tools/tracing/rtla/src/timerlat.bpf.c

@@ -28,6 +28,13 @@ struct {
 	__type(value, unsigned long long);
 } summary_irq SEC(".maps"), summary_thread SEC(".maps"), summary_user SEC(".maps");
 
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, unsigned int);
+	__type(value, unsigned long long);
+} stop_tracing SEC(".maps");
+
 struct {
 	__uint(type, BPF_MAP_TYPE_RINGBUF);
 	__uint(max_entries, 1);
@@ -41,8 +48,6 @@ const volatile int irq_threshold;
 const volatile int thread_threshold;
 const volatile bool aa_only;
 
-int stop_tracing;
-
 nosubprog unsigned long long map_get(void *map,
 				     unsigned int key)
 {
@@ -109,7 +114,7 @@ nosubprog void set_stop_tracing(void)
 	int value = 0;
 
 	/* Suppress further sample processing */
-	stop_tracing = 1;
+	map_set(&stop_tracing, 0, 1);
 
 	/* Signal to userspace */
 	bpf_ringbuf_output(&signal_stop_tracing, &value, sizeof(value), 0);
@@ -121,7 +126,7 @@ int handle_timerlat_sample(struct trace_event_raw_timerlat_sample *tp_args)
 	unsigned long long latency, latency_us;
 	int bucket;
 
-	if (stop_tracing)
+	if (map_get(&stop_tracing, 0))
 		return 0;
 
 	latency = tp_args->timer_latency / output_divisor;

tools/tracing/rtla/src/timerlat_bpf.c

@@ -106,6 +106,19 @@ int timerlat_bpf_wait(int timeout)
 	return retval;
 }
 
+/*
+ * timerlat_bpf_restart_tracing - restart stopped tracing
+ */
+int timerlat_bpf_restart_tracing(void)
+{
+	unsigned int key = 0;
+	unsigned long long value = 0;
+
+	return bpf_map__update_elem(bpf->maps.stop_tracing,
+				    &key, sizeof(key),
+				    &value, sizeof(value), BPF_ANY);
+}
+
 static int get_value(struct bpf_map *map_irq,
 		     struct bpf_map *map_thread,
 		     struct bpf_map *map_user,

tools/tracing/rtla/src/timerlat_bpf.h

@@ -18,6 +18,7 @@ int timerlat_bpf_attach(void);
 void timerlat_bpf_detach(void);
 void timerlat_bpf_destroy(void);
 int timerlat_bpf_wait(int timeout);
+int timerlat_bpf_restart_tracing(void);
 int timerlat_bpf_get_hist_value(int key,
 				long long *value_irq,
 				long long *value_thread,
@@ -28,6 +29,7 @@ int timerlat_bpf_get_summary_value(enum summary_field key,
 				   long long *value_thread,
 				   long long *value_user,
 				   int cpus);
+
 static inline int have_libbpf_support(void) { return 1; }
 #else
 static inline int timerlat_bpf_init(struct timerlat_params *params)
@@ -38,6 +40,7 @@ static inline int timerlat_bpf_attach(void) { return -1; }
 static inline void timerlat_bpf_detach(void) { };
 static inline void timerlat_bpf_destroy(void) { };
 static inline int timerlat_bpf_wait(int timeout) { return -1; }
+static inline int timerlat_bpf_restart_tracing(void) { return -1; };
 static inline int timerlat_bpf_get_hist_value(int key,
 					      long long *value_irq,
 					      long long *value_thread,