rtla/timerlat: Support tail call from BPF program

Add a map to the rtla-timerlat BPF program that holds a file descriptor
of another BPF program, to be executed on threshold overflow.

timerlat_bpf_set_action() is added as an interface to set the program.

Link: https://lore.kernel.org/r/20251126144205.331954-2-tglozar@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>

Author: lenticularis39

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

@@ -40,6 +40,17 @@ struct {
 	__uint(max_entries, 1);
 } signal_stop_tracing SEC(".maps");
 
+struct {
+	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
+	__uint(key_size, sizeof(unsigned int));
+	__uint(max_entries, 1);
+	__array(values, unsigned int (void *));
+} bpf_action SEC(".maps") = {
+	.values = {
+		[0] = 0
+	},
+};
+
 /* Params to be set by rtla */
 const volatile int bucket_size = 1;
 const volatile int output_divisor = 1000;
@@ -109,7 +120,7 @@ nosubprog void update_summary(void *map,
 	map_set(map, SUMMARY_SUM, map_get(map, SUMMARY_SUM) + latency);
 }
 
-nosubprog void set_stop_tracing(void)
+nosubprog void set_stop_tracing(struct trace_event_raw_timerlat_sample *tp_args)
 {
 	int value = 0;
 
@@ -118,6 +129,12 @@ nosubprog void set_stop_tracing(void)
 
 	/* Signal to userspace */
 	bpf_ringbuf_output(&signal_stop_tracing, &value, sizeof(value), 0);
+
+	/*
+	 * Call into BPF action program, if attached.
+	 * Otherwise, just silently fail.
+	 */
+	bpf_tail_call(tp_args, &bpf_action, 0);
 }
 
 SEC("tp/osnoise/timerlat_sample")
@@ -138,19 +155,19 @@ int handle_timerlat_sample(struct trace_event_raw_timerlat_sample *tp_args)
 		update_summary(&summary_irq, latency, bucket);
 
 		if (irq_threshold != 0 && latency_us >= irq_threshold)
-			set_stop_tracing();
+			set_stop_tracing(tp_args);
 	} else if (tp_args->context == 1) {
 		update_main_hist(&hist_thread, bucket);
 		update_summary(&summary_thread, latency, bucket);
 
 		if (thread_threshold != 0 && latency_us >= thread_threshold)
-			set_stop_tracing();
+			set_stop_tracing(tp_args);
 	} else {
 		update_main_hist(&hist_user, bucket);
 		update_summary(&summary_user, latency, bucket);
 
 		if (thread_threshold != 0 && latency_us >= thread_threshold)
-			set_stop_tracing();
+			set_stop_tracing(tp_args);
 	}
 
 	return 0;

tools/tracing/rtla/src/timerlat_bpf.c

@@ -59,6 +59,19 @@ int timerlat_bpf_init(struct timerlat_params *params)
 	return 0;
 }
 
+/*
+ * timerlat_bpf_set_action - set action on threshold executed on BPF side
+ */
+static int timerlat_bpf_set_action(struct bpf_program *prog)
+{
+	unsigned int key = 0, value = bpf_program__fd(prog);
+
+	return bpf_map__update_elem(bpf->maps.bpf_action,
+				    &key, sizeof(key),
+				    &value, sizeof(value),
+				    BPF_ANY);
+}
+
 /*
  * timerlat_bpf_attach - attach BPF program to collect timerlat data
  */

tools/tracing/rtla/src/timerlat_bpf.h

@@ -12,6 +12,7 @@ enum summary_field {
 };
 
 #ifndef __bpf__
+#include <bpf/libbpf.h>
 #ifdef HAVE_BPF_SKEL
 int timerlat_bpf_init(struct timerlat_params *params);
 int timerlat_bpf_attach(void);