[PATCH v2 1/9] rtla/timerlat: Introduce enum timerlat_tracing_mode

Tomas Glozar
2025-06-26T12:36:11Z

After the introduction of BPF-based sample collection, rtla-timerlat
effectively runs in one of three modes:

- Pure BPF mode, with tracefs only being used to set up the timerlat
tracer. Sample processing and stop on threshold are handled by BPF.

- tracefs mode. BPF is unsupported or kernel is lacking the necessary
trace event (osnoise:timerlat_sample). Stop on theshold is handled by
timerlat tracer stopping tracing in all instances.

- BPF/tracefs mixed mode - BPF is used for sample collection for top or
histogram, tracefs is used for trace output and/or auto-analysis. Stop
on threshold is handled both through BPF program, which stops sample
collection for top/histogram and wakes up rtla, and by timerlat
tracer, which stops tracing for trace output/auto-analysis instances.

Add enum timerlat_tracing_mode, with three values:

- TRACING_MODE_BPF
- TRACING_MODE_TRACEFS
- TRACING_MODE_MIXED

Those represent the modes described above. A field of this type is added
to struct timerlat_params, named "mode", replacing the no_bpf variable.
params->mode is set in timerlat_{top,hist}_parse_args to
TRACING_MODE_BPF or TRACING_MODE_MIXED based on whether trace output
and/or auto-analysis is requested. timerlat_{top,hist}_main then checks
if BPF is not unavailable or disabled, in that case, it sets
params->mode to TRACING_MODE_TRACEFS.

A condition is added to timerlat_apply_config that skips setting
timerlat tracer thresholds if params->mode is TRACING_MODE_BPF (those
are unnecessary, since they only turn off tracing, which is already
turned off in that case, since BPF is used to collect samples).

Signed-off-by: Tomas Glozar <tglozar@redhat.com>
---
tools/tracing/rtla/src/timerlat.c | 24 +++++++----
tools/tracing/rtla/src/timerlat.h | 18 ++++++++
tools/tracing/rtla/src/timerlat_hist.c | 51 +++++++++++++----------
tools/tracing/rtla/src/timerlat_top.c | 57 +++++++++++++++-----------
4 files changed, 97 insertions(+), 53 deletions(-)

diff --git a/tools/tracing/rtla/src/timerlat.c b/tools/tracing/rtla/src/timerlat.c
index c29e2ba2d7d8..63d6d43eafff 100644
--- a/tools/tracing/rtla/src/timerlat.c
+++ b/tools/tracing/rtla/src/timerlat.c
@@ -40,16 +40,22 @@ timerlat_apply_config(struct osnoise_tool *tool, struct timerlat_params *params)
CPU_SET(i, &params->monitored_cpus);
}

- retval = osnoise_set_stop_us(tool->context, params->stop_us);
- if (retval) {
- err_msg("Failed to set stop us\n");
- goto out_err;
- }
+ if (params->mode != TRACING_MODE_BPF) {
+ /*
+ * In tracefs and mixed mode, timerlat tracer handles stopping
+ * on threshold
+ */
+ retval = osnoise_set_stop_us(tool->context, params->stop_us);
+ if (retval) {
+ err_msg("Failed to set stop us\n");
+ goto out_err;
+ }

- retval = osnoise_set_stop_total_us(tool->context, params->stop_total_us);
- if (retval) {
- err_msg("Failed to set stop total us\n");
- goto out_err;
+ retval = osnoise_set_stop_total_us(tool->context, params->stop_total_us);
+ if (retval) {
+ err_msg("Failed to set stop total us\n");
+ goto out_err;
+ }
}

diff --git a/tools/tracing/rtla/src/timerlat.h b/tools/tracing/rtla/src/timerlat.h
index 73045aef23fa..e0a553545d03 100644
--- a/tools/tracing/rtla/src/timerlat.h
+++ b/tools/tracing/rtla/src/timerlat.h
@@ -1,6 +1,23 @@
// SPDX-License-Identifier: GPL-2.0
#include "osnoise.h"

+/*
+ * Define timerlat tracing mode.
+ *
+ * There are three tracing modes:
+ * - tracefs-only, used when BPF is unavailable.
+ * - BPF-only, used when BPF is available and neither trace saving nor
+ * auto-analysis are enabled.
+ * - mixed mode, used when BPF is available and either trace saving or
+ * auto-analysis is enabled (which rely on sample collection through
+ * tracefs).
+ */
+enum timerlat_tracing_mode {
+ TRACING_MODE_BPF,
+ TRACING_MODE_TRACEFS,
+ TRACING_MODE_MIXED,
+};
+
struct timerlat_params {
/* Common params */
char *cpus;
@@ -30,6 +47,7 @@ struct timerlat_params {
cpu_set_t hk_cpu_set;
struct sched_attr sched_param;
struct trace_events *events;
+ enum timerlat_tracing_mode mode;
union {
struct {
/* top only */
diff --git a/tools/tracing/rtla/src/timerlat_hist.c b/tools/tracing/rtla/src/timerlat_hist.c
index 36d2294c963d..6cf260e8553b 100644
--- a/tools/tracing/rtla/src/timerlat_hist.c
+++ b/tools/tracing/rtla/src/timerlat_hist.c
@@ -802,6 +802,9 @@ static struct timerlat_params
params->bucket_size = 1;
params->entries = 256;

+ /* default to BPF mode */
+ params->mode = TRACING_MODE_BPF;
+
while (1) {
static struct option long_options[] = {
{"auto", required_argument, 0, 'a'},
@@ -1054,6 +1057,13 @@ static struct timerlat_params
if (params->kernel_workload && params->user_workload)
timerlat_hist_usage("--kernel-threads and --user-threads are mutually exclusive!");

+ /*
+ * If auto-analysis or trace output is enabled, switch from BPF mode to
+ * mixed mode
+ */
+ if (params->mode == TRACING_MODE_BPF && params->trace_output && !params->no_aa)
+ params->mode = TRACING_MODE_MIXED;
+
return params;
}

@@ -1149,7 +1159,6 @@ int timerlat_hist_main(int argc, char *argv[])
pthread_t timerlat_u;
int retval;
int nr_cpus, i;
- bool no_bpf = false;

params = timerlat_hist_parse_args(argc, argv);
if (!params)
@@ -1161,12 +1170,6 @@ int timerlat_hist_main(int argc, char *argv[])
goto out_exit;
}

- retval = timerlat_hist_apply_config(tool, params);
- if (retval) {
- err_msg("Could not apply config\n");
- goto out_free;
- }
-
trace = &tool->trace;
/*
* Save trace instance into global variable so that SIGINT can stop
@@ -1175,24 +1178,30 @@ int timerlat_hist_main(int argc, char *argv[])
*/
hist_inst = trace;

+ /*
+ * Try to enable BPF, unless disabled explicitly.
+ * If BPF enablement fails, fall back to tracefs mode.
+ */
if (getenv("RTLA_NO_BPF") && strncmp(getenv("RTLA_NO_BPF"), "1", 2) == 0) {
debug_msg("RTLA_NO_BPF set, disabling BPF\n");
- no_bpf = true;
- }
-
- if (!no_bpf && !tep_find_event_by_name(trace->tep, "osnoise", "timerlat_sample")) {
+ params->mode = TRACING_MODE_TRACEFS;
+ } else if (!tep_find_event_by_name(trace->tep, "osnoise", "timerlat_sample")) {
debug_msg("osnoise:timerlat_sample missing, disabling BPF\n");
- no_bpf = true;
- }
-
- if (!no_bpf) {
+ params->mode = TRACING_MODE_TRACEFS;
+ } else {
retval = timerlat_bpf_init(params);
if (retval) {
debug_msg("Could not enable BPF\n");
- no_bpf = true;
+ params->mode = TRACING_MODE_TRACEFS;
}
}

+ retval = timerlat_hist_apply_config(tool, params);
+ if (retval) {
+ err_msg("Could not apply config\n");
+ goto out_free;
+ }
+
retval = enable_timerlat(trace);
if (retval) {
err_msg("Failed to enable timerlat tracer\n");
@@ -1320,7 +1329,7 @@ int timerlat_hist_main(int argc, char *argv[])
trace_instance_start(&record->trace);
if (!params->no_aa)
trace_instance_start(&aa->trace);
- if (no_bpf) {
+ if (params->mode == TRACING_MODE_TRACEFS) {
trace_instance_start(trace);
} else {
retval = timerlat_bpf_attach();
@@ -1333,7 +1342,7 @@ int timerlat_hist_main(int argc, char *argv[])
tool->start_time = time(NULL);
timerlat_hist_set_signals(params);

- if (no_bpf) {
+ if (params->mode == TRACING_MODE_TRACEFS) {
while (!stop_tracing) {
sleep(params->sleep_time);

@@ -1362,7 +1371,7 @@ int timerlat_hist_main(int argc, char *argv[])
} else
timerlat_bpf_wait(-1);

- if (!no_bpf) {
+ if (params->mode != TRACING_MODE_TRACEFS) {
timerlat_bpf_detach();
retval = timerlat_hist_bpf_pull_data(tool);
if (retval) {
@@ -1409,10 +1418,10 @@ int timerlat_hist_main(int argc, char *argv[])
osnoise_destroy_tool(aa);
osnoise_destroy_tool(record);
osnoise_destroy_tool(tool);
+ if (params->mode != TRACING_MODE_TRACEFS)
+ timerlat_bpf_destroy();
free(params);
free_cpu_idle_disable_states();
- if (!no_bpf)
- timerlat_bpf_destroy();
out_exit:
exit(return_value);
}
diff --git a/tools/tracing/rtla/src/timerlat_top.c b/tools/tracing/rtla/src/timerlat_top.c
index 7365e08fe986..1644eeb60181 100644
--- a/tools/tracing/rtla/src/timerlat_top.c
+++ b/tools/tracing/rtla/src/timerlat_top.c
@@ -559,6 +559,9 @@ static struct timerlat_params
/* display data in microseconds */
params->output_divisor = 1000;

+ /* default to BPF mode */
+ params->mode = TRACING_MODE_BPF;
+
while (1) {
static struct option long_options[] = {
{"auto", required_argument, 0, 'a'},
@@ -790,6 +793,13 @@ static struct timerlat_params
if (params->kernel_workload && params->user_workload)
timerlat_top_usage("--kernel-threads and --user-threads are mutually exclusive!");

+ /*
+ * If auto-analysis or trace output is enabled, switch from BPF mode to
+ * mixed mode
+ */
+ if (params->mode == TRACING_MODE_BPF && params->trace_output && !params->no_aa)
+ params->mode = TRACING_MODE_MIXED;
+
return params;
}

@@ -994,7 +1004,6 @@ int timerlat_top_main(int argc, char *argv[])
char *max_lat;
int retval;
int nr_cpus, i;
- bool no_bpf = false;

params = timerlat_top_parse_args(argc, argv);
if (!params)
@@ -1006,38 +1015,38 @@ int timerlat_top_main(int argc, char *argv[])
goto out_exit;
}

- retval = timerlat_top_apply_config(top, params);
- if (retval) {
- err_msg("Could not apply config\n");
- goto out_free;
- }
-
trace = &top->trace;
/*
- * Save trace instance into global variable so that SIGINT can stop
- * the timerlat tracer.
- * Otherwise, rtla could loop indefinitely when overloaded.
- */
+ * Save trace instance into global variable so that SIGINT can stop
+ * the timerlat tracer.
+ * Otherwise, rtla could loop indefinitely when overloaded.
+ */
top_inst = trace;

+ /*
+ * Try to enable BPF, unless disabled explicitly.
+ * If BPF enablement fails, fall back to tracefs mode.
+ */
if (getenv("RTLA_NO_BPF") && strncmp(getenv("RTLA_NO_BPF"), "1", 2) == 0) {
debug_msg("RTLA_NO_BPF set, disabling BPF\n");
- no_bpf = true;
- }
-
- if (!no_bpf && !tep_find_event_by_name(trace->tep, "osnoise", "timerlat_sample")) {
+ params->mode = TRACING_MODE_TRACEFS;
+ } else if (!tep_find_event_by_name(trace->tep, "osnoise", "timerlat_sample")) {
debug_msg("osnoise:timerlat_sample missing, disabling BPF\n");
- no_bpf = true;
- }
-
- if (!no_bpf) {
+ params->mode = TRACING_MODE_TRACEFS;
+ } else {
retval = timerlat_bpf_init(params);
if (retval) {
debug_msg("Could not enable BPF\n");
- no_bpf = true;
+ params->mode = TRACING_MODE_TRACEFS;
}
}

+ retval = timerlat_top_apply_config(top, params);
+ if (retval) {
+ err_msg("Could not apply config\n");
+ goto out_free;
+ }
+
retval = enable_timerlat(trace);
if (retval) {
err_msg("Failed to enable timerlat tracer\n");
@@ -1166,7 +1175,7 @@ int timerlat_top_main(int argc, char *argv[])
trace_instance_start(&record->trace);
if (!params->no_aa)
trace_instance_start(&aa->trace);
- if (no_bpf) {
+ if (params->mode == TRACING_MODE_TRACEFS) {
trace_instance_start(trace);
} else {
retval = timerlat_bpf_attach();
@@ -1179,7 +1188,7 @@ int timerlat_top_main(int argc, char *argv[])
top->start_time = time(NULL);
timerlat_top_set_signals(params);

- if (no_bpf)
+ if (params->mode == TRACING_MODE_TRACEFS)
retval = timerlat_top_main_loop(top, record, params, &params_u);
else
retval = timerlat_top_bpf_main_loop(top, record, params, &params_u);
@@ -1187,7 +1196,7 @@ int timerlat_top_main(int argc, char *argv[])
if (retval)
goto out_top;

- if (!no_bpf)
+ if (params->mode != TRACING_MODE_TRACEFS)
timerlat_bpf_detach();

if (params->user_workload && !params_u.stopped_running) {
@@ -1239,6 +1248,8 @@ int timerlat_top_main(int argc, char *argv[])
osnoise_destroy_tool(aa);
osnoise_destroy_tool(record);
osnoise_destroy_tool(top);
+ if (params->mode != TRACING_MODE_TRACEFS)
+ timerlat_bpf_destroy();
free(params);
free_cpu_idle_disable_states();
out_exit:
--
2.49.0