[PATCH v4 3/6] rtla/osnoise: Leverage IPI event filters when tracing a subset of CPUs

[Valentin Schneider <vschneid@redhat.com>]

Leverage the kernel event filtering infrastructure to only emit IPI events
if they target CPUs that are being traced, as specified by the -c cmdline
option.

Note that some post-processing is still required for the ipi_send_cpumask
event, as the event being emitted means *some* CPUs targeted by that event
are monitored, but not all of them - userspace has to recompute that
intersection.

Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
 tools/tracing/rtla/src/osnoise_top.c | 38 ++++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 5 deletions(-)

diff --git a/tools/tracing/rtla/src/osnoise_top.c b/tools/tracing/rtla/src/osnoise_top.c
index 7b5ae5336cf08..353b435fefcf8 100644
--- a/tools/tracing/rtla/src/osnoise_top.c
+++ b/tools/tracing/rtla/src/osnoise_top.c
@@ -302,17 +302,13 @@ osnoise_ipi_cpu_handler(struct trace_seq *s, struct tep_record *record,
 		     struct tep_event *event, void *context)
 {
 	struct osnoise_tool *tool;
-	struct osnoise_params *params;
 	unsigned long long dst_cpu;
 	struct trace_instance *trace = context;
 
 	tool = container_of(trace, struct osnoise_tool, trace);
-	params = to_osnoise_params(tool->params);
-
 	tep_get_field_val(s, event, "cpu", record, &dst_cpu, 1);
 
-	if (CPU_ISSET(dst_cpu, &params->common.monitored_cpus))
-		account_ipi(tool, dst_cpu);
+	account_ipi(tool, dst_cpu);
 
 	return 0;
 }
@@ -346,6 +342,11 @@ osnoise_ipi_cpumask_handler(struct trace_seq *s, struct tep_record *record,
 		return 0;
 	}
 
+	/*
+	 * Despite already filtering for such an intersection, we need to compute
+	 * the intersection here as the @cpumask field may contain non-monitored
+	 * CPUs.
+	 */
 	CPU_AND(&cpumask_tmp_cpus, event_cpus, &params->common.monitored_cpus);
 
 	/*
@@ -398,6 +399,33 @@ struct osnoise_tool *osnoise_init_top(struct common_params *params)
 		goto out_err;
 	}
 
+	/*
+	 * If tracing on a subset of possible CPUs, leverage the kernel filtering
+	 * infrastructure to only generate events on traced CPUs.
+	 */
+	if (params->cpus) {
+		char filter[MAX_PATH];
+
+		snprintf(filter, ARRAY_SIZE(filter), "cpu & CPUS{%s}\n", params->cpus);
+		retval = tracefs_event_file_write(tool->trace.inst,
+						  "ipi", "ipi_send_cpu", "filter",
+						  filter);
+		if (retval < 0) {
+			err_msg("Could not set ipi_send_cpu CPU filter\n");
+			goto out_err;
+		}
+
+
+		snprintf(filter, ARRAY_SIZE(filter), "cpumask & CPUS{%s}\n", params->cpus);
+		retval = tracefs_event_file_write(tool->trace.inst,
+						  "ipi", "ipi_send_cpumask", "filter",
+						  filter);
+		if (retval < 0) {
+			err_msg("Could not set ipi_send_cpumask CPU filter\n");
+			goto out_err;
+		}
+	}
+
 	tep_register_event_handler(tool->trace.tep, -1, "ipi", "ipi_send_cpu",
 				   osnoise_ipi_cpu_handler, NULL);
 
-- 
2.55.0