Re: [RFC PATCH v2 8/8] sched, smp: Trace smp callback causing an IPI

On Thu, Nov 17, 2022 at 02:45:29PM +0000, Valentin Schneider wrote:

quoted

+	if (trace_ipi_send_cpumask_enabled()) {
+		call_single_data_t *csd;
+		smp_call_func_t func;
+
+		csd = container_of(node, call_single_data_t, node.llist);
+
+		func = sched_ttwu_pending;
+		if (CSD_TYPE(csd) != CSD_TYPE_TTWU)
+			func = csd->func;
+
+		if (raw_smp_call_single_queue(cpu, node))
+			trace_ipi_send_cpumask(cpumask_of(cpu), _RET_IP_, func);

So I went with the tracepoint being placed *before* the actual IPI gets
sent to have a somewhat sane ordering between trace_ipi_send_cpumask() and
e.g. trace_call_function_single_entry().

Packaging the call_single_queue logic makes the code less horrible, but it
does mix up the event ordering...

Keeps em sharp ;-)

quoted

+		return;
+	}
+
+	raw_smp_call_single_queue(cpu, node);
 }

 /*

@@ -983,10 +1017,13 @@ static void smp_call_function_many_cond(
               * number of CPUs might be zero due to concurrent changes to the
               * provided mask.
               */
-		if (nr_cpus == 1)
+		if (nr_cpus == 1) {
+			trace_ipi_send_cpumask(cpumask_of(last_cpu), _RET_IP_, func);
                      send_call_function_single_ipi(last_cpu);

This'll yield an IPI event even if no IPI is sent due to the idle task
polling, no?

Oh, right..