tracing/filters: Optimise CPU vs cpumask filtering when the user mask is a single CPU

Steven noted that when the user-provided cpumask contains a single CPU,
then the filtering function can use a scalar as input instead of a
full-fledged cpumask.

In this case we can directly re-use filter_pred_cpu(), we just need to
transform '&' into '==' before executing it.

Link: https://lkml.kernel.org/r/20230707172155.70873-8-vschneid@redhat.com

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Leonardo Bras <leobras@redhat.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace_events_filter.c

@@ -1750,7 +1750,7 @@ static int parse_pred(const char *str, void *data,
 		 * then we can treat it as a scalar input.
 		 */
 		single = cpumask_weight(pred->mask) == 1;
-		if (single && field->filter_type != FILTER_CPU) {
+		if (single) {
 			pred->val = cpumask_first(pred->mask);
 			kfree(pred->mask);
 		}
@@ -1760,7 +1760,12 @@ static int parse_pred(const char *str, void *data,
 				FILTER_PRED_FN_CPUMASK_CPU :
 				FILTER_PRED_FN_CPUMASK;
 		} else if (field->filter_type == FILTER_CPU) {
+			if (single) {
+				pred->op = pred->op == OP_BAND ? OP_EQ : pred->op;
+				pred->fn_num = FILTER_PRED_FN_CPU;
+			} else {
 				pred->fn_num = FILTER_PRED_FN_CPU_CPUMASK;
+			}
 		} else if (single) {
 			pred->op = pred->op == OP_BAND ? OP_EQ : pred->op;
 			pred->fn_num = select_comparison_fn(pred->op, field->size, false);