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Subject: [RFC PATCH v2 05/20] tracing/filters: Optimise cpumask vs cpumask filtering when user mask is a single CPU
Date:   Thu, 20 Jul 2023 17:30:41 +0100
Message-Id: <20230720163056.2564824-6-vschneid@redhat.com>
In-Reply-To: <20230720163056.2564824-1-vschneid@redhat.com>
References: <20230720163056.2564824-1-vschneid@redhat.com>
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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.

Reuse do_filter_scalar_cpumask() when the input mask has a weight of one.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
 kernel/trace/trace_events_filter.c | 35 +++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 3009d0c61b532..2fe65ddeb34ef 100644
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -70,6 +70,7 @@ enum filter_pred_fn {
 	FILTER_PRED_FN_CPU,
 	FILTER_PRED_FN_CPU_CPUMASK,
 	FILTER_PRED_FN_CPUMASK,
+	FILTER_PRED_FN_CPUMASK_CPU,
 	FILTER_PRED_FN_FUNCTION,
 	FILTER_PRED_FN_,
 	FILTER_PRED_TEST_VISITED,
@@ -957,6 +958,22 @@ static int filter_pred_cpumask(struct filter_pred *pred, void *event)
 	return do_filter_cpumask(pred->op, mask, cmp);
 }
 
+/* Filter predicate for cpumask field vs user-provided scalar  */
+static int filter_pred_cpumask_cpu(struct filter_pred *pred, void *event)
+{
+	u32 item = *(u32 *)(event + pred->offset);
+	int loc = item & 0xffff;
+	const struct cpumask *mask = (event + loc);
+	unsigned int cpu = pred->val;
+
+	/*
+	 * This inverts the usual usage of the function (field is first element,
+	 * user parameter is second), but that's fine because the (scalar, mask)
+	 * operations used are symmetric.
+	 */
+	return do_filter_scalar_cpumask(pred->op, cpu, mask);
+}
+
 /* Filter predicate for COMM. */
 static int filter_pred_comm(struct filter_pred *pred, void *event)
 {
@@ -1453,6 +1470,8 @@ static int filter_pred_fn_call(struct filter_pred *pred, void *event)
 		return filter_pred_cpu_cpumask(pred, event);
 	case FILTER_PRED_FN_CPUMASK:
 		return filter_pred_cpumask(pred, event);
+	case FILTER_PRED_FN_CPUMASK_CPU:
+		return filter_pred_cpumask_cpu(pred, event);
 	case FILTER_PRED_FN_FUNCTION:
 		return filter_pred_function(pred, event);
 	case FILTER_PRED_TEST_VISITED:
@@ -1666,6 +1685,7 @@ static int parse_pred(const char *str, void *data,
 
 	} else if (!strncmp(str + i, "CPUS", 4)) {
 		unsigned int maskstart;
+		bool single;
 		char *tmp;
 
 		switch (field->filter_type) {
@@ -1724,8 +1744,21 @@ static int parse_pred(const char *str, void *data,
 
 		/* Move along */
 		i++;
+
+		/*
+		 * Optimisation: if the user-provided mask has a weight of one
+		 * then we can treat it as a scalar input.
+		 */
+		single = cpumask_weight(pred->mask) == 1;
+		if (single && field->filter_type == FILTER_CPUMASK) {
+			pred->val = cpumask_first(pred->mask);
+			kfree(pred->mask);
+		}
+
 		if (field->filter_type == FILTER_CPUMASK) {
-			pred->fn_num = FILTER_PRED_FN_CPUMASK;
+			pred->fn_num = single ?
+				FILTER_PRED_FN_CPUMASK_CPU :
+				FILTER_PRED_FN_CPUMASK;
 		} else if (field->filter_type == FILTER_CPU) {
 			pred->fn_num = FILTER_PRED_FN_CPU_CPUMASK;
 		} else {
-- 
2.31.1