trace2: add global counter mechanism

Add global counters mechanism to Trace2.

The Trace2 counters mechanism adds the ability to create a set of
global counter variables and an API to increment them efficiently.
Counters can optionally report per-thread usage in addition to the sum
across all threads.

Counter events are emitted to the Trace2 logs when a thread exits and
at process exit.

Counters are an alternative to `data` and `data_json` events.

Counters are useful when you want to measure something across the life
of the process, when you don't want per-measurement events for
performance reasons, when the data does not fit conveniently within a
region, or when your control flow does not easily let you write the
final total.  For example, you might use this to report the number of
calls to unzip() or the number of de-delta steps during a checkout.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 101 insertions, 0 deletions

diff --git a/trace2/tr2_ctr.c b/trace2/tr2_ctr.c
new file mode 100644
index 0000000000..483ca7c308
--- /dev/null
+++ b/trace2/tr2_ctr.c
@@ -0,0 +1,101 @@
+#include "cache.h"
+#include "thread-utils.h"
+#include "trace2/tr2_tgt.h"
+#include "trace2/tr2_tls.h"
+#include "trace2/tr2_ctr.h"
+
+/*
+ * A global counter block to aggregrate values from the partial sums
+ * from each thread.
+ */
+static struct tr2_counter_block final_counter_block; /* access under tr2tls_mutex */
+
+/*
+ * Define metadata for each global counter.
+ *
+ * This array must match the "enum trace2_counter_id" and the values
+ * in "struct tr2_counter_block.counter[*]".
+ */
+static struct tr2_counter_metadata tr2_counter_metadata[TRACE2_NUMBER_OF_COUNTERS] = {
+	[TRACE2_COUNTER_ID_TEST1] = {
+		.category = "test",
+		.name = "test1",
+		.want_per_thread_events = 0,
+	},
+	[TRACE2_COUNTER_ID_TEST2] = {
+		.category = "test",
+		.name = "test2",
+		.want_per_thread_events = 1,
+	},
+
+	/* Add additional metadata before here. */
+};
+
+void tr2_counter_increment(enum trace2_counter_id cid, uint64_t value)
+{
+	struct tr2tls_thread_ctx *ctx = tr2tls_get_self();
+	struct tr2_counter *c = &ctx->counter_block.counter[cid];
+
+	c->value += value;
+
+	ctx->used_any_counter = 1;
+	if (tr2_counter_metadata[cid].want_per_thread_events)
+		ctx->used_any_per_thread_counter = 1;
+}
+
+void tr2_update_final_counters(void)
+{
+	struct tr2tls_thread_ctx *ctx = tr2tls_get_self();
+	enum trace2_counter_id cid;
+
+	if (!ctx->used_any_counter)
+		return;
+
+	/*
+	 * Access `final_counter_block` requires holding `tr2tls_mutex`.
+	 * We assume that our caller is holding the lock.
+	 */
+
+	for (cid = 0; cid < TRACE2_NUMBER_OF_COUNTERS; cid++) {
+		struct tr2_counter *c_final = &final_counter_block.counter[cid];
+		const struct tr2_counter *c = &ctx->counter_block.counter[cid];
+
+		c_final->value += c->value;
+	}
+}
+
+void tr2_emit_per_thread_counters(tr2_tgt_evt_counter_t *fn_apply)
+{
+	struct tr2tls_thread_ctx *ctx = tr2tls_get_self();
+	enum trace2_counter_id cid;
+
+	if (!ctx->used_any_per_thread_counter)
+		return;
+
+	/*
+	 * For each counter, if the counter wants per-thread events
+	 * and this thread used it (the value is non-zero), emit it.
+	 */
+	for (cid = 0; cid < TRACE2_NUMBER_OF_COUNTERS; cid++)
+		if (tr2_counter_metadata[cid].want_per_thread_events &&
+		    ctx->counter_block.counter[cid].value)
+			fn_apply(&tr2_counter_metadata[cid],
+				 &ctx->counter_block.counter[cid],
+				 0);
+}
+
+void tr2_emit_final_counters(tr2_tgt_evt_counter_t *fn_apply)
+{
+	enum trace2_counter_id cid;
+
+	/*
+	 * Access `final_counter_block` requires holding `tr2tls_mutex`.
+	 * We assume that our caller is holding the lock.
+	 */
+
+	for (cid = 0; cid < TRACE2_NUMBER_OF_COUNTERS; cid++)
+		if (final_counter_block.counter[cid].value)
+			fn_apply(&tr2_counter_metadata[cid],
+				 &final_counter_block.counter[cid],
+				 1);
+}