From 35c7c72601cee9692846de13117411d80b3d13f6 Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Wed, 27 Nov 2024 20:10:19 -0500
Subject: ioq: Add an ioq_nop() operation for benchmarking

---
 src/bftw.c |  4 ++++
 src/ioq.c  | 28 ++++++++++++++++++++++++++++
 src/ioq.h  | 30 ++++++++++++++++++++++++++++++
 3 files changed, 62 insertions(+)

(limited to 'src')

diff --git a/src/bftw.c b/src/bftw.c
index 61193d5..5725825 100644
--- a/src/bftw.c
+++ b/src/bftw.c
@@ -1051,6 +1051,10 @@ static int bftw_ioq_pop(struct bftw_state *state, bool block) {
 
 		bftw_queue_attach(&state->fileq, file, true);
 		break;
+
+	default:
+		bfs_bug("Unexpected ioq op %d", (int)op);
+		break;
 	}
 
 	ioq_free(ioq, ent);
diff --git a/src/ioq.c b/src/ioq.c
index 1c5524e..5668a83 100644
--- a/src/ioq.c
+++ b/src/ioq.c
@@ -136,6 +136,7 @@
 #include <stdint.h>
 #include <stdlib.h>
 #include <sys/stat.h>
+#include <unistd.h>
 
 #if BFS_WITH_LIBURING
 #  include <liburing.h>
@@ -532,6 +533,14 @@ static bool ioq_check_cancel(struct ioq *ioq, struct ioq_ent *ent) {
 /** Dispatch a single request synchronously. */
 static void ioq_dispatch_sync(struct ioq *ioq, struct ioq_ent *ent) {
 	switch (ent->op) {
+		case IOQ_NOP:
+			if (ent->nop.type == IOQ_NOP_HEAVY) {
+				// A fast, no-op syscall
+				getpid();
+			}
+			ent->result = 0;
+			return;
+
 		case IOQ_CLOSE:
 			ent->result = try(xclose(ent->close.fd));
 			return;
@@ -587,6 +596,13 @@ static struct io_uring_sqe *ioq_dispatch_async(struct ioq_ring_state *state, str
 	struct io_uring_sqe *sqe = NULL;
 
 	switch (ent->op) {
+	case IOQ_NOP:
+		if (ent->nop.type == IOQ_NOP_HEAVY) {
+			sqe = io_uring_get_sqe(ring);
+			io_uring_prep_nop(sqe);
+		}
+		return sqe;
+
 	case IOQ_CLOSE:
 		if (ops & IOQ_RING_CLOSE) {
 			sqe = io_uring_get_sqe(ring);
@@ -1010,6 +1026,18 @@ static struct ioq_ent *ioq_request(struct ioq *ioq, enum ioq_op op, void *ptr) {
 	return ent;
 }
 
+int ioq_nop(struct ioq *ioq, enum ioq_nop_type type, void *ptr) {
+	struct ioq_ent *ent = ioq_request(ioq, IOQ_NOP, ptr);
+	if (!ent) {
+		return -1;
+	}
+
+	ent->nop.type = type;
+
+	ioqq_push(ioq->pending, ent);
+	return 0;
+}
+
 int ioq_close(struct ioq *ioq, int fd, void *ptr) {
 	struct ioq_ent *ent = ioq_request(ioq, IOQ_CLOSE, ptr);
 	if (!ent) {
diff --git a/src/ioq.h b/src/ioq.h
index cb14be4..fce1d7f 100644
--- a/src/ioq.h
+++ b/src/ioq.h
@@ -22,6 +22,8 @@ struct ioq;
  * I/O queue operations.
  */
 enum ioq_op {
+	/** ioq_nop(). */
+	IOQ_NOP,
 	/** ioq_close(). */
 	IOQ_CLOSE,
 	/** ioq_opendir(). */
@@ -32,6 +34,16 @@ enum ioq_op {
 	IOQ_STAT,
 };
 
+/**
+ * ioq_nop() types.
+ */
+enum ioq_nop_type {
+	/** A lightweight nop that avoids syscalls. */
+	IOQ_NOP_LIGHT,
+	/** A heavyweight nop that involves a syscall. */
+	IOQ_NOP_HEAVY,
+};
+
 /**
  * The I/O queue implementation needs two tag bits in each pointer to a struct
  * ioq_ent, so we need to ensure at least 4-byte alignment.  The natural
@@ -54,6 +66,10 @@ struct ioq_ent {
 
 	/** Operation-specific arguments. */
 	union {
+		/** ioq_nop() args. */
+		struct ioq_nop {
+			enum ioq_nop_type type;
+		} nop;
 		/** ioq_close() args. */
 		struct ioq_close {
 			int fd;
@@ -97,6 +113,20 @@ struct ioq *ioq_create(size_t depth, size_t nthreads);
  */
 size_t ioq_capacity(const struct ioq *ioq);
 
+/**
+ * A no-op, for benchmarking.
+ *
+ * @ioq
+ *         The I/O queue.
+ * @type
+ *         The type of operation to perform.
+ * @ptr
+ *         An arbitrary pointer to associate with the request.
+ * @return
+ *         0 on success, or -1 on failure.
+ */
+int ioq_nop(struct ioq *ioq, enum ioq_nop_type type, void *ptr);
+
 /**
  * Asynchronous close().
  *
-- 
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