[RFC net-next v3 28/29] io_uring: batch submission notif referencing

[Pavel Begunkov]

Batch get notifier references and use ->msg_ubuf_ref to hand off one ref
per sendzc request to the network layer. This ammortises the submission
side net_zcopy_get() atomics. Note that we always keep at least one
reference in the cache because we do only post send checks on
whether ->msg_ubuf_ref was consumed or not.

Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
---
 fs/io_uring.c | 32 +++++++++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 08c98a4d9bd2..78990a130b66 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -374,6 +374,7 @@ struct io_ev_fd {
 };
 
 #define IO_NOTIF_MAX_SLOTS	(1U << 10)
+#define IO_NOTIF_REF_CACHE_NR	64
 
 struct io_notif {
 	struct ubuf_info	uarg;
@@ -384,6 +385,8 @@ struct io_notif {
 	u64			tag;
 	/* see struct io_notif_slot::seq */
 	u32			seq;
+	/* extra uarg->refcnt refs */
+	int			cached_refs;
 	/* hook into ctx->notif_list and ctx->notif_list_locked */
 	struct list_head	cache_node;
 
@@ -2949,14 +2952,30 @@ static struct io_notif *io_alloc_notif(struct io_ring_ctx *ctx,
 
 	notif->seq = slot->seq++;
 	notif->tag = slot->tag;
+	notif->cached_refs = IO_NOTIF_REF_CACHE_NR;
 	/* master ref owned by io_notif_slot, will be dropped on flush */
-	refcount_set(&notif->uarg.refcnt, 1);
+	refcount_set(&notif->uarg.refcnt, IO_NOTIF_REF_CACHE_NR + 1);
 	percpu_ref_get(&ctx->refs);
 	notif->rsrc_node = ctx->rsrc_node;
 	io_charge_rsrc_node(ctx);
 	return notif;
 }
 
+static inline void io_notif_consume_ref(struct io_notif *notif)
+	__must_hold(&ctx->uring_lock)
+{
+	notif->cached_refs--;
+
+	/*
+	 * Issue sends without looking at notif->cached_refs first, so we
+	 * always have to have at least one ref cached
+	 */
+	if (unlikely(!notif->cached_refs)) {
+		refcount_add(IO_NOTIF_REF_CACHE_NR, &notif->uarg.refcnt);
+		notif->cached_refs += IO_NOTIF_REF_CACHE_NR;
+	}
+}
+
 static inline struct io_notif *io_get_notif(struct io_ring_ctx *ctx,
 					    struct io_notif_slot *slot)
 {
@@ -2979,13 +2998,15 @@ static void io_notif_slot_flush(struct io_notif_slot *slot)
 	__must_hold(&ctx->uring_lock)
 {
 	struct io_notif *notif = slot->notif;
+	int refs = notif->cached_refs + 1;
 
 	slot->notif = NULL;
+	notif->cached_refs = 0;
 
 	if (WARN_ON_ONCE(in_interrupt()))
 		return;
-	/* drop slot's master ref */
-	if (refcount_dec_and_test(&notif->uarg.refcnt))
+	/* drop all cached refs and the slot's master ref */
+	if (refcount_sub_and_test(refs, &notif->uarg.refcnt))
 		io_notif_complete(notif);
 }
 
@@ -6653,6 +6674,7 @@ static int io_sendzc(struct io_kiocb *req, unsigned int issue_flags)
 	msg.msg_controllen = 0;
 	msg.msg_namelen = 0;
 	msg.msg_managed_data = 1;
+	msg.msg_ubuf_ref = 1;
 
 	if (req->msgzc.zc_flags & IORING_SENDZC_FIXED_BUF) {
 		ret = __io_import_fixed(WRITE, &msg.msg_iter, req->imu,
@@ -6686,6 +6708,10 @@ static int io_sendzc(struct io_kiocb *req, unsigned int issue_flags)
 	msg.msg_ubuf = &notif->uarg;
 	ret = sock_sendmsg(sock, &msg);
 
+	/* check if the send consumed an additional ref */
+	if (likely(!msg.msg_ubuf_ref))
+		io_notif_consume_ref(notif);
+
 	if (likely(ret >= min_ret)) {
 		unsigned zc_flags = req->msgzc.zc_flags;
 
-- 
2.36.1