[PATCH io_uring 04/16] io_uring/zcrx: cache RQ tail

The RQ tail is updated by the user space. Cache it to reduce cache line
bouncing. Refilling now tries to exhaust the previous batch of rqes, but
since it could be too low, the iterator is allowed to recalculate the
rqes to process once after synching the tail value.

Signed-off-by: Pavel Begunkov <[email protected]>
---
 io_uring/zcrx.c | 27 +++++++++++++++++++++------
 io_uring/zcrx.h |  1 +
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
index 805424fdd573..38557a9194ef 100644
--- a/io_uring/zcrx.c
+++ b/io_uring/zcrx.c
@@ -1090,16 +1090,22 @@ void io_unregister_zcrx(struct io_ring_ctx *ctx)
 
 struct zcrx_rq_iter {
 	int rqes_left;
+	bool flushed;
 };
 
-static inline u32 zcrx_rq_entries(struct zcrx_rq *rq)
+static inline u32 __zcrx_rq_entries(struct zcrx_rq *rq)
 {
-	u32 entries;
+	u32 entries = rq->cached_tail - rq->cached_head;
 
-	entries = smp_load_acquire(&rq->ring->tail) - rq->cached_head;
 	return min(entries, rq->nr_entries);
 }
 
+static inline u32 zcrx_rq_entries(struct zcrx_rq *rq)
+{
+	rq->cached_tail = smp_load_acquire(&rq->ring->tail);
+	return __zcrx_rq_entries(rq);
+}
+
 static struct io_uring_zcrx_rqe *zcrx_next_rqe(struct zcrx_rq *rq, unsigned mask)
 {
 	unsigned int idx = rq->cached_head++ & mask;
@@ -1110,7 +1116,8 @@ static struct io_uring_zcrx_rqe *zcrx_next_rqe(struct zcrx_rq *rq, unsigned mask
 static inline void zcrx_rq_iter_init(struct zcrx_rq_iter *it,
 				     struct zcrx_rq *rq)
 {
-	it->rqes_left = min_t(unsigned, zcrx_rq_entries(rq), ZCRX_REFILL_CAP);
+	it->rqes_left = min_t(unsigned, __zcrx_rq_entries(rq), ZCRX_REFILL_CAP);
+	it->flushed = false;
 }
 
 static inline bool zcrx_rq_iter_next(struct zcrx_rq_iter *it,
@@ -1118,8 +1125,16 @@ static inline bool zcrx_rq_iter_next(struct zcrx_rq_iter *it,
 				     struct io_uring_zcrx_rqe **rqe)
 {
 	it->rqes_left--;
-	if (unlikely(it->rqes_left < 0))
-		return false;
+	if (unlikely(it->rqes_left < 0)) {
+		if (it->flushed)
+			return false;
+		rq->cached_tail = smp_load_acquire(&rq->ring->tail);
+		it->rqes_left = min_t(unsigned, __zcrx_rq_entries(rq),
+				      ZCRX_REFILL_CAP);
+		it->flushed = true;
+		if (--it->rqes_left < 0)
+			return false;
+	}
 
 	*rqe = zcrx_next_rqe(rq, rq->nr_entries - 1);
 	return true;
diff --git a/io_uring/zcrx.h b/io_uring/zcrx.h
index 9ae0bf1c632f..4f8428db4aa4 100644
--- a/io_uring/zcrx.h
+++ b/io_uring/zcrx.h
@@ -54,6 +54,7 @@ struct zcrx_rq {
 	struct zcrx_rq_hdr		*ring;
 	struct io_uring_zcrx_rqe	*rqes;
 	u32				cached_head;
+	u32				cached_tail;
 	u32				nr_entries;
 };
 
-- 
2.54.0