io_uring: optimise request allocation

Even after fully inlining io_alloc_req() my compiler does a NULL check
in the path of successful allocation, no hacks like an empty dereference
help it. Restructure io_alloc_req() by splitting out refilling part, so
the compiler generate a slightly better binary.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/eda17571bdc7248d8e617b23e7132a5416e4680b.1633373302.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 045474b..614c982 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1912,18 +1912,17 @@ static bool io_flush_cached_reqs(struct io_ring_ctx *ctx)
  * Because of that, io_alloc_req() should be called only under ->uring_lock
  * and with extra caution to not get a request that is still worked on.
  */
-static struct io_kiocb *io_alloc_req(struct io_ring_ctx *ctx)
+static bool __io_alloc_req_refill(struct io_ring_ctx *ctx)
        __must_hold(&ctx->uring_lock)
 {
        struct io_submit_state *state = &ctx->submit_state;
        gfp_t gfp = GFP_KERNEL | __GFP_NOWARN;
        void *reqs[IO_REQ_ALLOC_BATCH];
-       struct io_wq_work_node *node;
        struct io_kiocb *req;
        int ret, i;
 
        if (likely(state->free_list.next || io_flush_cached_reqs(ctx)))
-               goto got_req;
+               return true;
 
        ret = kmem_cache_alloc_bulk(req_cachep, gfp, ARRAY_SIZE(reqs), reqs);
 
@@ -1934,7 +1933,7 @@ static struct io_kiocb *io_alloc_req(struct io_ring_ctx *ctx)
        if (unlikely(ret <= 0)) {
                reqs[0] = kmem_cache_alloc(req_cachep, gfp);
                if (!reqs[0])
-                       return NULL;
+                       return false;
                ret = 1;
        }
 
@@ -1944,8 +1943,21 @@ static struct io_kiocb *io_alloc_req(struct io_ring_ctx *ctx)
                io_preinit_req(req, ctx);
                wq_stack_add_head(&req->comp_list, &state->free_list);
        }
-got_req:
-       node = wq_stack_extract(&state->free_list);
+       return true;
+}
+
+static inline bool io_alloc_req_refill(struct io_ring_ctx *ctx)
+{
+       if (unlikely(!ctx->submit_state.free_list.next))
+               return __io_alloc_req_refill(ctx);
+       return true;
+}
+
+static inline struct io_kiocb *io_alloc_req(struct io_ring_ctx *ctx)
+{
+       struct io_wq_work_node *node;
+
+       node = wq_stack_extract(&ctx->submit_state.free_list);
        return container_of(node, struct io_kiocb, comp_list);
 }
 
@@ -7217,12 +7229,12 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
                const struct io_uring_sqe *sqe;
                struct io_kiocb *req;
 
-               req = io_alloc_req(ctx);
-               if (unlikely(!req)) {
+               if (unlikely(!io_alloc_req_refill(ctx))) {
                        if (!submitted)
                                submitted = -EAGAIN;
                        break;
                }
+               req = io_alloc_req(ctx);
                sqe = io_get_sqe(ctx);
                if (unlikely(!sqe)) {
                        wq_stack_add_head(&req->comp_list, &ctx->submit_state.free_list);