io_uring/msg_ring: add an alloc cache for io_kiocb entries

With slab accounting, allocating and freeing memory has considerable
overhead. Add a basic alloc cache for the io_kiocb allocations that
msg_ring needs to do. Unlike other caches, this one is used by the
sender, grabbing it from the remote ring. When the remote ring gets
the posted completion, it'll free it locally. Hence it is separately
locked, using ctx->msg_lock.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/io_uring_types.h

@@ -397,6 +397,9 @@ struct io_ring_ctx {
 	struct callback_head		poll_wq_task_work;
 	struct list_head		defer_list;
 
+	struct io_alloc_cache		msg_cache;
+	spinlock_t			msg_lock;
+
 #ifdef CONFIG_NET_RX_BUSY_POLL
 	struct list_head	napi_list;	/* track busy poll napi_id */
 	spinlock_t		napi_lock;	/* napi_list lock */

io_uring/io_uring.c

@@ -95,6 +95,7 @@
 #include "futex.h"
 #include "napi.h"
 #include "uring_cmd.h"
+#include "msg_ring.h"
 #include "memmap.h"
 
 #include "timeout.h"
@@ -315,6 +316,9 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 			    sizeof(struct io_async_rw));
 	ret |= io_alloc_cache_init(&ctx->uring_cache, IO_ALLOC_CACHE_MAX,
 			    sizeof(struct uring_cache));
+	spin_lock_init(&ctx->msg_lock);
+	ret |= io_alloc_cache_init(&ctx->msg_cache, IO_ALLOC_CACHE_MAX,
+			    sizeof(struct io_kiocb));
 	ret |= io_futex_cache_init(ctx);
 	if (ret)
 		goto err;
@@ -351,6 +355,7 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 	io_alloc_cache_free(&ctx->netmsg_cache, io_netmsg_cache_free);
 	io_alloc_cache_free(&ctx->rw_cache, io_rw_cache_free);
 	io_alloc_cache_free(&ctx->uring_cache, kfree);
+	io_alloc_cache_free(&ctx->msg_cache, io_msg_cache_free);
 	io_futex_cache_free(ctx);
 	kfree(ctx->cancel_table.hbs);
 	kfree(ctx->cancel_table_locked.hbs);
@@ -2599,6 +2604,7 @@ static __cold void io_ring_ctx_free(struct io_ring_ctx *ctx)
 	io_alloc_cache_free(&ctx->netmsg_cache, io_netmsg_cache_free);
 	io_alloc_cache_free(&ctx->rw_cache, io_rw_cache_free);
 	io_alloc_cache_free(&ctx->uring_cache, kfree);
+	io_alloc_cache_free(&ctx->msg_cache, io_msg_cache_free);
 	io_futex_cache_free(ctx);
 	io_destroy_buffers(ctx);
 	mutex_unlock(&ctx->uring_lock);

io_uring/msg_ring.c

@@ -11,6 +11,7 @@
 #include "io_uring.h"
 #include "rsrc.h"
 #include "filetable.h"
+#include "alloc_cache.h"
 #include "msg_ring.h"
 
 /* All valid masks for MSG_RING */
@@ -75,7 +76,13 @@ static void io_msg_tw_complete(struct io_kiocb *req, struct io_tw_state *ts)
 	struct io_ring_ctx *ctx = req->ctx;
 
 	io_add_aux_cqe(ctx, req->cqe.user_data, req->cqe.res, req->cqe.flags);
-	kmem_cache_free(req_cachep, req);
+	if (spin_trylock(&ctx->msg_lock)) {
+		if (io_alloc_cache_put(&ctx->msg_cache, req))
+			req = NULL;
+		spin_unlock(&ctx->msg_lock);
+	}
+	if (req)
+		kfree(req);
 	percpu_ref_put(&ctx->refs);
 }
 
@@ -91,6 +98,19 @@ static void io_msg_remote_post(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	io_req_task_work_add_remote(req, ctx, IOU_F_TWQ_LAZY_WAKE);
 }
 
+static struct io_kiocb *io_msg_get_kiocb(struct io_ring_ctx *ctx)
+{
+	struct io_kiocb *req = NULL;
+
+	if (spin_trylock(&ctx->msg_lock)) {
+		req = io_alloc_cache_get(&ctx->msg_cache);
+		spin_unlock(&ctx->msg_lock);
+	}
+	if (req)
+		return req;
+	return kmem_cache_alloc(req_cachep, GFP_KERNEL | __GFP_NOWARN);
+}
+
 static int io_msg_data_remote(struct io_kiocb *req)
 {
 	struct io_ring_ctx *target_ctx = req->file->private_data;
@@ -98,7 +118,7 @@ static int io_msg_data_remote(struct io_kiocb *req)
 	struct io_kiocb *target;
 	u32 flags = 0;
 
-	target = kmem_cache_alloc(req_cachep, GFP_KERNEL);
+	target = io_msg_get_kiocb(req->ctx);
 	if (unlikely(!target))
 		return -ENOMEM;
 
@@ -296,3 +316,10 @@ int io_msg_ring(struct io_kiocb *req, unsigned int issue_flags)
 	io_req_set_res(req, ret, 0);
 	return IOU_OK;
 }
+
+void io_msg_cache_free(const void *entry)
+{
+	struct io_kiocb *req = (struct io_kiocb *) entry;
+
+	kmem_cache_free(req_cachep, req);
+}

io_uring/msg_ring.h

@@ -3,3 +3,4 @@
 int io_msg_ring_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
 int io_msg_ring(struct io_kiocb *req, unsigned int issue_flags);
 void io_msg_ring_cleanup(struct io_kiocb *req);
+void io_msg_cache_free(const void *entry);