[PATCH 15/17] io_uring/uring_cmd: defer SQE copying until we need it

[Jens Axboe <axboe@kernel.dk>]

The previous commit turned on async data for uring_cmd, and did the
basic conversion of setting everything up on the prep side. However, for
a lot of use cases, we'll get -EIOCBQUEUED on issue, which means we do
not need a persistent big SQE copied.

Unless we're going async immediately, defer copying the double SQE until
we know we have to.

This greatly reduces the overhead of such commands, as evidenced by
a perf diff from before and after this change:

    10.60%     -8.58%  [kernel.vmlinux]  [k] io_uring_cmd_prep

where the prep side drops from 10.60% to ~2%, which is more expected.
Performance also rises from ~113M IOPS to ~122M IOPS, bringing us back
to where it was before the async command prep.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

~# Last command done (1 command done):
---
 io_uring/uring_cmd.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
index 9bd0ba87553f..92346b5d9f5b 100644
--- a/io_uring/uring_cmd.c
+++ b/io_uring/uring_cmd.c
@@ -182,12 +182,18 @@ static int io_uring_cmd_prep_setup(struct io_kiocb *req,
 	struct uring_cache *cache;
 
 	cache = io_uring_async_get(req);
-	if (cache) {
-		memcpy(cache->sqes, sqe, uring_sqe_size(req->ctx));
-		ioucmd->sqe = req->async_data;
+	if (unlikely(!cache))
+		return -ENOMEM;
+
+	if (!(req->flags & REQ_F_FORCE_ASYNC)) {
+		/* defer memcpy until we need it */
+		ioucmd->sqe = sqe;
 		return 0;
 	}
-	return -ENOMEM;
+
+	memcpy(req->async_data, sqe, uring_sqe_size(req->ctx));
+	ioucmd->sqe = req->async_data;
+	return 0;
 }
 
 int io_uring_cmd_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
@@ -245,8 +251,15 @@ int io_uring_cmd(struct io_kiocb *req, unsigned int issue_flags)
 	}
 
 	ret = file->f_op->uring_cmd(ioucmd, issue_flags);
-	if (ret == -EAGAIN || ret == -EIOCBQUEUED)
-		return ret;
+	if (ret == -EAGAIN) {
+		struct uring_cache *cache = req->async_data;
+
+		if (ioucmd->sqe != (void *) cache)
+			memcpy(cache, ioucmd->sqe, uring_sqe_size(req->ctx));
+		return -EAGAIN;
+	} else if (ret == -EIOCBQUEUED) {
+		return -EIOCBQUEUED;
+	}
 
 	if (ret < 0)
 		req_set_fail(req);
-- 
2.43.0