nvme/io_uring: optimize IOPOLL completions for local ring context

When multiple io_uring rings poll on the same NVMe queue, one ring can
find completions belonging to another ring. The current code always
uses task_work to handle this, but this adds overhead for the common
single-ring case.

This patch passes the polling io_ring_ctx through io_comp_batch's new
poll_ctx field. In io_do_iopoll(), the polling ring's context is stored
in iob.poll_ctx before calling the iopoll callbacks.

In nvme_uring_cmd_end_io(), we now compare iob->poll_ctx with the
request's owning io_ring_ctx (via io_uring_cmd_ctx_handle()). If they
match (local context), we complete inline with io_uring_cmd_done32().
If they differ (remote context) or iob is NULL (non-iopoll path), we
use task_work as before.

This optimization eliminates task_work scheduling overhead for the
common case where a ring polls and finds its own completions.

~10% IOPS improvement is observed in the following benchmark:

fio/t/io_uring -b512 -d128 -c32 -s32 -p1 -F1 -O0 -P1 -u1 -n1 /dev/ng0n1

Signed-off-by: Ming Lei <[email protected]>

Author: Ming Lei

drivers/nvme/host/ioctl.c

@@ -426,14 +426,20 @@ static enum rq_end_io_ret nvme_uring_cmd_end_io(struct request *req,
 	pdu->result = le64_to_cpu(nvme_req(req)->result.u64);
 
 	/*
-	 * IOPOLL could potentially complete this request directly, but
-	 * if multiple rings are polling on the same queue, then it's possible
-	 * for one ring to find completions for another ring. Punting the
-	 * completion via task_work will always direct it to the right
-	 * location, rather than potentially complete requests for ringA
-	 * under iopoll invocations from ringB.
+	 * For IOPOLL, check if this completion is happening in the context
+	 * of the same io_ring that owns the request (local context). If so,
+	 * we can complete inline without task_work overhead. Otherwise, we
+	 * must punt to task_work to ensure completion happens in the correct
+	 * ring's context.
 	 */
-	io_uring_cmd_do_in_task_lazy(ioucmd, nvme_uring_task_cb);
+	if (blk_rq_is_poll(req) && iob &&
+	    iob->poll_ctx == io_uring_cmd_ctx_handle(ioucmd)) {
+		if (pdu->bio)
+			blk_rq_unmap_user(pdu->bio);
+		io_uring_cmd_done32(ioucmd, pdu->status, pdu->result, 0);
+	} else {
+		io_uring_cmd_do_in_task_lazy(ioucmd, nvme_uring_task_cb);
+	}
 	return RQ_END_IO_FREE;
 }
 

include/linux/blkdev.h

@@ -1822,6 +1822,7 @@ struct io_comp_batch {
 	struct rq_list req_list;
 	bool need_ts;
 	void (*complete)(struct io_comp_batch *);
+	void *poll_ctx;
 };
 
 static inline bool blk_atomic_write_start_sect_aligned(sector_t sector,

io_uring/rw.c

@@ -1320,6 +1320,12 @@ int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin)
 	DEFINE_IO_COMP_BATCH(iob);
 	int nr_events = 0;
 
+	/*
+	 * Store the polling io_ring_ctx so drivers can detect if they're
+	 * completing a request in the same ring context that's polling.
+	 */
+	iob.poll_ctx = ctx;
+
 	/*
 	 * Only spin for completions if we don't have multiple devices hanging
 	 * off our complete list.