drbd: don't recvmsg with zero length (c13f7e1a) · Commits · e / devices / android_kernel_fairphone_FP4

This should fix a performance degradation we observed recently. If we don't expect any subheader, we should not call into the tcp stack, as that may add considerable latency if there is no data available at this point. For a synthetic synchronous write load with single outstanding writes, this additional latency when processing the "unplug remote" packet added up to a performance degradation factor >= 10. Signed-off-by:

Philipp Reisner <philipp.reisner@linbit.com> Signed-off-by:

Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_receiver.c

+8 −6

Original line number	Diff line number	Diff line
		@@ -3627,15 +3627,17 @@ static void drbdd(struct drbd_conf *mdev)
		}

		shs = drbd_cmd_handler[cmd].pkt_size - sizeof(union p_header);
		if (packet_size - shs > 0 && !drbd_cmd_handler[cmd].expect_payload) {
		dev_err(DEV, "No payload expected %s l:%d\n", cmdname(cmd), packet_size);
		goto err_out;
		}

		if (shs) {
		rv = drbd_recv(mdev, &header->h80.payload, shs);
		if (unlikely(rv != shs)) {
		dev_err(DEV, "short read while reading sub header: rv=%d\n", rv);
		goto err_out;
		}

		if (packet_size - shs > 0 && !drbd_cmd_handler[cmd].expect_payload) {
		dev_err(DEV, "No payload expected %s l:%d\n", cmdname(cmd), packet_size);
		goto err_out;
		}

		rv = drbd_cmd_handler[cmd].function(mdev, cmd, packet_size - shs);