Drivers: hv: vmbus: vmbus_sendpacket_ctl: hvsock: avoid unnecessary signaling

When the hvsock channel's outbound ringbuffer is full (i.e.,
hv_ringbuffer_write() returns -EAGAIN), we should avoid the unnecessary
signaling the host.

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/channel.c

@@ -659,6 +659,9 @@ int vmbus_sendpacket_ctl(struct vmbus_channel *channel, void *buffer,
 	 * If we cannot write to the ring-buffer; signal the host
 	 * even if we may not have written anything. This is a rare
 	 * enough condition that it should not matter.
+	 * NOTE: in this case, the hvsock channel is an exception, because
+	 * it looks the host side's hvsock implementation has a throttling
+	 * mechanism which can hurt the performance otherwise.
 	 */
 
 	if (channel->signal_policy)
@@ -666,7 +669,8 @@ int vmbus_sendpacket_ctl(struct vmbus_channel *channel, void *buffer,
 	else
 		kick_q = true;
 
-	if (((ret == 0) && kick_q && signal) || (ret))
+	if (((ret == 0) && kick_q && signal) ||
+	    (ret && !is_hvsock_channel(channel)))
 		vmbus_setevent(channel);
 
 	return ret;