On Mon, Jan 31, 2011 at 06:24:34PM -0600, Steve Dobbelstein wrote:

"Michael S. Tsirkin" [off-list ref] wrote on 01/28/2011 06:16:16 AM:

quoted

OK, so thinking about it more, maybe the issue is this:
tx becomes full. We process one request and interrupt the guest,
then it adds one request and the queue is full again.

Maybe the following will help it stabilize?
By itself it does nothing, but if you set
all the parameters to a huge value we will
only interrupt when we see an empty ring.
Which might be too much: pls try other values
in the middle: e.g. make bufs half the ring,
or bytes some small value, or packets some
small value etc.

Warning: completely untested.

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index aac05bc..6769cdc 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c

@@ -32,6 +32,13 @@
  * Using this limit prevents one virtqueue from starving others. */
 #define VHOST_NET_WEIGHT 0x80000

+int tx_bytes_coalesce = 0;
+module_param(tx_bytes_coalesce, int, 0644);
+int tx_bufs_coalesce = 0;
+module_param(tx_bufs_coalesce, int, 0644);
+int tx_packets_coalesce = 0;
+module_param(tx_packets_coalesce, int, 0644);
+
 enum {
    VHOST_NET_VQ_RX = 0,
    VHOST_NET_VQ_TX = 1,

@@ -127,6 +134,9 @@ static void handle_tx(struct vhost_net *net)
    int err, wmem;
    size_t hdr_size;
    struct socket *sock;
+   int bytes_coalesced = 0;
+   int bufs_coalesced = 0;
+   int packets_coalesced = 0;

    /* TODO: check that we are running from vhost_worker? */
    sock = rcu_dereference_check(vq->private_data, 1);

@@ -196,14 +206,26 @@ static void handle_tx(struct vhost_net *net)
       if (err != len)
          pr_debug("Truncated TX packet: "
              " len %d != %zd\n", err, len);
-      vhost_add_used_and_signal(&net->dev, vq, head, 0);
       total_len += len;
+      packets_coalesced += 1;
+      bytes_coalesced += len;
+      bufs_coalesced += in;

Should this instead be:
      bufs_coalesced += out;

Correct.

Perusing the code I see that earlier there is a check to see if "in" is not
zero, and, if so, error out of the loop.  After the check, "in" is not
touched until it is added to bufs_coalesced, effectively not changing
bufs_coalesced, meaning bufs_coalesced will never trigger the conditions
below.

Or am I missing something?

quoted

+      if (unlikely(packets_coalesced > tx_packets_coalesce ||
+              bytes_coalesced > tx_bytes_coalesce ||
+              bufs_coalesced > tx_bufs_coalesce))
+         vhost_add_used_and_signal(&net->dev, vq, head, 0);
+      else
+         vhost_add_used(vq, head, 0);
       if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
          vhost_poll_queue(&vq->poll);
          break;
       }
    }

+   if (likely(packets_coalesced > tx_packets_coalesce ||
+         bytes_coalesced > tx_bytes_coalesce ||
+         bufs_coalesced > tx_bufs_coalesce))
+      vhost_signal(&net->dev, vq);
    mutex_unlock(&vq->mutex);
 }

Steve D.