netdev - Re: Slow inbound traffic on macvtap interfaces

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Message-ID: <20120830082057.GA18072@alpha.arachsys.com>
Date:	Thu, 30 Aug 2012 09:20:57 +0100
From:	Richard Davies <richard.j.davies@...il.com>
To:	Chris Webb <chris@...chsys.com>
Cc:	netdev@...r.kernel.org, qemu-devel@...gnu.org,
	Jason Wang <jasowang@...hat.com>,
	Arnd Bergmann <arnd@...db.de>, Michael Tsirkin <mst@...hat.com>
Subject: Re: Slow inbound traffic on macvtap interfaces

Chris Webb wrote:
> I found that on my laptop, the single change of host kernel config
>
> -CONFIG_INTEL_IDLE=y
> +# CONFIG_INTEL_IDLE is not set
>
> is sufficient to turn transfers into guests from slow to full wire speed

I am not deep enough in this code to write a patch, but I wonder if
macvtap_forward in macvtap.c is missing a call to kill_fasync, which I
understand is used to signal to interested processes when data arrives?


Here is the end of macvtap_forward:

  skb_queue_tail(&q->sk.sk_receive_queue, skb);
  wake_up_interruptible_poll(sk_sleep(&q->sk), POLLIN | POLLRDNORM | POLLRDBAND);
  return NET_RX_SUCCESS;


Compared to this end of tun_net_xmit in tun.c:

  /* Enqueue packet */
  skb_queue_tail(&tun->socket.sk->sk_receive_queue, skb);

  /* Notify and wake up reader process */
  if (tun->flags & TUN_FASYNC)
          kill_fasync(&tun->fasync, SIGIO, POLL_IN);
  wake_up_interruptible_poll(&tun->wq.wait, POLLIN |
                             POLLRDNORM | POLLRDBAND);
  return NETDEV_TX_OK;


Richard.
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