linux-kernel - Re: [PATCH] virtio_net: free transmit skbs in a timer

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Message-ID: <48294776.3010504@qumranet.com>
Date:	Tue, 13 May 2008 10:47:02 +0300
From:	Avi Kivity <avi@...ranet.com>
To:	Mark McLoughlin <markmc@...hat.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	Anthony Liguori <aliguori@...ibm.com>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] virtio_net: free transmit skbs in a timer

Mark McLoughlin wrote:
> virtio_net currently only frees old transmit skbs just
> before queueing new ones. If the queue is full, it then
> enables interrupts and waits for notification that more
> work has been performed.
>
> However, a side-effect of this scheme is that there are
> always xmit skbs left dangling when no new packets are
> sent, against the Documentation/networking/driver.txt
> guideline:
>
>   "... it is not allowed for your TX mitigation scheme
>    to let TX packets "hang out" in the TX ring unreclaimed
>    forever if no new TX packets are sent."
>
> Add a timer to ensure that any time we queue new TX
> skbs, we will shortly free them again.
>
> This fixes an easily reproduced hang at shutdown where
> iptables attempts to unload nf_conntrack and nf_conntrack
> waits for an skb it is tracking to be freed, but virtio_net
> never frees it.
>   

Sorry to barge in late, but IMO the timer should be on the host, which 
is cheaper than on the guest (well, a 100ms timer is likely zero cost, 
but  I still don't like it).

the host should fire a tx completion interrupt whenever the completion 
queue has "enough" entries, where we can define "enough" now as the 
halfway mark or a timer expiry, whichever comes earlier.

We can later improve "enough" to be "just enough so the timer never 
triggers" and adjust it dynamically.  It probably doesn't matter for 
Linux, but I don't want to punish guests that can do true async 
networking and depend on timely completion notification.

-- 
error compiling committee.c: too many arguments to function

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