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Message-ID: <20141023114017.GG9188@zion.uk.xensource.com>
Date: Thu, 23 Oct 2014 12:40:17 +0100
From: Wei Liu <wei.liu2@...rix.com>
To: David Vrabel <david.vrabel@...rix.com>
CC: <netdev@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
Ian Campbell <ian.campbell@...rix.com>,
Wei Liu <wei.liu2@...rix.com>
Subject: Re: [PATCH 2/3] xen-netback: fix unlimited guest Rx internal queue
and carrier flapping
On Wed, Oct 22, 2014 at 02:08:54PM +0100, David Vrabel wrote:
> Netback needs to discard old to-guest skb's (guest Rx queue drain) and
> it needs detect guest Rx stalls (to disable the carrier so packets are
> discarded earlier), but the current implementation is very broken.
>
> 1. The check in hard_start_xmit of the slot availability did not
> consider the number of packets that were already in the guest Rx
> queue. This could allow the queue to grow without bound.
>
> The guest stops consuming packets and the ring was allowed to fill
> leaving S slot free. Netback queues a packet requiring more than S
> slots (ensuring that the ring stays with S slots free). Netback
> queue indefinately packets provided that then require S or fewer
> slots.
>
> 2. The Rx stall detection is not triggered in this case since the
> (host) Tx queue is not stopped.
>
> 3. If the Tx queue is stopped and a guest Rx interrupt occurs, netback
> will consider this an Rx purge event which may result in it taking
> the carrier down unnecessarily. It also considers a queue with
> only 1 slot free as unstalled (even though the next packet might
> not fit in this).
>
> The internal guest Rx queue is limited by a byte length (to 512 Kib,
> enough for half the ring). The (host) Tx queue is stopped and started
> based on this limit. This sets an upper bound on the amount of memory
> used by packets on the internal queue.
>
> This allows the estimatation of the number of slots for an skb to be
> removed (it wasn't a very good estimate anyway). Instead, the guest
+1 for this.
> Rx thread just waits for enough free slots for a maximum sized packet.
>
> skbs queued on the internal queue have an 'expires' time (set to the
> current time plus the drain timeout). The guest Rx thread will detect
> when the skb at the head of the queue has expired and discard expired
> skbs. This sets a clear upper bound on the length of time an skb can
> be queued for. For a guest being destroyed the maximum time needed to
> wait for all the packets it sent to be dropped is still the drain
> timeout (10 s) since it will not be sending new packets.
>
> Rx stall detection is reintroduced in a later commit.
>
> Signed-off-by: David Vrabel <david.vrabel@...rix.com>
Reviewed-by: Wei Liu <wei.liu2@...rix.com>
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