netdev - Re: [PATCH v2 00/46] Clean up RX copybreak and DMA handling

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Date:	Mon, 11 Jul 2011 11:47:23 +0200
From:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH v2 00/46] Clean up RX copybreak and DMA handling

On Mon, Jul 11, 2011 at 02:24:03AM -0700, David Miller wrote:
> From: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> Date: Mon, 11 Jul 2011 11:16:49 +0200
> > Packet is indicated in queue N, theres no memory for new skb, so its
> > dropped, and the buffer goes back to free list. In parallel, queue M
> > (!= N) indicates new packet. Still, there's no memory for new skb so
> > its also dropped and its buffer is reused. The effect is that all
> > packets are dropped, whatever queue they appear on.
> Why would queue M (!= N) fail just because N did?  They may be
> allocating out of different NUMA nodes, and thus succeed.
> 
> > The HOL blocking does not matter here, because there's only one head
> > --- the system memory. If I misunderstood this point, please explain
> > it further.
> Multiqueue drivers are moving towards placing the queues on different
> NUMA nodes, and in that scenerio one queue might succeed even if the
> other fails.

I assumed that all queues get buffers from the same rx free ring. If
queues have their own free list, then we can treat them as separate NICs
for this discussion. Queues on one NUMA node stall, others go on normally.

> Back to the hardware hanging issue, it's real.  Getting into a
> situation where the RX ring lacks any buffers at all is the least
> tested path for these chips.
> 
> Testing fate is a really bad idea, and this is why I always propose to
> keep the hardware with RX buffers to use in all circumstances.

Catch 22: The chips are not tested because they have always free buffers,
they are provided with endless rx buffers because they are not being
tested. I'd rather test them well rather than workaround a phantom issue.
Tripping on empty free rx buffer ring is still possible even with scheme
#1 (eg. with lots of NICs receiving heavy traffic) --- just harder to
recognise and debug (if it breaks at all) when it happens.

Best Regards,
Michał Mirosław
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