netdev - Re: [PATCH] vmxnet3: fix starving rx ring when alloc

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Date:	Fri, 17 Jun 2011 13:14:20 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	David Miller <davem@...emloft.net>
Cc:	scottjg@...are.com, netdev@...r.kernel.org, pv-drivers@...are.com
Subject: Re: [PATCH] vmxnet3: fix starving rx ring when alloc_skb fails

On Fri, 2011-06-17 at 00:53 -0400, David Miller wrote:
> From: Scott Goldman <scottjg@...are.com>
> Date: Thu, 16 Jun 2011 21:40:15 -0700
> 
> > I'm sorry if my patch description was unclear, but as I understand it, what you are describing is what this patch implements. The patched rx handler does something like:
> > 1) poll the rx ring, peel off a pending skb (don't pass the packet up the stack yet)
> > 2) if the ring needs to be repopulated, we do that
> > 3) if the ring was repopulated successfully, then that's great, and we pass up the pending skb to the network stack.
> > 4) if not and there are no skbs left on the ring, we reuse the pending skb and recycle it back on the ring (effectively dropping the packet we were about to pass up to the network stack)
> 
> You shouldn't restrict this logic to when the ring is empty, you
> should never leave any RX ring slots empty.
> 
> Every RX ring entry should be processed with a "alloc_skb()" first,
> and if the allocation fails you recycle the RX ring's SKB.

But this means the driver drops received packets as soon as there is an
allocation failure, rather than only if there are repeated failures.  I
don't see how that's preferable.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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