Re: [RFC] niu: RX queues should rotate if budget exhausted

[David Miller <davem@davemloft.net>]

From: David Miller <davem@davemloft.net>
Date: Fri, 27 Mar 2009 00:55:46 -0700 (PDT)

> From: Eric Dumazet <dada1@cosmosbay.com>
> Date: Thu, 26 Mar 2009 19:26:20 +0100
> 
> > I dont have NIU hardware but it seems this driver could suffer from 
> > starvation of high numbered RX queues under flood. I suspect other
> > multiqueue drivers might have same problem.
> > 
> > With a standard budget of 64, we can consume all credit when handling
> > first queue(s), and last queues might discard packets.
> > 
> > Solve this by rotating the starting point, so that we garantee to scan at
> > least one new queue at each niu_poll_core() invocation, even under stress.
> > 
> > Also, change logic calling niu_sync_rx_discard_stats() to take
> > into account the device qlen, not bounded by budget (which could be 0)
> > 
> > Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
> 
> This change looks good to me, thanks for spotting this and
> suggesting the fix.
> 
> It only hits people who don't have usable MSI-X interrupts,
> and hey if you have one of these cards what are you doing
> sticking it into such a machine :-)
> 
> Anyways, I'll apply this directly, thanks a lot!

Actually, I take that back... I want to do this slightly
differently.

That niu_poll_core() is used in both the MSI-X and non-MSI
case.

I always meant to fix this driver to not scan over all the
RX rings when we use MSI-X and the event can only be for
one specific queue.

Then we can add your rotating logic to the non-MSI-X interrupt
paths.