netdev - Re: [PATCH net-next-2.6] ixgbe: fix multi-ring polling [V2]

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Message-Id: <20090617.044026.244800766.davem@davemloft.net>
Date:	Wed, 17 Jun 2009 04:40:26 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	jesse.brandeburg@...el.com
Cc:	andy@...yhouse.net, netdev@...r.kernel.org,
	peter.p.waskiewicz.jr@...el.com, jeffrey.t.kirsher@...el.com
Subject: Re: [PATCH net-next-2.6] ixgbe: fix multi-ring polling [V2]

From: "Brandeburg, Jesse" <jesse.brandeburg@...el.com>
Date: Tue, 16 Jun 2009 15:02:24 -0700 (Pacific Daylight Time)

> On Tue, 16 Jun 2009, Andy Gospodarek wrote:
>> Adding a check is no problem, but that means we need to save the
>> original budget.  It would be good to do that to avoid the WARN_ON_ONCE
>> in net_rx_action as well, but should we be cheating like that?  Here's
>> the new patch:
> 
> I hope davem can comment on that.
> 
>> [PATCH net-next-2.6] ixgbe: fix multi-ring polling V2
> 
> I think technically I'm okay with V2, the outstanding questions about what 
> exactly we should return need to be answered.

If you aren't going to complete the NAPI run, you must indicate
to the caller of ->poll() that you've consumed the entire budget.

This is the second driver where the multi-queue-in-one-irq "issue"
has been noticed.  Eric Dumazet posted a similar patch for NIU.

There are a few other ways to approach this problem, now that I've
thought about it for some time:

1) Use multiple NAPI contexts to represent the queues even if
   they are backed by a single interrupt.

2) Use only "1" queue if you only have "1" interrupt.  (replace
   "1" with "N" for all valid values of "N" :-)

Those approaches are a lot cleaner and keeps us from needing all
of this gross starvation-avoidance and budget faking code.
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