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

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Message-ID: <20090617210652.GD8515@gospo.rdu.redhat.com>
Date:	Wed, 17 Jun 2009 17:06:52 -0400
From:	Andy Gospodarek <andy@...yhouse.net>
To:	David Miller <davem@...emloft.net>
Cc:	jesse.brandeburg@...el.com, 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]

On Wed, Jun 17, 2009 at 04:40:26AM -0700, David Miller wrote:
> 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.

By 'complete the NAPI run' do you mean call napi_complete?  Looking at
net_rx_action I don't see where it really matters how much work was done
by ->poll as long as it's not more than the device weight (since that
will spring the WARNing).

> 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.

So multiple calls to napi_schedule in a single interrupt handler?
Interesting....


> 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.

I agree.  It also seems much cleaner to do it that way because then each
queue or device gets the full weight.

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