Date: Mon, 17 Sep 2012 15:03:12 -0400 From: John Baldwin <jhb@freebsd.org> To: Adrian Chadd <adrian@freebsd.org> Cc: freebsd-net@freebsd.org, Ryan Stone <rysto32@gmail.com> Subject: Re: What's the latest on fixing IFF_DRV_OACTIVE/if_start/etc? Message-ID: <201209171503.12517.jhb@freebsd.org> In-Reply-To: <CAJ-Vmon1iHpjx37=RDOqFPthLv5nTTyjmkOR246nBQ4VxwhVOA@mail.gmail.com> References: <CAFMmRNzkwbQpUZ3OOoMKVdrz=dePc5fkeX3m-5vXtiWJ7qXwVA@mail.gmail.com> <201209171316.45029.jhb@freebsd.org> <CAJ-Vmon1iHpjx37=RDOqFPthLv5nTTyjmkOR246nBQ4VxwhVOA@mail.gmail.com>
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On Monday, September 17, 2012 1:45:12 pm Adrian Chadd wrote:
> On 17 September 2012 10:16, John Baldwin <jhb@freebsd.org> wrote:
>
> > I think for if_bridge the fix is that it no longer uses if_start. :)
>
> :)
>
> > For real hardware you will get some sort of TX completion interrupt that will
> > restart the transmit queue. Virtual software-only interfaces such as vlan(4)
> > and if_bridge(4) don't have that luxury though, and the best bet for them is
> > to probably have them use if_transmit instead. vlan(4) and if_bridge(4) are
> > already fixed for that (if_bridge was only fixed a week or so ago in HEAD).
>
> I'm still not convinced that going the if_start route (with
> process-to-completion) is going to work well when forwarding gobs of
> packets on anything bar ${BIG_IRON}, but that aside..
Eh? For virtual interfaces, if_transmit introduces less overhead than
using if_start (no locking, queueing, dequeueing, etc.). I expect that
to be a net win for smaller boards.
> It may be nice to introduce a virtual TX completion callback? Ie, a
> child driver could signal that it's successfully drained its TX queue,
> notifying any parent drivers that they can send more?
That could work, but I generally think if_transmit is a better route for
these sorts of things. That turns these interfaces into simple filters
rather than building up their own queue, etc.
--
John Baldwin
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