netdev - Re: [PATCH] ucc_geth: Move freeing of TX packets to NAPI context.

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Date:	Mon, 30 Mar 2009 18:24:07 +0800
From:	Li Yang <leoli@...escale.com>
To:	Joakim Tjernlund <Joakim.Tjernlund@...nsmode.se>,
	David Miller <davem@...emloft.net>
Cc:	avorontsov@...mvista.com,
	linuxppc-dev Development <linuxppc-dev@...abs.org>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] ucc_geth: Move freeing of TX packets to NAPI context.

On Mon, Mar 30, 2009 at 6:01 PM, Joakim Tjernlund
<Joakim.Tjernlund@...nsmode.se> wrote:
> pku.leo@...il.com wrote on 30/03/2009 11:36:36:
>>
>> On Mon, Mar 30, 2009 at 5:21 PM, Joakim Tjernlund
>> <Joakim.Tjernlund@...nsmode.se> wrote:
>> > pku.leo@...il.com wrote on 30/03/2009 10:34:47:
>> >>
>> >> On Thu, Mar 26, 2009 at 1:51 AM, Joakim Tjernlund
>> >> <Joakim.Tjernlund@...nsmode.se> wrote:
>> >> > Anton Vorontsov <avorontsov@...mvista.com> wrote on 25/03/2009
>> > 15:25:40:
>> >> >> On Wed, Mar 25, 2009 at 02:30:49PM +0100, Joakim Tjernlund wrote:
>> >> >> > >>From 1c2f23b1f37f4818c0fd0217b93eb38ab6564840 Mon Sep 17
> 00:00:00
>> >> > 2001
>> >> >> > From: Joakim Tjernlund <Joakim.Tjernlund@...nsmode.se>
>> >> >> > Date: Tue, 24 Mar 2009 10:19:27 +0100
>> >> >> > Subject: [PATCH] ucc_geth: Move freeing of TX packets to NAPI
>> > context.
>> >> >> >  Also increase NAPI weight somewhat.
>> >> >> >  This will make the system alot more responsive while
>> >> >> >  ping flooding the ucc_geth ethernet interaface.
>> >> >>
>> >> >> Some time ago I've tried a similar thing for this driver, but
> during
>> >> >> tcp (or udp I don't quite remember) netperf tests I was getting tx
>> >> >> watchdog timeouts after ~2-5 minutes of work. I was testing with a
>> >> >> gigabit and 100 Mbit link, with 100 Mbit link the issue was not
>> >> >> reproducible.
>> >> >>
>> >> >> Though, I recalling I was doing a bit more than your patch: I was
>> >> >> also clearing the TX events in the ucce register before calling
>> >> >> ucc_geth_tx, that way I was trying to avoid stale interrupts. That
>> >> >> helped to increase an overall performance (not only
> responsiveness),
>> >> >> but as I said my approach didn't pass the tests.
>> >> >>
>> >> >> I don't really think that your patch may cause this, but can you
>> >> >> try netperf w/ this patch applied anyway? And see if it really
>> >> >> doesn't cause any issues under stress?
>> >> >
>> >> > Does the line(in ucc_geth_tx()) look OK to you:
>> >> >        if ((bd == ugeth->txBd[txQ]) && (netif_queue_stopped(dev) ==
>> > 0))
>> >> >                        break;
>> >> >
>> >> > Sure does look fishy to me.
>> >>
>> >> There are two cases when txBd=ConfBd: the BD ring is full or empty.
>> >> The condition used here ensures that it is the empty case.  Because
> in
>> >> hard_start_xmit, the queue will be stopped when the BD ring is full.
>> >> Maybe some comment is needed here.
>> >
>> > But how do you know that the queue hasn't been stopped by someone else
>> > than
>> > the driver?
>> > If it is stopped by higher layers, the if stmt will fail.
>>
>> It looks like from existing code that only the driver can legally stop
>> the queue.  I'm not 100% sure though.  Correct me if I'm wrong.
>
> I don't know. But the question you should ask is: Does the networking
> code promise this now and for the future?

Right.  But it's beyond my knowledge to answer this question.  If not,
adding a device specific flag is not very costing.

Hi Dave,

Can we assume that the netif_stop_queue() and netif_wake_queue() are
only used by the netdev driver?  And the queue state will not be
changed by other part of the networking subsystem?

- Leo
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