netdev - Re: [PATCH] ppp: Don't stop and restart queue on every TX packet

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Date:	Sun, 08 Apr 2012 21:58:24 +0200
From:	David Woodhouse <dwmw2@...radead.org>
To:	David Miller <davem@...emloft.net>
Cc:	paulus@...ba.org, netdev@...r.kernel.org
Subject: Re: [PATCH] ppp: Don't stop and restart queue on every TX packet

On Tue, 2012-04-03 at 17:29 -0400, David Miller wrote:
> From: David Woodhouse <dwmw2@...radead.org>
> Date: Mon, 26 Mar 2012 11:03:42 +0100
> 
> > For every transmitted packet, ppp_start_xmit() will stop the netdev
> > queue and then, if appropriate, restart it. This causes the TX softirq
> > to run, entirely gratuitously.
> > 
> > This is "only" a waste of CPU time in the normal case, but it's actively
> > harmful when the PPP device is a TEQL slave ― the wakeup will cause the
> > offending device to receive the next TX packet from the TEQL queue, when
> > it *should* have gone to the next slave in the list. We end up seeing
> > large bursts of packets on just *one* slave device, rather than using
> > the full available bandwidth over all slaves.
> > 
> > This patch fixes the problem by *not* unconditionally stopping the queue
> > in ppp_start_xmit(). It adds a return value from ppp_xmit_process()
> > which indicates whether the queue should be stopped or not.
> > 
> > It *doesn't* remove the call to netif_wake_queue() from
> > ppp_xmit_process(), because other code paths (especially from
> > ppp_output_wakeup()) need it there and it's messy to push it out to the
> > other callers to do it based on the return value. So we leave it in
> > place ― it's a no-op in the case where the queue wasn't stopped, so it's
> > harmless in the TX path.
> > 
> > Signed-off-by: David Woodhouse <David.Woodhouse@...el.com>
> 
> Applied, thanks. 

Hmm. After going through the locking issues on wakeup, for the PPPoATM
patch I just sent, I think that the above has introduced a bug. I'll
send a patch with a full explanation in a few moments...

-- 
dwmw2

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