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

[David Miller <davem@xxxxxxxxxxxxx>]

From: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
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@xxxxxxxxx>

Applied, thanks.  But:

> --- drivers/net/ppp/ppp_generic.c~	2012-01-26 00:39:32.000000000 +0000
> +++ drivers/net/ppp/ppp_generic.c	2012-03-26 10:32:31.286744147 +0100

Please -p1 root your patches in the future.
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