netdev - Re: [PATCH] net: reduce net_rx

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Date:	Thu, 21 Mar 2013 14:45:44 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Paul Gortmaker <paul.gortmaker@...driver.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, stable <stable@...r.kernel.org>,
	Willy Tarreau <w@....eu>, Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCH] net: reduce net_rx_action() latency to 2 HZ

On Thu, 2013-03-21 at 10:43 -0700, Eric Dumazet wrote:
> On Thu, 2013-03-21 at 13:25 -0400, Paul Gortmaker wrote:
> 
> > That is also reasonably portable back to 2.6.34.  And it is more
> > interesting too -- it will be interesting in a preempt_rt context
> > too, once RT moves ahead off the current 3.6 baseline, which still
> > has the old count-limit of 10 vs the new 2ms time limit.
> > 
> > RT (3.4 and 3.6 based) currently has this patch from Steven:
> > http://git.kernel.org/cgit/linux/kernel/git/paulg/3.6-rt-patches.git/tree/net-tx-action-avoid-livelock-on-rt.patch
> 
> Interesting, as Google has an internal patch removing this trylock() as
> well.
> 
> I think I should upstream it eventually ;)

Yes please :-)

-- Steve


> 
> commit 2f0a3f573b531dc57c268fd809dc65169edae369
> Author: Eric Dumazet <edumazet@...gle.com>
> Date:   Thu Dec 13 09:18:01 2012 -0800
> 
>     net-dev_xmit_hold_queues: fix a busy loop in net_tx_action
>     
>     Under load, net_tx_action() fails to acquire qdisc lock
>     and reschedules qdisc in a never ending loop.
>     
>     The spin_trylock() has almost no chance to complete because
>     of ticket spinlock and xmit_hold_queue holding the lock for long
>     period of times.
>     
> 


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