netdev - Re: [PATCH] net: reduce net_rx

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Date:	Thu, 21 Mar 2013 10:43:14 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, stable <stable@...r.kernel.org>,
	Willy Tarreau <w@....eu>, Tom Herbert <therbert@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] net: reduce net_rx_action() latency to 2 HZ

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 ;)

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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