linux-kernel - Re: bad networking related lag in v2.6.22-rc2

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Message-ID: <20070521204606.GA4354@elte.hu>
Date:	Mon, 21 May 2007 22:46:06 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Anant Nitya <kernel@...chanda.info>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: bad networking related lag in v2.6.22-rc2


* Anant Nitya <kernel@...chanda.info> wrote:

> I am posting links to the information you asked for. One more thing, 
> after digging a bit more I found its QoS shaping that is making the 
> box crawl. Once I disabled the traffic shaping everything comes back 
> to smooth and normal. Shaping being done on very low speed residential 
> ADSL 256/64 Kbps connection. If you want me to post shaping rules, 
> please free to ask. BTW its a simple HTB/SFQ rules.
[...]
> http://cybertek.info/taitai/trace-to-ingo.txt.bz2

thanks! This trace indeed includes the smoking gun, htb_dequeue() and 
__qdisc_run():

   privoxy-12926 1.Ns1 1597us : rb_first (htb_dequeue)

this goes on, non-preemptible, for 160 milliseconds (!):

 privoxy-12926 1.Ns1 161568us : rb_first (htb_dequeue)
 privoxy-12926 1.Ns1 161568us : qdisc_watchdog_schedule (htb_dequeue)

and finally manages to escape the loop:

 privoxy-12926 1.Ns1 161597us : rb_first (htb_dequeue)
 privoxy-12926 1.Ns1 161597us : rb_first (htb_dequeue)
 privoxy-12926 1.Ns1 161599us : htb_safe_rb_erase (htb_dequeue)
 privoxy-12926 1.Ns1 161599us : rb_erase (htb_safe_rb_erase)
 privoxy-12926 1.Ns1 161600us : htb_change_class_mode (htb_dequeue)
 privoxy-12926 1.Ns1 161601us : htb_activate_prios (htb_change_class_mode)

and the system recovers.

David, any ideas about what's wrong with htb_dequeue(), based on this 
trace?

	Ingo
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