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

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Date:	Mon, 21 May 2007 23:02:10 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Anant Nitya <kernel@...chanda.info>, 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

Ingo Molnar wrote:
> * 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?


This looks like fallout from the switch to hrtimers. Anant, please
send me your HTB script, I'll try to reproduce it.

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