netdev - Re: [PATCH 1/2] cbq: incorrectly low bandwidth setting blocks limited traffic

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Message-ID: <53EB5FFE.1070604@parallels.com>
Date:	Wed, 13 Aug 2014 16:54:22 +0400
From:	Vasily Averin <vvs@...allels.com>
To:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
CC:	netdev@...r.kernel.org, Jamal Hadi Salim <jhs@...atatu.com>,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>
Subject: Re: [PATCH 1/2] cbq: incorrectly low bandwidth setting blocks limited
 traffic

On 08/13/2014 04:45 PM, Sergei Shtylyov wrote:
> Hello.
> 
> On 8/13/2014 4:38 PM, Vasily Averin wrote:
> 
>> Mainstream commit f0f6ee1f70c4eaab9d52cf7d255df4bd89f8d1c2 have side effect:
> 
>    Please also specify that commit's summary line in parens.
cbq: incorrect processing of high limits

>> if cbq bandwidth setting is less than real interface throughput
>> non-limited traffic can delay limited traffic for a very long time.
> 
>> This happen because of q->now changes incorrectly in cbq_dequeue():
>> in described scenario L2T is much greater than real time delay,
>> and q->now gets an extra boost for each transmitted packet.
> 
>> Accumulated boost prevents update q->now, and blocked class can wait
>> very long time until (q->now >= cl->undertime) will be true again.
> 
>> To fix the problem the patch updates q->now on each cbq_update() call.
>> L2T-related pre-modification q->now was moved to cbq_update().
> 
>> My testing confirmed that it fixes the problem and did not discovered
> 
>   Discover.
> 
>> any side-effects.
> 
>> Signed-off-by: Vasily Averin <vvs@...nvz.org>
> 
> WBR, Sergei
> 
> 

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