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Date:	Wed, 13 Aug 2014 16:54:22 +0400
From:	Vasily Averin <>
To:	Sergei Shtylyov <>
CC:, Jamal Hadi Salim <>,
	"David S. Miller" <>,
	Alexey Kuznetsov <>
Subject: Re: [PATCH 1/2] cbq: incorrectly low bandwidth setting blocks limited

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 <>
> WBR, Sergei

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