netdev - Re: [RFC PATCH] ipv4: release dev refcnt early when destroying inetdev

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Date:	Fri, 10 Sep 2010 16:57:16 +0200
From:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	netdev <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH] ipv4: release dev refcnt early when destroying inetdev

Le 10.09.2010 16:24, Eric Dumazet a écrit :
> Le vendredi 10 septembre 2010 à 15:35 +0200, Nicolas Dichtel a écrit :
>> Hi all,
>>
>> We got a scalability problem when we try to remove a lot of virtual interfaces. 
>> After analysis, we found that a refcnt on a device was released too late.
>> Here is a proposal patch. If we are not missing something, the refcnt can be 
>> release before call_rcu(). In IPv6, this is already the case.
>>
>> Comments are welcome.
>>
>>
>> Regards,
>> Nicolas
>> pièce jointe différences entre fichiers
>> (0001-ipv4-release-dev-refcnt-early-when-destroying-inetd.patch)
>> From 6fe291ff56b1f94599dfaa57dfb0ed4c168b603f Mon Sep 17 00:00:00 2001
>> From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
>> Date: Fri, 10 Sep 2010 14:52:15 +0200
>> Subject: [PATCH] ipv4: release dev refcnt early when destroying inetdev
>>
>> When a virtual device is removed, refcnt on dev is released
>> after rcu barrier, hence we fall always in the msleep(250)
>> of netdev_wait_allrefs(). This causes a long delay when
>> a lot of interfaces are removed.
>> Refcnt can be released before this rcu barrier, this allows
>> to accelerate the removing of virtual interfaces.
>>
>> Test of removing 50 ipip tunnel interfaces:
>>  Before the patch:
>>   real    0m12.804s
>>   user    0m0.020s
>>   sys     0m0.000s
>>
>>  After the patch:
>>   real    0m0.988s
>>   user    0m0.004s
>>   sys     0m0.016s
>>
>> Signed-off-by: Wang Xuefu <xuefu.wang@...nd.com>
>> Signed-off-by: Nicolas Dichtel <nicolas.dichtel@...nd.com>
>> ---
> 
> This is a well known problem, (many patches were sent some months ago)
> but your patch is not the right solution.
> 
> As long as the idev is not yet freed, it can be used and we need to
> access idev->dev

Is this not true in IPv6? What is the difference?


Regards,
Nicolas
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