Date:	Tue, 24 Mar 2009 14:29:29 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Joakim Tjernlund <Joakim.Tjernlund@...nsmode.se>
CC:	avorontsov@...mvista.com, Patrick McHardy <kaber@...sh.net>,
	netdev@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] conntrack: Reduce conntrack count in nf_conntrack_free()

Joakim Tjernlund a écrit :
> Eric Dumazet <dada1@...mosbay.com> wrote on 24/03/2009 13:07:16:
>> Joakim Tjernlund a écrit :
>>> Eric Dumazet <dada1@...mosbay.com> wrote on 24/03/2009 10:12:53:
>>>> Joakim Tjernlund a écrit :
>>>>> Patrick McHardy <kaber@...sh.net> wrote on 23/03/2009 18:49:15:
>>>>>> Joakim Tjernlund wrote:
>>>>>>> Patrick McHardy <kaber@...sh.net> wrote on 23/03/2009 13:29:33:
>>>>>>>
>>>>>>>
>>>>>>>>> There is no /proc/net/netfilter/nf_conntrack. There is a
>>>>>>>>> /proc/net/nf_conntrack though and it is empty. If I telnet
>>>>>>>>> to the board I see:
>>>>>>>>>
>>>>>>>> That means that something is leaking conntrack references, most 
>>>>> likely
>>>>>>>> by leaking skbs. Since I haven't seen any other reports, my guess 
> 
>>>>> would
>>>>>>>> be the ucc_geth driver.
>>>>>>>>
>>>>>>> Mucking around with the ucc_geth driver I found that if I:
>>>>>>>  - Move TX from IRQ to NAPI context
>>>>>>>  - double the weight.
>>>>>>>  - after booting up, wait a few mins until the JFFS2 GC kernel 
>>> thread 
>>>>> has 
>>>>>>> stopped
>>>>>>>    scanning the FS 
>>>>>>>
>>>>>>> Then the "nf_conntrack: table full, dropping packet." msgs stops.
>>>>>>> Does this seem right to you guys?
>>>>>> No. As I said, something seems to be leaking packets. You should be
>>>>>> able to confirm that by checking the sk_buff slabs in 
> /proc/slabinfo.
>>>>>> If that *doesn't* show any signs of a leak, please run "conntrack 
> -E"
>>>>>> to capture the conntrack events before the "table full" message
>>>>>> appears and post the output.
>>>>> skbuff does not differ much, but others do
>>>>>
>>>>> Before ping:
>>>>>   skbuff_fclone_cache    0      0    352   11    1 : tunables   54 
> 27 
>>> 0 
>>>>> : slabdata      0      0      0
>>>>>   skbuff_head_cache     20     20    192   20    1 : tunables  120 
> 60 
>>> 0 
>>>>> : slabdata      1      1      0
>>>>>   size-64              731    767     64   59    1 : tunables  120 
> 60 
>>> 0 
>>>>> : slabdata     13     13      0
>>>>>   nf_conntrack          10     19    208   19    1 : tunables  120 
> 60 
>>> 0 
>>>>> : slabdata      1      1      0
>>>>>
>>>>> During ping: 
>>>>>   skbuff_fclone_cache    0      0    352   11    1 : tunables   54 
> 27 
>>> 0 
>>>>> : slabdata      0      0      0
>>>>>   skbuff_head_cache     40     40    192   20    1 : tunables  120 
> 60 
>>> 0 
>>>>> : slabdata      2      2      0
>>>>>   size-64             8909   8909     64   59    1 : tunables  120 
> 60 
>>> 0 
>>>>> : slabdata    151    151      0
>>>>>   nf_conntrack        5111   5111    208   19    1 : tunables  120 
> 60 
>>> 0 
>>>>> : slabdata    269    269      0
>>>>>
>>>>> This feels more like the freeing of conntrack objects are delayed 
> and 
>>>>> builds up when ping flooding.
>>>>>
>>>>> Don't have "conntrack -E" for my embedded board so that will have to 
> 
>>> wait 
>>>>> a bit longer.
>>>> I dont understand how your ping can use so many conntrack entries...
>>>>
>>>> Then, as I said yesterday, I believe you have a RCU delay, because of
>>>> a misbehaving driver or something...
>>>>
>>>> grep RCU .config
>>> grep RCU .config
>>> # RCU Subsystem
>>> CONFIG_CLASSIC_RCU=y
>>> # CONFIG_TREE_RCU is not set
>>> # CONFIG_PREEMPT_RCU is not set
>>> # CONFIG_TREE_RCU_TRACE is not set
>>> # CONFIG_PREEMPT_RCU_TRACE is not set
>>> # CONFIG_RCU_TORTURE_TEST is not set
>>> # CONFIG_RCU_CPU_STALL_DETECTOR is not set
>>>
>>>> grep CONFIG_SMP .config
>>> grep CONFIG_SMP .config
>>> # CONFIG_SMP is not set
>>>
>>>> You could change qhimark from 10000 to 1000 in kernel/rcuclassic.c 
> (line 
>>> 80)
>>>> as a workaround. It should force a quiescent state after 1000 freed 
>>> conntracks.
>>>
>>> right, doing this almost killed all conntrack messages, had to stress 
> it 
>>> pretty
>>> hard before I saw handful "nf_conntrack: table full, dropping packet"
>>>
>>> RCU is not my cup of tea, do you have any ideas were to look?
>> In a stress situation, you feed more deleted conntracks to call_rcu() 
> than
>> the blimit (10 real freeing per RCU softirq invocation). 
>>
>> So with default qhimark being 10000, this means about 10000 conntracks
>> can sit in RCU (per CPU) before being really freed.
>>
>> Only when hitting 10000, RCU enters a special mode to free all queued 
> items, instead
>> of a small batch of 10
>>
>> To solve your problem we can :
>>
>> 1) reduce qhimark from 10000 to 1000 (for example)
>>    Probably should be done to reduce some spikes in RCU code when 
> freeing
>>    whole 10000 elements...
>> OR
>> 2) change conntrack tunable (max conntrack entries on your machine)
>> OR
>> 3) change net/netfilter/nf_conntrack_core.c to decrement net->ct.count
>>   in nf_conntrack_free() instead of callback.
>>
>> [PATCH] conntrack: Reduce conntrack count in nf_conntrack_free()
> 
> The patch fixes the problem and the system feels a bit more responsive 
> too, thanks.
> I guess I should probably do both 1) and 3) as my board is pretty slow 
> too.
> 
> Been trying to figure out a good value for NAPI weigth too. Currently my
> HW RX and TX queues are 16 pkgs deep and weigth is 16 too. If I move TX 
> processing
> to NAPI context AND increase weigth to 32, the system is a lot more 
> responsive during
> ping flooding. Does weigth 32 make sense when the HW TX and RX queues are 
> 16?

If you only have one NIC, I dont understand why changing weight should make
a difference. Are you referring to dev_weight or netdev_budget ?

# cat /proc/sys/net/core/dev_weight
64
# cat /proc/sys/net/core/netdev_budget
300


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