Date:	Tue, 24 Mar 2009 14:20:36 +0100
From:	Joakim Tjernlund <Joakim.Tjernlund@...nsmode.se>
To:	Eric Dumazet <dada1@...mosbay.com>
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()

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?

 Jocke

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