netdev - Re: [net-next PATCH V3-evictor] net: frag evictor, avoid killing warm frag queues

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Date:	Thu, 06 Dec 2012 06:47:56 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc:	Florian Westphal <fw@...len.de>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Thomas Graf <tgraf@...g.ch>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Cong Wang <amwang@...hat.com>,
	Herbert Xu <herbert@...dor.hengli.com.au>
Subject: Re: [net-next PATCH V3-evictor] net: frag evictor, avoid killing
 warm frag queues

On Thu, 2012-12-06 at 14:55 +0100, Jesper Dangaard Brouer wrote:

> Perhaps.  But do note my bashing of the LRU list were wrong.  I planned
> to explain that in a separate mail, but basically I were causing a DoS
> attack with incomplete fragments on my self, because I had disabled
> Ethernet flow-control.  Which led me to some false assumptions on the
> LRU list behavior (sorry).
> 
> The LRU might be the correct solution after all.  If I enable Ethernet
> flow-control again, then I have a hard time "activating" the evictor
> code (with thresh 4M/3M) .  I'll need a separate DoS program, which can
> send incomplete fragments (in back-to-back bursts) to provoke the
> evictor and LRU.
> 
> My cheap DoS reproducer-hack is to disable Ethernet flow-control on only
> one interface (out of 3), to cause packet drops and the incomplete
> fragments. The current preliminary results is that the two other
> interfaces still gets packets through, we don't get the zero throughput
> situation.
>  Two interfaces and no DoS: 15342 Mbit/s
>  Three interfaces and DoS:   7355 Mbit/s
> 
> The reduction might look big, but you have to take into account, that
> "activating" the evictor code, is also causing scalability issues of its
> own (which could account for the performance drop it self).

I would try removing the LRU, but keeping the age information (jiffie of
last valid frag received on one inet_frag_queue)

The eviction would be a function of the current memory used for the
frags (percpu_counter for good SMP scalability), divided by the max
allowed size, and ipfrag_time.

Under load, we would evict inet_frag_queue before the ipfrag_time timer,
without necessarily having to scan whole frags, only the ones we find in
the bucket we need to parse anyway (and lock)

The whole idea of a full garbage collect under softirq is not scalable,
as it locks a CPU in a non preemptible section for too long.



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