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Date:	Wed, 19 Sep 2012 13:09:17 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Neil Horman <nhorman@...driver.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Maciej ┼╗enczykowski <maze@...gle.com>,
	Tom Herbert <therbert@...gle.com>,
	Lorenzo Colitti <lorenzo@...gle.com>
Subject: Re: [PATCH net-next] net: only run neigh_forced_gc() from one cpu

On Wed, 2012-09-19 at 13:07 +0200, Eric Dumazet wrote:
> On Wed, 2012-09-19 at 06:50 -0400, Neil Horman wrote:
> 
> > This is going to cause callers in neigh_alloc to immediately fail their
> > allocation attempts.  Would it be a good idea to modify that call site so that
> > instead of returning NULL, instead reread tbl->entries before comparing to
> > gc_thresh3, on the hope that the cpu in the garbage collecting routine has freed
> > some entries?
> 
> neigh_alloc() fails only if gc_thresh3 is hit, and if it is hit, we are
> under attack by definition.
> 
> (the gc is run every 5 seconds is above gc_thresh2, and below
> gc_thresh3)
> 
> No matter what you try, the attacker is going to be the winner.
> 
> The best thing here is to drop packets, not spending several milli
> seconds to serve one packet, as queues are going to tail drop anyway.
> 

I meant several hundred of milli seconds per packet.

In our tests we even trigger a softlockup, so thats more than 10 seconds
waiting for the rwlock, for a single packet.




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