netdev - Re: [PATCH] fib_info_hashfn leads to long hash chains

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Date:	Mon, 07 May 2007 16:09:11 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	bcrl@...ck.org
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] fib_info_hashfn leads to long hash chains

From: Benjamin LaHaise <bcrl@...ck.org>
Date: Sun, 29 Apr 2007 11:03:36 -0400

> Hello
> 
> The patch below fixes a case where fib_find_info() is consume excessive 
> amounts of CPU during the creation of 10000 PPP interfaces.  In access 
> servers, each point to point link has the same local address, but a different 
> destination and interface.  Because the device is not included in the hash 
> calculation, the chain grows excessively large and we end up spinning the 
> CPU walking the list.  As near as I can tell, this shouldn't have any negative 
> sideeffects, but someone with a better understanding of fib_semantics.c will 
> need to check over it.  Cheers,

fib_create_info() makes sure that fi->fib_nhs is always at least one.
So the conditional can be removed.  After a fib_info is create, it's
fi->fib_nhs is immutable, it doesn't change.

In fact there are some checks for fi->fib_nhs == 0 elsewhere in this
code, those are bogus too, and I imagine you saw those when coding
up this patch. :-)
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