netdev - Re: [PATCH] net: make ip_rt_acct a normal percpu var

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Message-Id: <20081119.151754.44649102.davem@davemloft.net>
Date:	Wed, 19 Nov 2008 15:17:54 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	rusty@...tcorp.com.au
Cc:	dada1@...mosbay.com, netdev@...r.kernel.org
Subject: Re: [PATCH] net: make ip_rt_acct a normal percpu var

From: Rusty Russell <rusty@...tcorp.com.au>
Date: Thu, 20 Nov 2008 09:43:21 +1030

> On Thursday 20 November 2008 08:50:23 David Miller wrote:
> > Do you really need this to forward some work you are doing?  If not
> > can we just let sleeping dogs lie on this one? :)
> 
> Yes, I have patches to convert the dynamic percpu data to use the same 
> mechanism as static percpu data.  Unfortunately we don't have a mechanism for 
> enlarging the percpu region (which is why this wasn't done earlier), so we use 
> a heuristic to figure out how much extra percpu region to allocate at boot.
> 
> And 4k makes this one of the Big Pigs in dynamic per-cpu allocations.
> 
> (SNMP mibs are even worse, but that's a separate debate...)

We make a big fuss (rightly) about a few hundred bytes and this sucker
is FOUR KILOBYTES.

Really for the time being I'd rather see this converted to a
num_possible_cpus() sized normal kzalloc() and direct indexing.  I
don't want the networking to bloat up the main kernel image by so
much.

> I can try to implement a bss-like DEFINE_PER_CPU_ZERO(), but it seems silly to 
> talk about tight boot loader size restrictions for SMP kernels.

SMP these days is candy.  And sparc is what has the bootloader restrictions
btw :)
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