linux-kernel - Re: [PATCH 1/2]percpu: introduce read mostly percpu API

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Message-ID: <4CBF603D.5040808@zytor.com>
Date:	Wed, 20 Oct 2010 14:33:49 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Shaohua Li <shaohua.li@...el.com>,
	lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <andi@...stfloor.org>,
	"Chen, Tim C" <tim.c.chen@...el.com>
Subject: Re: [PATCH 1/2]percpu: introduce read mostly percpu API

On 10/19/2010 10:18 PM, Eric Dumazet wrote:
> 
> We can see many holes because of 2^5 alignments of
> individual .o  .data..percpu sections.
> 
> find . -name "*.o"|xargs objdump -h|grep percpu
> 
> Linker promotes a section alignment from natural alignment to 2^5 as
> soon as the size reaches 2^5
> 
> For example in net/ipv4/route.o, we have a per_cpu structure
> (rt_cache_stat), that is an array of 16 integers. The natural alignement
> should be 4 (alignof(int)), but we get :
> 
> # objdump -h net/ipv4/route.o|grep percpu
>  19 .data..percpu 00000040  0000000000000000  0000000000000000  00007a80  2**5
> 
> For a section replicated N times, this really is a concern.
> 

That wouldn't be the linker, that would be the compiler or assembler
-- I suspect it's the compiler -- and that needs to be fixed.

To reduce linker-induced padding, we may want to use SORT_BY_ALIGNMENT()
in the linker script.

	-hpa
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