linux-kernel - Re: [PATCH] module: ppc64 module CRC relocation fix causes perf issues

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Date:	Mon, 15 Jul 2013 14:09:49 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Anton Blanchard <anton@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Neil Horman <nhorman@...driver.com>
Cc:	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] module: ppc64 module CRC relocation fix causes perf issues

Anton Blanchard <anton@...ba.org> writes:
> Module CRCs are implemented as absolute symbols that get resolved by
> a linker script. We build an intermediate .o that contains an
> unresolved symbol for each CRC. genksysms parses this .o, calculates
> the CRCs and writes a linker script that "resolves" the symbols to
> the calculated CRC.
>
> Unfortunately the ppc64 relocatable kernel sees these CRCs as symbols
> that need relocating and relocates them at boot. Commit d4703aef
> (module: handle ppc64 relocating kcrctabs when CONFIG_RELOCATABLE=y)
> added a hook to reverse the bogus relocations. Part of this patch
> created a symbol at 0x0:
>
> # head -2 /proc/kallsyms 
> 0000000000000000 T reloc_start
> c000000000000000 T .__start
>
> This reloc_start symbol is causing lots of confusion to perf. It
> thinks reloc_start is a massive function that stretches from 0x0 to
> 0xc000000000000000 and we get various cryptic errors out of perf,
> including:
>
> problem incrementing symbol count, skipping event
>
> This patch removes the  reloc_start linker script label and instead
> defines it as PHYSICAL_START. We also need to wrap it with
> CONFIG_PPC64 because the ppc32 kernel can set a non zero
> PHYSICAL_START at compile time and we wouldn't want to subtract
> it from the CRCs in that case.
>
> Signed-off-by: Anton Blanchard <anton@...ba.org>
> Cc: <stable@...nel.org>

Acked-by: Rusty Russell <rusty@...tcorp.com.au>

Ben?

Cheers,
Rusty.
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