linux-kernel - Re: [PATCH 10/10] powerpc: select DYNAMIC_DEBUG_RELATIVE

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Date:   Wed, 24 Apr 2019 08:46:40 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Christophe Leroy <christophe.leroy@....fr>
Cc:     linuxppc-dev@...ts.ozlabs.org, Jason Baron <jbaron@...mai.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/10] powerpc: select DYNAMIC_DEBUG_RELATIVE_POINTERS for
 PPC64

On 23/04/2019 21.36, Andrew Morton wrote:
> On Tue, 23 Apr 2019 17:37:33 +0200 Christophe Leroy <christophe.leroy@....fr> wrote:
> 
>>> --- a/arch/powerpc/Kconfig
>>> +++ b/arch/powerpc/Kconfig
>>> @@ -155,6 +155,7 @@ config PPC
>>>   	select BUILDTIME_EXTABLE_SORT
>>>   	select CLONE_BACKWARDS
>>>   	select DCACHE_WORD_ACCESS		if PPC64 && CPU_LITTLE_ENDIAN
>>> +	select DYNAMIC_DEBUG_RELATIVE_POINTERS	if PPC64
>>
>> Why only PPC64 ? Won't it work with PPC32 ? Or would it be 
>> counter-performant on 32 bits ?
> 
> Ditto arm and i386?
> 

It's pointless on 32-bit platforms - I'm replacing absolute const char*
pointers with a relative s32 offset from the _ddebug descriptor, so if
sizeof(void*)==4 there's no gain.

And yes, the current implementation also wouldn't work out-of-the-box
for 32-bit platforms, since the asm needs to know how to properly
initialize a whole struct _ddebug, which (often) contains a static_key,
which in turn contains a pointer member, which both affects its size as
well as its placement inside _ddebug. The C code in dynamic_debug.c
would likely Just Work, but there's no point in complicating the asm
part for no gain, so there are static_assert()s in place to ensure
BITS_PER_LONG==64 (as well as checking the various offsetof()s etc.).

[I don't think performance matters at all, it's one extra addition to
access these fields, and that is only done in the rare cases where
someone interacts with the dynamic_debug/control sysfs file, and when
one of the activated pr_debug()s is actually hit (so a few extra
instructions drown in the printk overhead).]

I do now see that PPC64 does not select GENERIC_BUG_RELATIVE_POINTERS,
so maybe this scheme simply doesn't work on PPC64, or nobody has done
the work to reduce the sizeof(struct bug_entry) on PPC64? As I said,
I've only compile-tested arm64 and ppc64.

Rasmus