From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Andrew Morton <akpm@linux-foundation.org>,
Christophe Leroy <christophe.leroy@c-s.fr>
Cc: linuxppc-dev@lists.ozlabs.org, Jason Baron <jbaron@akamai.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 10/10] powerpc: select DYNAMIC_DEBUG_RELATIVE_POINTERS for PPC64
Date: Wed, 24 Apr 2019 08:46:40 +0200 [thread overview]
Message-ID: <0fbffd5d-7e2a-1b12-5c94-b75f3f5d7ac1@rasmusvillemoes.dk> (raw)
In-Reply-To: <20190423123611.1f7276cc4eb9f7a7005899a9@linux-foundation.org>
On 23/04/2019 21.36, Andrew Morton wrote:
> On Tue, 23 Apr 2019 17:37:33 +0200 Christophe Leroy <christophe.leroy@c-s.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
next prev parent reply other threads:[~2019-04-24 6:46 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-09 21:25 [PATCH 00/10] implement DYNAMIC_DEBUG_RELATIVE_POINTERS Rasmus Villemoes
2019-04-09 21:25 ` [PATCH 01/10] linux/device.h: use unique identifier for each struct _ddebug Rasmus Villemoes
2019-04-09 21:25 ` [PATCH 02/10] linux/net.h: " Rasmus Villemoes
2019-04-09 21:25 ` [PATCH 03/10] linux/printk.h: " Rasmus Villemoes
2019-04-09 21:25 ` [PATCH 04/10] dynamic_debug: introduce accessors for string members of " Rasmus Villemoes
2019-04-09 21:25 ` [PATCH 05/10] dynamic_debug: drop use of bitfields in " Rasmus Villemoes
2019-04-09 21:25 ` [PATCH 06/10] dynamic_debug: introduce CONFIG_DYNAMIC_DEBUG_RELATIVE_POINTERS Rasmus Villemoes
2019-04-09 21:25 ` [PATCH 07/10] dynamic_debug: add asm-generic implementation for DYNAMIC_DEBUG_RELATIVE_POINTERS Rasmus Villemoes
2019-04-09 21:25 ` [PATCH 08/10] x86-64: select DYNAMIC_DEBUG_RELATIVE_POINTERS Rasmus Villemoes
2019-04-10 7:31 ` Ingo Molnar
2019-04-09 21:25 ` [PATCH 09/10] arm64: " Rasmus Villemoes
2019-04-26 9:39 ` Arnd Bergmann
2019-04-26 10:05 ` Rasmus Villemoes
2019-04-26 13:00 ` Nathan Chancellor
2019-04-26 19:06 ` [PATCH 11/10] arm64: unbreak DYNAMIC_DEBUG=y build with clang Rasmus Villemoes
2019-04-26 19:06 ` [PATCH 12/10] powerpc: " Rasmus Villemoes
2019-04-29 17:34 ` Nick Desaulniers
2019-04-26 19:27 ` [PATCH 11/10] arm64: " Rasmus Villemoes
2019-04-26 21:58 ` Konstantin Ryabitsev
2019-04-26 22:07 ` Konstantin Ryabitsev
2019-04-29 17:32 ` Nick Desaulniers
2019-04-30 18:22 ` Nick Desaulniers
2019-05-02 8:57 ` Rasmus Villemoes
2019-04-09 21:25 ` [PATCH 10/10] powerpc: select DYNAMIC_DEBUG_RELATIVE_POINTERS for PPC64 Rasmus Villemoes
2019-04-23 15:37 ` Christophe Leroy
2019-04-23 19:36 ` Andrew Morton
2019-04-24 6:46 ` Rasmus Villemoes [this message]
2019-05-06 6:48 ` [PATCH 00/10] implement DYNAMIC_DEBUG_RELATIVE_POINTERS Rasmus Villemoes
2019-05-06 7:05 ` Ingo Molnar
2019-05-06 7:34 ` Rasmus Villemoes
2019-05-06 7:48 ` Ingo Molnar
2019-05-06 14:48 ` Segher Boessenkool
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