linux-kernel - Re: [PATCH v3 7/7] dynamic_debug: add jump label support

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Date:	Mon, 11 Jul 2016 15:52:26 -0400
From:	Jason Baron <jbaron@...mai.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	<joe@...ches.com>, <peterz@...radead.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 7/7] dynamic_debug: add jump label support

On 07/11/2016 03:23 PM, Andrew Morton wrote:
> On Mon, 11 Jul 2016 09:18:21 -0400 Jason Baron <jbaron@...mai.com> wrote:
>
>> On 07/08/2016 05:41 PM, Andrew Morton wrote:
>>> On Wed,  6 Jul 2016 17:42:36 -0400 Jason Baron <jbaron@...mai.com> wrote:
>>>
>>>> Although dynamic debug is often only used for debug builds, sometimes its
>>>> enabled for production builds as well. Minimize its impact by using jump
>>>> labels. This reduces the text section by 7000+ bytes in the kernel image
>>>> below. It does increase data, but this should only be referenced when
>>>> changing the direction of the branches, and hence usually not in cache.
>>>>
>>>> ...
>>>>
>>>> +#ifdef HAVE_JUMP_LABEL
>>>> +
>>>> +#define dd_key_init(key, init) key = (init)
>>>>
>>>> ...
>>>>
>>>> +#else
>>>> +
>>>> +#define dd_key_init(key, init)
>>>> +
>>> umm, lazy.  One is an lval and returns a value and the other does
>>> neither.  Lack of parenthesization in the first version doubtless
>>> exposes various horrors.
>>>
>>> Care to do something more robust and conventional here?  Presumably use
>>> symmetrical do{}while(0) things, neither of which is an lval, both of
>>> which don't return anything.
>>>
>> Hi,
>>
>> The 'dd_key_init()' macro is being used here to help initialize
>> the 'key' field in the 'struct _ddebug', and its not being used as a
>> statement.
>>
>> In the 'HAVE_JUMP_LABEL' case, we are initializing the 'key' field, while in
>> the not-'HAVE_JUMP_LABEL' case, the 'key' field is simply not present
>> in the structure (to conserve space).
> Well yeah.  And it's doing it wrongly, isn't it?
>
> : @@ -68,13 +78,51 @@ void __dynamic_netdev_dbg(struct _ddebug
> :  		.filename = __FILE__,				\
> :  		.format = (fmt),				\
> :  		.lineno = __LINE__,				\
> : -		.flags =  _DPRINTK_FLAGS_DEFAULT,		\
> : +		.flags = _DPRINTK_FLAGS_DEFAULT,		\
> : +		dd_key_init(key, init)				\
> :  	}
> :
> : +#ifdef HAVE_JUMP_LABEL
> : +
> : +#define dd_key_init(key, init) key = (init)
>
> Shouldn't it be ".key = (init)"?
>
> Anyway, it's odd-looking.  I guess something like
>
> #define DD_KEY_INIT(init)	.key = (init)
>
> would be more idiomatic.

Ok, so the 'key' field is a union and so the patch is effectively
calling (after substitution):

dd_key_init(.key.dd_key_true, STATIC_KEY_TRUE_INIT)

and:

dd_key_init(.key.dd_key_true, STATIC_KEY_FALSE_INIT)

So we could have variations such as:

#define dd_key_true_init() .key.dd_key_true = (STATIC_KEY_TRUE_INIT)
#define dd_key_false_init()

and

#define dd_key_true_init()
#define dd_key_false_init() .key.dd_key_false = (STATIC_KEY_FALSE_INIT)

and finally:

#define dd_key_true_init()
#define dd_key_false_init()

and then have both dd_key_true_init()  and dd_key_fase_init() in
the structure definition. It adds a bunch more definitions and I'm
not sure if its more readable, but maybe it would look cleaner?

Thanks,

-Jason