linux-kernel - Re: [RFC PATCH] printk: Make functions of pr

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Message-ID: <1488434334.2179.1.camel@perches.com>
Date:   Wed, 01 Mar 2017 21:58:54 -0800
From:   Joe Perches <joe@...ches.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH] printk: Make functions of pr_<level> macros

On Thu, 2017-03-02 at 14:35 +0900, Sergey Senozhatsky wrote:
> Hello Joe,
> 
> On (02/28/17 19:17), Joe Perches wrote:
> > Can save the space that the KERN_<LEVEL> headers require.
> > 
> > The biggest negative here is the %pV use which needs
> > recursion and adds stack depth.
> > 
> > $ size vmlinux.o* (defconfig, x86-64)
> >    text     data     bss      dec     hex  filename
> > 12586135  1909841  777528 15273504  e90e20 vmlinux.o.new
> > 12590348  1909841  777528 15277717  e91e95 vmlinux.o.old
> 
> interesting. 4K.

Yeah, more when more calls are converted,

Maybe more like 12k, still the goal is to 
create singletons for the pr_fmt prefixes
and whatever __func__ uses that are most
common via a SOH + flag and using
__builtin_return_address where possible and
appropriate.  That could shrink another 10k
or so.

> [..]
> > +#define define_pr_func(func, level)			\
> > +asmlinkage __visible int func(const char *fmt, ...)	\
> > +{							\ 
> > +	va_list args;					\
> > +	int r;						\
> > +	struct va_format vaf;				\
> > +							\
> > +	va_start(args, fmt);				\
> > +	vaf.fmt = fmt;					\
> > +	vaf.va = &args;					\
> > +							\
> > +	r = printk(level "%pV", &vaf);			\
> > +							\
> > +	va_end(args);					\
> > +							\
> > +	return r;					\
> > +}							\
> 
> hm. that's really hacky (which is a compliment) and a bit complicated.
> my quick thought was to tweak vprintk_emit() for 'facility != 0' so it
> could get loglevel (and adjust lflags) from the passed level, not from
> the text, and then do something like this


> #define define_pr_func(func, level) asmlinkage __visible int func(const char *fmt, ...)
> {
> 	va_start(args, fmt);
> 	r = vprintk_emit(level[0], level[1], NULL, 0, fmt, args);
> 	va_end();
> }
> 
> but this won't do the trick. because func()->vprintk_emit() shortcut
> disables the printk-safe mechanism:
> 	func()->printk()->vprintk_func()->this_cpu(printk_context)::print()

That was what I had done originally a while ago
https://lkml.org/lkml/2016/6/23/652

Now the with "safe" version, it's a bit more complicated.

[stack depth can be high, ~400 bytes per recursion]

> dunno, at the moment I'm not really comfortable with %pV recursion
> for every pr_foo() call

Me neither really.  I'd much prefer a direct vprintk_emit.