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

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Date:   Thu, 27 Apr 2017 17:11:21 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.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 Wed 2017-03-01 21:58:54, Joe Perches wrote:
> 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

BTW: The above mentioned commit adds one argument to
vprintk_default(). But the symbol is exported. I am
not sure if we could break the API.

We might need to create alternative vprintk_* functions
(called vlprintk_* or so) that would have the extra parameter.
And call them from the existing vprintk_* ones. Sigh.

Also note that we might need to pass more information
via the extra parameter, for example, KERN_ERR + KERN_CONT.

> 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

Same here. We should avoid the recursion.

Best Regards,
Petr