linux-kernel - Re: [PATCH v2] kunit: fix failure to build without printk

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Date:   Fri, 30 Aug 2019 16:37:10 -0700
From:   Brendan Higgins <brendanhiggins@...gle.com>
To:     Tim.Bird@...y.com
Cc:     joe@...ches.com, shuah@...nel.org,
        sergey.senozhatsky.work@...il.com, kunit-dev@...glegroups.com,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        frowand.list@...il.com, sboyd@...nel.org, pmladek@...e.com,
        sergey.senozhatsky@...il.com, rostedt@...dmis.org,
        rdunlap@...radead.org, sfr@...b.auug.org.au
Subject: Re: [PATCH v2] kunit: fix failure to build without printk

On Fri, Aug 30, 2019 at 11:22:43PM +0000, Tim.Bird@...y.com wrote:
> > -----Original Message-----
> > From: Brendan Higgins 
> > 
> > On Fri, Aug 30, 2019 at 3:46 PM Joe Perches <joe@...ches.com> wrote:
> > >
> > > On Fri, 2019-08-30 at 21:58 +0000, Tim.Bird@...y.com wrote:
> > > > > From: Joe Perches
> > > []
> > > > IMHO %pV should be avoided if possible.  Just because people are
> > > > doing it doesn't mean it should be used when it is not necessary.
> > >
> > > Well, as the guy that created %pV, I of course
> > > have a different opinion.
> > >
> > > > >  then wouldn't it be easier to pass in the
> > > > > > kernel level as a separate parameter and then strip off all printk
> > > > > > headers like this:
> > > > >
> > > > > Depends on whether or not you care for overall
> > > > > object size.  Consolidated formats with the
> > > > > embedded KERN_<LEVEL> like suggested are smaller
> > > > > overall object size.
> > > >
> > > > This is an argument I can agree with.  I'm generally in favor of
> > > > things that lessen kernel size creep. :-)
> > >
> > > As am I.
> > 
> > Sorry, to be clear, we are talking about the object size penalty due
> > to adding a single parameter to a function. Is that right?
> 
> Not exactly.  The argument is that pre-pending the different KERN_LEVEL
> strings onto format strings can result in several versions of nearly identical strings
> being compiled into the object file.  By parameterizing this (that is, adding
> '%s' into the format string, and putting the level into the string as an argument),
> it prevents this duplication of format strings.
> 
> I haven't seen the data on duplication of format strings, and how much this
> affects it, but little things can add up.  Whether it matters in this case depends
> on whether the format strings that kunit uses are also used elsewhere in the kernel,
> and whether these same format strings are used with multiple kernel message levels.
>  -- Tim

I thought this portion of the discussion was about whether Joe's version
of kunit_printk was better or my critique of his version of kunit_printk:

Joe's:
> > > > -void kunit_printk(const char *level,
> > > > -		  const struct kunit *test,
> > > > -		  const char *fmt, ...)
> > > > +void kunit_printk(const struct kunit *test, const char *fmt, ...)
> > > >  {
> > > > +	char lvl[PRINTK_MAX_SINGLE_HEADER_LEN + 1] = "\0";
> > > >  	struct va_format vaf;
> > > >  	va_list args;
> > > > +	int kern_level;
> > > >
> > > >  	va_start(args, fmt);
> > > >
> > > > +	while ((kern_level = printk_get_level(fmt)) != 0) {
> > > > +		size_t size = printk_skip_level(fmt) - fmt;
> > > > +
> > > > +		if (kern_level >= '0' && kern_level <= '7') {
> > > > +			memcpy(lvl, fmt,  size);
> > > > +			lvl[size] = '\0';
> > > > +		}
> > > > +		fmt += size;
> > > > +	}
> > > > +
> > > >  	vaf.fmt = fmt;
> > > >  	vaf.va = &args;
> > > >
> > > > -	kunit_vprintk(test, level, &vaf);
> > > > +	printk("%s\t# %s %pV\n", lvl, test->name, &vaf);
> > > >
> > > >  	va_end(args);
> > > >  }

Mine:
>  void kunit_printk(const char *level,
>  		  const struct kunit *test,
>  		  const char *fmt, ...)
>  {
>  	struct va_format vaf;
>  	va_list args;
> 
>  	va_start(args, fmt);
> 
> +	fmt = printk_skip_headers(fmt);
> +
>  	vaf.fmt = fmt;
>  	vaf.va = &args;
> 
> -	kunit_vprintk(test, level, &vaf);
> +	printk("%s\t# %s %pV\n", level, test->name, &vaf);
> 
>  	va_end(args);
>  }

I thought you and Joe were arguing that "Joe's" resulted in a smaller
object size than "Mine" (not to be confused with the actual patch I
presented here, which is what Sergey suggested I do on a different
thread).

I really don't feel strongly about what Sergey suggested I do (which is
what this patch originally introduced), versus, what Joe suggested,
versus what I suggested in response to Joe (or any of the things
suggested on other threads). I just want to pick one, fix the breakage
in linux-next, and move on with my life.

Cheers