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From:   Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Date:   Fri, 14 Sep 2018 20:50:28 +0900
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Alexander Potapenko <glider@google.com>,
        Dmitriy Vyukov <dvyukov@google.com>,
        kbuild test robot <fengguang.wu@intel.com>,
        syzkaller <syzkaller@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [PATCH] printk: inject caller information into the body of
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On (09/14/18 19:37), Tetsuo Handa wrote:
> > @@ -20,6 +20,9 @@
> >   * Annotation for a "continued" line of log printout (only done after a
> >   * line that had no enclosing \n). Only to be used by core/arch code
> >   * during early bootup (a continued line is not SMP-safe otherwise).
> > + *
> > + * Please consider pr_line()/vpr_line() functions for SMP-safe continued
> > + * line printing.
> 
> I think the advantage is not limited to SMP-safeness. Reducing the frequency of
> calling printk() will reduce overhead. Also, latency for netconsole will be
> reduced by sending a whole line in one printk().

Hmm. These are very good points, indeed. But do we want to list all
advantages here? I just wanted to mention SMP-unsafe pr_cont/printk(KERN_CONT),
because I also mention pr_line in kern_levels.h.

> > + * Defines a new pr_line varialbe, which would use an implicit
> 
> s/varialbe/variable/ .

Thanks.

> > +#define DEFINE_PR_LINE(lev, name)				\
> > +	char		__line_##name[__PR_LINE_BUF_SZ];	\
> > +	struct pr_line	name = {				\
> > +		.sb	= __SEQ_BUF_INITIALIZER(__line_##name,	\
> > +					__PR_LINE_BUF_SZ),	\
> > +		.level	= lev,					\
> > +	}
> 
> Want a note that
> 
>   static DEFINE_PR_LINE(lev, name);
> 
> won't make "name" variable "static" ?

Interesting point. Any hint what the comment should look like?
Do we want to have static pr_line buffers?

> > +#define DEFINE_PR_LINE_BUF(lev, name, buf, sz)			\
> > +	struct pr_line	name = {				\
> > +		.sb	= __SEQ_BUF_INITIALIZER(buf, (sz)),	\
> > +		.level	= lev,					\
> > +	}
> > +
> 
> I would use this one for the OOM killer. 80 bytes is too short.

80 bytes is quite short for OOM, agreed.

>   static char oom_print_buf[1024];
>   DEFINE_PR_LINE_BUF(level, oom_print_buf);

Do I get it right that you suggest to drop the "size" param?
Do OOM people agree on 1024 bytes stack usage?


> > @@ -131,4 +187,8 @@ extern int
> >  seq_buf_bprintf(struct seq_buf *s, const char *fmt, const u32 *binary);
> >  #endif
> >  
> > +extern __printf(2, 0)
> > +int vpr_line(struct pr_line *pl, const char *fmt, va_list args);
> > +extern __printf(2, 3)
> > +int pr_line(struct pr_line *pl, const char *fmt, ...);
> 
> Do we want to mark "asmlinkage" like printk() ?

Dunno, do we? Does code written in assembly call pr_cont that often?
We are not turning pr_line() into syscall anyway.

> > @@ -324,3 +324,60 @@ int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, int cnt)
> >  	s->readpos += cnt;
> >  	return cnt;
> >  }
> > +
> > +/**
> > + * vpr_line - Append data to the printk() line buffer
> > + * @pl: the pr_line descriptor
> 
> s/descriptor/structure/ ?

Yeah, I used the term "descriptor", just because it's used in seq_buf.c.
So, it's sort of common in seq_buf.
E.g.
   seq_buf_vprintf(), seq_buf_print_seq(), seq_buf_can_fit() and so on.

> > + * @fmt: printf format string
> > + * @args: va_list of arguments from a printf() type function
> > + *
> > + * Writes a vnprintf() format into the printk() pr_line buffer.
> 
> s/vnprintf/vprintf/ ?

Indeed.
We also need to fix a typo in seq_buf_vprintf() comment then.

	-ss