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Subject: Re: [PATCH] printk: inject caller information into the body of
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To:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>
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From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
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Date:   Fri, 14 Sep 2018 19:37:21 +0900
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On 2018/09/14 15:57, Sergey Senozhatsky wrote:
> On (09/13/18 23:28), Sergey Senozhatsky wrote:
>> Not that I see any problems with pr_line_flush(). But can drop it, sure.
>> pr_line() is a replacement for pr_cont() and as such it's not for multi-line
>> buffering.
> 
> OK, attached.
> Let me know if anything needs to improved (including broken English).
> Will we keep in the printk tree or shall I send a formal patch to Andrew?


> @@ -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().


> +/**
> + * DEFINE_PR_LINE - define a new pr_line variable
> + * @lev:	printk() log level
> + * @name:	variable name
> + *
> + * Defines a new pr_line varialbe, which would use an implicit

s/varialbe/variable/ .

> + * stack buffer of size __PR_LINE_BUF_SZ.
> + */
> +#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" ?


> +/**
> + * DEFINE_PR_LINE_BUF - define a new pr_line variable
> + * @lev:	printk() log level
> + * @name:	variable name
> + * @buf:	external buffer
> + * @sz:	external buffer size
> + *
> + * Defines a new pr_line variable, which would use an external
> + * buffer for printk line.
> + */
> +#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.

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


> @@ -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() ?

> @@ -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/ ?

> + * @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/ ?