Re: [PATCH v6 1/3] printk: Add line-buffered printk() API.

[Sergey Senozhatsky]

On (11/02/18 22:31), Tetsuo Handa wrote:
>   (1) Call get_printk_buffer() and acquire "struct printk_buffer *".
> 
>   (2) Rewrite printk() calls in the following way. The "ptr" is
>       "struct printk_buffer *" obtained in step (1).
> 
>       printk(fmt, ...)     => printk_buffered(ptr, fmt, ...)
>       vprintk(fmt, args)   => vprintk_buffered(ptr, fmt, args)
>       pr_emerg(fmt, ...)   => bpr_emerg(ptr, fmt, ...)
>       pr_alert(fmt, ...)   => bpr_alert(ptr, fmt, ...)
>       pr_crit(fmt, ...)    => bpr_crit(ptr, fmt, ...)
>       pr_err(fmt, ...)     => bpr_err(ptr, fmt, ...)
>       pr_warning(fmt, ...) => bpr_warning(ptr, fmt, ...)
>       pr_warn(fmt, ...)    => bpr_warn(ptr, fmt, ...)
>       pr_notice(fmt, ...)  => bpr_notice(ptr, fmt, ...)
>       pr_info(fmt, ...)    => bpr_info(ptr, fmt, ...)
>       pr_cont(fmt, ...)    => bpr_cont(ptr, fmt, ...)
> 
>   (3) Release "struct printk_buffer" by calling put_printk_buffer().

[..]

> Since we want to remove "struct cont" eventually, we will try to remove
> both "implicit printk() users who are expecting KERN_CONT behavior" and
> "explicit pr_cont()/printk(KERN_CONT) users". Therefore, converting to
> this API is recommended.

- The printk-fallback sounds like a hint that the existing 'cont' handling
  better stay in the kernel. I don't see how the existing 'cont' is
  significantly worse than
		bpr_warn(NULL, ...)->printk() // no 'cont' support
  I don't see why would we want to do it, sorry. I don't see "it takes 16
  printk-buffers to make a thing go right" as a sure thing.

A question.

How bad would it actually be to:

- Allocate seq_buf 512-bytes buffer (GFP_ATOMIC) just-in-time, when we
  need it.
    // How often systems cannot allocate a 512-byte buffer? //

- OK, assuming that systems around the world are so badly OOM like all the
  time and even kmalloc(512) is absolutely impossible, then have a fallback
  to the existing 'cont' handling; it just looks to me better than a plain
  printk()-fallback with removed 'cont' support.

- Do not allocate seq_buf if we are in printk-safe or in printk-nmi mode.
  To avoid "buffering for the sake of buffering". IOW, when in printk-safe
  use printk-safe.

	-ss