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

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Date:   Tue, 6 Nov 2018 23:35:02 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexander Potapenko <glider@...gle.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v6 1/3] printk: Add line-buffered printk() API.

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