Re: printk meeting at LPC

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
To: John Ogness <john.ogness@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Petr Mladek <pmladek@suse.com>,
	Andrea Parri <parri.andrea@gmail.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Brendan Higgins <brendanhiggins@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Theodore Ts'o" <tytso@mit.edu>, Paul Turner <pjt@google.com>,
	Daniel Vetter <daniel.vetter@ffwll.ch>,
	Prarit Bhargava <prarit@redhat.com>
Subject: Re: printk meeting at LPC
Date: Mon, 16 Sep 2019 13:30:17 +0900	[thread overview]
Message-ID: <cfc7b1fa-e629-19a6-154b-0dd4f5604aa7@I-love.SAKURA.ne.jp> (raw)
In-Reply-To: <87k1acz5rx.fsf@linutronix.de>

On 2019/09/13 22:26, John Ogness wrote:
> 6. A new may-sleep function pr_flush() will be made available to wait
> for all previously printk'd messages to be output on all consoles before
> proceeding. For example:
> 
>     pr_cont("Running test ABC... ");
>     pr_flush();
> 
>     do_test();
> 
>     pr_cont("PASSED\n");
>     pr_flush();

Don't we need to allow printk() callers to know the sequence number which
the printk() has queued? Something like

  u64 seq;
  pr_info(...);
  pr_info(...);
  pr_info(...);
  seq = pr_current_seq();
  pr_wait_seq(seq);

in case concurrently executed printk() flooding keeps adding a lot of
pending output?

By the way, do we need to keep printk() return bytes like printf() ?
Maybe we can make printk() return "void", for almost nobody can do
meaningful things with the return value.

> 9. Support for printk dictionaries will be discontinued. I will look
> into who is using this and why. If printk dictionaries are important for
> you, speak up now!

I think that dev_printk() is using "const char *dict, size_t dictlen," part
via create_syslog_header(). Some userspace programs might depend on
availability of such information.