linux-kernel - Re: printk meeting at LPC

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Date:   Mon, 16 Sep 2019 13:30:17 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Theodore Ts'o" <tytso@....edu>, Paul Turner <pjt@...gle.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Prarit Bhargava <prarit@...hat.com>
Subject: Re: printk meeting at LPC

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.