linux-kernel - Re: linux.git: printk() problem

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Date:   Tue, 25 Oct 2016 10:42:56 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: linux.git: printk() problem

On Sun, Oct 23, 2016 at 11:11:18AM -0700, Linus Torvalds wrote:
> 
> For example, one of the really historical uses for partial lines is this:
> 
>        pr_info("Checking 'hlt' instruction... ");
> 
>        if (!boot_cpu_data.hlt_works_ok) {
>                pr_cont("disabled\n");
>                return;
>        }
>        halt();
>        halt();
>        halt();
>        halt();
>        pr_cont("OK\n");
> 
> and the point was that there used to be some really old i386 machines
> that hung on the "hlt" instruction (probably not because of a CPU bug,
> but because of either power supply issues or some DMA issues).
> 
> To support that, we really *had* to print out the continuation lines
> even when they were partial. And that complicates the printk logic a
> lot.

Note, my ftrace start up tests still does exactly this (e.g.):

	pr_info("Testing dynamic ftrace ops: #%d: ", cnt);

	[ do lots of testing ]

	printk(KERN_CONT "PASSED\n");

Previously a change was made to buffer lines without \n, and I wasted an
entire day debugging why a crash happened because I was looking at the wrong
test.

-- Steve

> 
> Now, that "hlt" case is long long gone, and maybe we should just say
> "screw that". It would be really quite easy to say "we don't print out
> continuation lines immediately, we just buffer them for 0.1s instead,
> and KERN_CONT only works for things that really happen more or less
> immediately".
> 
> Maybe that really is the right answer. Because the original cause of
> us having to bend over backwards in this case is really no longer
> there. And it would simplify printk a *lot*.
> 
> Let me whip up a minimal patch for you to try.