linux-kernel - Re: printk: what is going on with additional newlines?

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Date:   Mon, 4 Sep 2017 14:22:46 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Joe Perches <joe@...ches.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Pavel Machek <pavel@....cz>, Petr Mladek <pmladek@...e.com>,
        Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jiri Slaby <jslaby@...e.com>, Andreas Mohr <andi@...as.de>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: printk: what is going on with additional newlines?

Hello,

I'll answer to both Linus and Joe, just to keep it all one place.

On (09/01/17 13:21), Linus Torvalds wrote:
> On Fri, Sep 1, 2017 at 10:32 AM, Joe Perches <joe@...ches.com> wrote:
> >
> > Yes, it's a poor name.  At least keep using a pr_ prefix.
> 
> I'd suggest perhaps just "pr_line()".

ok, pr_line sound good.

> And instead of having those "err/info/cont" variations, the severity
> level should just be set at initialization time.  Not different
> versions of "pr_line()".
> 
> There's no point to having different severity variations, since the
> *only* reason for this would be for buffering. So "pr_cont()" is kind
> of assumed for everything but the first.
> 
> And even if you end up doing multiple lines, if you actually do
> different severities, you damn well shouldn't buffer them together.
> They are clearly different things!

hm... may be.
the main point of prbuf is not the support of cont lines, but the
fact that buffered messages are added to the logbuf atomically,
and thus are printed in consequent lines, not the usual way:

CPU0	because
CPU1	this
CPU0	it's easier
CPU1	might
CPU0	to read
CPU1	be
CPU1	inconvenient.
CPU0	seq messages.

some people want to be able to make it to look less spaghetti-like:

CPU0	because
CPU0	it's easier
CPU0	to read
CPU0	seq messages.
CPU1	this
CPU1	might
CPU1	be
CPU1	inconvenient.

and it's not something completely wrong to ask for, I think.
well, who knows.

there is only way to serialize printks against other printks -- take
the logbuf lock. and that's what pr_line/prbuf flush is doing.

now, pr_line/prbuf/pr_buf is, of course, very limited. it should NOT
be used for things that are really important and absolutely must [if
possible] appear in serial logs/on screens/etc. simply because panic
can happen on CPUA before we flush any pending pr_line/prbuf buffers
on other CPUs. and that's exactly the reason why I initially wanted
(and still do) to implement pr_line/prbuf using printk-safe
mechanism - because we flush printk-safe buffers from panic(). so
utilizing printk-safe buffers can make pr_line less fragile. apart
from that printk-safe buffers are always there, so OOM is not a show
stopper anymore. but, like I said in another email, printk-safe buffer
is per-CPU and is also used for actual printk-safe, hence it must be
used with local IRQs disabled when we "borrow" the buffer for pr_line
(disabled preemption is not enough due to possible IRQ printk-safe
print out). this can be a bit annoying.

in it's current form, pr_line/pr_buf is NOT a replacement for pr_cont
or printk(KERN_CONT). because pr_cont has no such thing as "we were
unable to flush the buffer from CPUB because of panic on CPUA". so
pr_cont/printk(KERN_CONT) beats pr_line/pr_buf here. This can be a
major limitation. am I wrong?

another thing,
if we eventually will decide to stick to "use a seq_buf with stack
allocated char buffer to hold a single line only" design, then I'm
not entirely sure I get it why do we want to add a new API at all.
I mean, the new API does not make anything simpler or shorter. we
need to declare the buffer, seq_buf, init seq_buf, append chars to
seq_buf, flush it:

	char buf[80];
	struct seq_buf cont_line;

	pr_line_init(&cont_line, buf, sizeof(buf));
	pr_line_printf(&cont_line, "....");
	pr_line_printf(&cont_line, "....");
	pr_line_flush(&cont_line);

this asks for  s/pr_line_/seq_buf_/g, no? well, except for the flush()
part. it can be replaced with printk("%s\n", cont_line->buffer).

so it seems that we need to re-think it.

	-ss