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

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Date:   Tue, 5 Sep 2017 22:42:28 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Joe Perches <joe@...ches.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Pavel Machek <pavel@....cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.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?

On (09/05/17 14:21), Petr Mladek wrote:
[..]
> > that's why I want buffered printk to re-use the printk-safe buffer
> > on that particular CPU [ if buffered printk will ever land ].
> > printk-safe buffer is not allocated on stack, or kmalloc-ed for
> > temp usafe, and, more importantly, we flush it from panic().
> > 
> > and I'm not sure that lost messages due to missing panic flush()
> > can really be an option even for a single cont line buffer. well,
> > may be it can. printk has a sort of guarantee that messages will
> > be at some well known location when pr_foo or printk function
> > returns. buffered printk kills it. and I don't want to have
> > several "flavors" of printk. printk-safe buffer seems to be the
> > way to preserve that guarantee.
> 
> But the well known locations would help only when they are flushed
> in panic() or when a crashdump is created. They do not help
> in other cases, especially where there is a sudden death.

if the system locked up and there is no panic()->flush_on_panic(),
no console_unlock(), crashdump, no nothing - then even having
messages in the logbuf is probably not really helpful. you can't
reach them anyway :)
so yes, I'm speaking here about the cases when we flush_on_panic()
or/and generate crash dump.


> There are many fears that printk offloading does not have enough
> guarantees to actually happen. IMHO, there must be similar fears
> that the messages in a temporary buffer will never get flushed.
> 
> And there are more risks with this approach:
> 
>   + soft-lockups caused by disabled preemption; we would
>     need this to stay on the same CPU and use the same buffer

well, yes. like any control path that disables IRQs there are
rules to follow. so printk-safe based solution has limitations.
I mentioned them probably every time I speak about printk-safe
buffering. but those limitations come with a bonus - flush on
panic and well known location of the messages.

one thing to notice, is that
printk-safe is usually faster than printk() or at least as fast as
the fastest printk() path. because, unlike printk, it does not take
spin on the logbuf lock; it does not console_trylock(), it does not
do console_unlock().


>   + broken preempt-count and missing message when one forgets
>     to close the buffered section or do it twice

yes, coding errors are possible.


>   + lost messages because a per-CPU buffer size limitations

which is true for any type of buffers. including logbuf. and
stack allocated buffers, any buffer. printk-safe buffer is at
least much-much bigger than any stack allocated buffer.


>   + races in printk_safe() that is not recursions safe
>
>   + not to say the problems mentioned by Linus as reply
>     to the Tetsuo's proposal, see
> https://lkml.kernel.org/r/CA+55aFx+5R-vFQfr7+Ok9Yrs2adQ2Ma4fz+S6nCyWHY_-2mrmw@mail.gmail.com

like "limited in where you can actually expect buffering to happen"?

sure. it does not come for free, it's not all beautiful and shiny.


[..]
> I wonder if all this is worth the effort, complexity, and risk.
> We are talking about cosmetic problems after all.

the thing about printk-safe buffering is that _mostly_ everything
is already in the kernel. especially if we talk about single cont
line buffering. just add public API printk_buffering_begin() and
printk_buffering_end() that will __printk_safe_enter() and
__printk_safe_exit(). and that's it. unless I'm missing something.

but I'm not super eager to have printk-safe based buffering.
that's why I never posted a patch set. this approach has its
limitations.


> Well, what do you think about the extra printed information?
> For example:
> 
>     <timestamp> <PID> <context> message
> 
> It looks straightforward to me. These information
> might be helpful on its own. So, it might be a
> win-win solution.

hm... don't know. frankly, I never found PID useful. I mostly look
at the serial logs postmortem. so lines
	12231 foo
	21331 bar

are not much better than just
	foo
	bar


I prepend every line with the CPU number that has printk()-ed it.
and that's helpful because one can grep and filter out messages
from other CPUs. it's quite OK thing to have given that messages
can be really mixed sometimes.

so adding extra information to `struct printk_log' could be helpful.
I think we had this discussion before and you didn't want to change
the size of `struct printk_log' because that might break gdb/crash/etc
user space tools. has it changed?

may be we can #ifdef CONFIG_PRINTK_ABC them.

	-ss