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From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Dmitriy Vyukov <dvyukov@google.com>,
	Alexander Potapenko <glider@google.com>,
	Fengguang Wu <fengguang.wu@intel.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep messages.
Date: Mon, 26 Nov 2018 13:34:45 +0900	[thread overview]
Message-ID: <20181126043445.GB540@jagdpanzerIV> (raw)
In-Reply-To: <6422717f-db27-8ba8-1183-ccb9f0400fc3@i-love.sakura.ne.jp>

On (11/24/18 09:24), Tetsuo Handa wrote:
> >> Steven told me on Plumbers conference that even few initial
> >> characters saved him a day few times.
> > 
> > Yes, and that has happened more than once. I would reboot and retest
> > code that is crashing, and due to a triple fault, the machine would
> > reboot because of some race, and the little output I get from the
> > console would help tremendously.
> > 
> > Remember, debugging the kernel is a lot like forensics, especially when
> > it's from a customer's site. You look at all the evidence that you can
> > get, and sometimes it's just 10 characters in the output that gives you
> > an idea of where things went wrong. I'm really not liking the buffering
> > idea because of this.
> 
> Then, we should not enforce buffering in a way that requires modification of
> printk() callers. That is, we should not ask printk() callers to use their
> private buffer. What we can do is to enable/disable line buffering inside
> printk() depending on the problem the user wants to debug.

Right; overall I tend to agree with what you guys are saying and
I like Petr's "I am more and more wondering if the buffered printk
is worth the effort" comment; and I like Steven's comment on flushes;
and admire Tetsuo's efforts.

I think that printk_seq_buf/printk_buffer was never going to replace
pr_cont() and I never liked the idea. The printk_safe proposal for
lockdep had one OK thing about it - it would pass our normal marshaling
before it would reach the buffering stage. Which means - no buffering
for people who "detest" printk buffering.

This looks better:
	printk->vprint_func->{early_printk/printk_safe/vprintk_emit}->buffering

Than this:
	pr_buffer->buffering->vprintk_func->{early_printk/printk_safe/vprintk_emit}

Another thing is - printk seq_buf/printk_buffer doesn't really solve any
problem. People, who can use seq_buf/char buf[256]/etc. buffering, already
can do so; people who cannot - won't switch to a new buffering printk
anyway.

The bad thing about printk_safe proposal is that it's per-CPU; which
is OK for some paths (like lockdep), but not OK in general (e.g. OOM).

IMO, try_buffered_printk() attempts to solve the problem at the right
place - printk. And it does not break our normal marshaling, so we don't
"fix" printk users and we keep people, who does vprintk_func->early_printk
thing, happy. So I don't dislike try_buffered_printk() approach. And unlike
before, now we are talking about a single line buffering.

If we'd walk this way, I would prefer to NOT introduce any structs and
any new code, or any new "split and log_store() in the middle" rules. Just
a bunch of "struct cont" buffers:

	static struct cont conts[N];

and cont_add()/cont_flush() to handle pr_cont, with all the flushes it
does; but on a per-context basis.
conts[0] should serve as a fallback cont buffer, in case if there are
no available cont buffers left. flush_on_panic() is still miserable,
for sure; probably we can do something about it.

Or... Instead.
We can just leave pr_cont() alone for now. And make it possible to
reconstruct messages - IOW, inject some info to printk messages. We
do this at Samsung (inject CPU number at the beginning of every
message. `cat serial.0 | grep "\[1\]"` to grep for all messages from
CPU1). Probably this would be the simplest thing.

> Also, we should allow disabling "struct cont" depending on the problem (in
> order to allow flushing the 10 characters in the "cont" buffer).
>
> By the way, is the comment
>
>   /*
>    * Continuation lines are buffered, and not committed to the record buffer
>    * until the line is complete, or a race forces it. The line fragments
>    * though, are printed immediately to the consoles to ensure everything has
>    * reached the console in case of a kernel crash.
>    */

printk does not do this anymore; you are right.

> appropriate despite we don't call cont_flush() upon a kernel crash?

I tend to count on flush_on_panic more than on a "last moment"
pr_cont->cont_flush(), which was guaranteed to happen immediately
only with early_con.

A kernel crash usually has enough pr_emerg/printk-s to force cont flush.
Even pr_info() will do. We look at the loglevel much later; so even
messages which never make it to the consoles still flush cont buffer.

	-ss

  reply	other threads:[~2018-11-26  4:34 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-02 13:31 [PATCH v6 1/3] printk: Add line-buffered printk() API Tetsuo Handa
2018-11-02 13:31 ` [PATCH 2/3] mm: Use line-buffered printk() for show_free_areas() Tetsuo Handa
2018-11-07 14:07   ` Petr Mladek
2018-11-02 13:31 ` [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep messages Tetsuo Handa
2018-11-02 13:36   ` Peter Zijlstra
2018-11-03  2:00     ` Tetsuo Handa
2018-11-06  8:38     ` Petr Mladek
2018-11-06  9:05       ` Sergey Senozhatsky
2018-11-06 12:57         ` Petr Mladek
2018-11-06  9:56       ` Tetsuo Handa
2018-11-07 15:19   ` Petr Mladek
2018-11-08  4:45     ` Sergey Senozhatsky
2018-11-08 11:37       ` Tetsuo Handa
2018-11-09  6:12         ` Sergey Senozhatsky
2018-11-09  9:55           ` Tetsuo Handa
2018-11-09 15:43             ` Petr Mladek
2018-11-10  2:42               ` Tetsuo Handa
2018-11-23 12:46                 ` Petr Mladek
2018-11-23 13:12                   ` Tetsuo Handa
2018-11-23 15:56                   ` Steven Rostedt
2018-11-24  0:24                     ` Tetsuo Handa
2018-11-26  4:34                       ` Sergey Senozhatsky [this message]
2018-11-28 13:29                     ` David Laight
2018-11-29 10:09                       ` Tetsuo Handa
2018-11-30 16:01                         ` Petr Mladek
2018-11-10  8:52               ` Tetsuo Handa
2018-11-23 12:52                 ` Petr Mladek
2018-11-09 14:08           ` Linus Torvalds
2018-11-09 14:42             ` Steven Rostedt
2018-11-08 11:53       ` Petr Mladek
2018-11-08 12:44         ` Sergey Senozhatsky
2018-11-08 14:21           ` Sergey Senozhatsky
2018-11-09  9:54     ` Tetsuo Handa
2018-11-02 14:40 ` [PATCH v6 1/3] printk: Add line-buffered printk() API Matthew Wilcox
2018-11-03  1:55   ` Tetsuo Handa
2018-11-02 18:12 ` kbuild test robot
2018-11-02 18:12   ` kbuild test robot
2018-11-06 14:35 ` Sergey Senozhatsky
2018-11-07 10:21   ` Petr Mladek
2018-11-07 12:54     ` Tetsuo Handa
2018-11-08  2:21     ` Sergey Senozhatsky
2018-11-08 11:24       ` Petr Mladek
2018-11-08 11:46         ` Tetsuo Handa
2018-11-08 12:30         ` Sergey Senozhatsky
2018-11-09 14:10           ` Petr Mladek
2018-11-12  7:59             ` Sergey Senozhatsky
2018-11-12 10:42               ` Tetsuo Handa
2018-11-17 10:14               ` Tetsuo Handa
2018-11-07 10:52   ` Tetsuo Handa
2018-11-07 11:01     ` David Laight
2018-11-07 12:00       ` Petr Mladek
2018-11-07 11:45     ` Petr Mladek
2018-11-08  2:30     ` Sergey Senozhatsky
2018-11-07 13:41 ` Petr Mladek
2018-11-07 14:06 ` Petr Mladek

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