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From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	Dmitriy Vyukov <dvyukov@google.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Alexander Potapenko <glider@google.com>,
	Fengguang Wu <fengguang.wu@intel.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.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 v6 1/3] printk: Add line-buffered printk() API.
Date: Thu, 8 Nov 2018 21:30:49 +0900	[thread overview]
Message-ID: <20181108123049.GA30440@jagdpanzerIV> (raw)
In-Reply-To: <20181108112443.huqkju4uwrenvtnu@pathway.suse.cz>

On (11/08/18 12:24), Petr Mladek wrote:
> I believe that I mentioned this more times. 16 buffers is the first
> attempt. We could improve it later in several ways

Sure. Like I said - maybe it is a normal development pattern; I really
don't know.

> > Let's have one more look at what we will fix and what we will break.
> > 
> > 'cont' has premature flushes.
> > 
> >   Why is it good.
> >   It preserves the correct order of events.
> > 
> >   pr_cont("calling foo->init()....");
> >   foo->init()
> >    printk("Can't allocate buffer\n");    // premature flush
> >   pr_cont("...blah\h");
> > 
> >  Will end up in the logbuf as:
> >  [12345.123] calling foo->init()....
> >  [12345.124] Can't allocate buffer
> >  [12345.125] ...blah
> > 
> >  Where buffered printk will endup as:
> >  [12345.123] Can't allocate buffer
> >  [12345.124] calling foo->init().......blah
> 
> We will always have this problem with API using explicit buffers.

Exactly.

> What do you suggest instead, please?

So this is why "let's not remove 'cont'" thing. Buffered printk
is not 100% identical to 'cont'. And 'cont' does a good job sometimes,
Because 'cont' looks like a buffered printk, but it behaves like a
normal printk when things go bad. Buffered printk is always buffered;
and not even aware of the fact that things go bad around.

> > If our problem is OOM and lockdep print outs, then let's address only
> > those two; let's not "fix" the rest of the kernel, especially the early
> > boot, - we can break more things than we can mend.
> 
> Do you have any alternative proposal how to handle OOM and lockdep, please?

You misunderstood me. I'm not against the buffered printk in lockdep and
OOM. Albeit I must admit that lockdep patch looks quite big. I don't like
the idea of "we will remove 'cont' and replace it with buffered printk
through out the kernel".

[..]
> > - It seems that buffered printk attempts to solve too many problems.
> >   I'd prefer it to address just one.
> 
> This API tries to handle continuous lines more reliably.
> Do I miss anything, please?

This part:

+       /* Flush already completed lines if any. */
+       for (pos = ptr->len - 1; pos >= 0; pos--) {
+               if (ptr->buf[pos] != '\n')
+                       continue;
+               ptr->buf[pos++] = '\0';
+               printk("%s\n", ptr->buf);
+               ptr->len -= pos;
+               memmove(ptr->buf, ptr->buf + pos, ptr->len);
+               /* This '\0' will be overwritten by next vsnprintf() above. */
+               ptr->buf[ptr->len] = '\0';
+               break;
+       }
+       return r;

If I'm not mistaken, this is for the futute "printk injection" work.

Right now printk("foo\nbar\n") will end up to be a single logbuf
entry, with \n in the middle and at the end. So it will look like
two lines on the serial console:

	[123.123] foo
	          bar

Tetsuo does this \n lookup (on every vprintk_buffered) and split lines
(via memove) for "printk injection", so the output will be

	[123.123] foo
	[123.124] bar

Which makes it simpler to "inject" printk origin into every printed
line.

Without it we can just do

	len = vsprintf();
	if (len && text[len - 1] == '\n' || overflow)
		flush();

Can't we?

	-ss

  parent reply	other threads:[~2018-11-08 12:30 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-02 13:31 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
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 [this message]
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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