linux-kernel - Re: [PATCH v6 1/3] printk: Add line-buffered printk() API.

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Date:   Thu, 8 Nov 2018 21:30:49 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexander Potapenko <glider@...gle.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v6 1/3] printk: Add line-buffered printk() API.

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