linux-kernel - Re: [PATCH v3] printk: Have printk() never buffer its data

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Date:	Mon, 09 Jul 2012 15:29:44 -0700
From:	Joe Perches <joe@...ches.com>
To:	Kay Sievers <kay@...y.org>
Cc:	Michael Neuling <mikey@...ling.org>, linuxppc-dev@...abs.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Wu Fengguang <fengguang.wu@...el.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH v3] printk: Have printk() never buffer its data

On Tue, 2012-07-10 at 00:10 +0200, Kay Sievers wrote:
> On Mon, Jul 9, 2012 at 11:42 PM, Joe Perches <joe@...ches.com> wrote:
> > On Sun, 2012-07-08 at 19:55 +0200, Kay Sievers wrote:
> >
> >> At the same time the CPU#2 prints the same warning with a continuation
> >> line, but the buffer from CPU#1 can not be flushed to the console, nor
> >> can the continuation line printk()s from CPU#2 be merged at this point.
> >> The consoles are still locked and busy with replaying the old log
> >> messages, so the new continuation data is just stored away in the record
> >> buffer as it is coming in.
> >> If the console would be registered a bit earlier, or the warning would
> >> happen a bit later, we would probably not see any of this.
> >>
> >> I can fake something like this just by holding the console semaphore
> >> over a longer time and printing continuation lines with different CPUs
> >> in a row.
> >>
> >> The patch below seems to work for me. It is also here:
> >>   http://git.kernel.org/?p=linux/kernel/git/kay/patches.git;a=blob;f=kmsg-merge-cont.patch;hb=HEAD
> >>
> >> It only applies cleanly on top of this patch:
> >>   http://git.kernel.org/?p=linux/kernel/git/kay/patches.git;a=blob;f=kmsg-syslog-1-byte-read.patch;hb=HEAD
> >>
> >
> > Hi Kay.
> >
> > I just ran a test with what's in Greg's driver-core -for-linus branch.
> >
> > One of the differences in dmesg is timestamping of consecutive
> >         pr_<level>("foo...)
> > followed directly by
> >         pr_cont("bar...")
> >
> > For instance: (dmesg is 3.4, dmesg.0 is 3.5-rc6+)
> >
> > # grep MAP /var/log/dm* -A1
> > dmesg:[    0.781687] ata_piix 0000:00:1f.2: MAP [ P0 P2 P1 P3 ]
> > dmesg-[    0.781707] ata2: port disabled--ignoring
> > --
> > dmesg.0:[    0.948881] ata_piix 0000:00:1f.2: MAP [
> > dmesg.0-[    0.948883]  P0 P2 P1 P3 ]
> >
> > These messages originate starting at
> > drivers/ata/ata_piix.c:1354
> >
> > All the continuations are emitted with pr_cont.
> >
> > I think this output should still be coalesced without
> > timestamp deltas.  Perhaps the timestamping code can
> > still be reworked to avoid too small a delta producing
> > a new timestamp and another dmesg line.
> 
> Hmm, I don't see that.
> 
> If I do:
>   pr_info("[");
>   for (i = 0; i < 4; i++)
>        pr_cont("%i ", i);
>   pr_cont("]\n");
> 
> I get:
>   6,173,0;[0 1 2 3 ]
> 
> And if I fill the cont buffer and forcefully hold the console sem
> during all that, and we can't merge anymore, I get:
>   6,167,0;[
>   4,168,0;0
>   4,169,0;1
>   4,170,0;2
>   4,171,0;3
>   4,172,0;]
> 
> But the output is still all fine for both lines:
>   [    0.000000] [0 1 2 3 ]
>   [    0.000000] [0 1 2 3 ]
> 
> What do I miss?

In this case the initial line is dev_info not pr_info
so there are the additional dict descriptors output to
/dev/kmsg as well.

Maybe that interferes with continuations.  Dunno.

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