linux-kernel - Re: RFC: android logger feedback request

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Date:	Wed, 21 Dec 2011 18:12:39 -0800
From:	Tim Bird <tim.bird@...sony.com>
To:	Greg KH <gregkh@...e.de>
CC:	linux-embedded <linux-embedded@...r.kernel.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	john stultz <johnstul@...ibm.com>,
	Brian Swetland <swetland@...gle.com>,
	Kay Sievers <kay.sievers@...y.org>,
	Lennart Poettering <lennart@...ttering.net>
Subject: Re: RFC: android logger feedback request

On 12/21/2011 05:47 PM, Greg KH wrote:
> On Wed, Dec 21, 2011 at 05:32:36PM -0800, Tim Bird wrote:
>> On 12/21/2011 04:51 PM, Greg KH wrote:
>>> On Wed, Dec 21, 2011 at 04:36:21PM -0800, Tim Bird wrote:
>>>> On 12/21/2011 03:19 PM, Greg KH wrote:
>>
>>> Huh, I'm not talking about syslogd, I'm talking about the syslog(2)
>>> syscall we have.
>>
>> OK - switching gears.  Since the kernel log buffer isn't normally
>> used to store use-space messages, I thought you were referring
>> to syslog(3) and the associated (logger(1) and syslogd(8)).
> 
> The kernel log buffer has been storing userspace messages for a while
> now, look at your boot log on the latest Fedora and openSUSE releases
> (or any other distro using systemd for booting).

Sorry - I don't have a distro that uses systemd.
I'm completely unaware of this use of the kernel log for user-space
message logging.  No wonder Lennart and Kay
are so hot on making a new logging system.  It seems sub-optimal
to me, to intermingle a structured log with a pure-ASCII log.

> Again, please see what we are already doing in the kernel and userspace,
> I think a lot of the above is already implemented.

I don't know what systemd has got going on in user-space.  I'm looking
at a very recent kernel, and I see no support for multiple log channels,
or an optimized open/write path.

Maybe Lennart could save me some time doing this research.

Lennart,

How does current systemd prevent user-space messages from crowding
out kernel messages?  (or vice-versa).  Since you've been doing
a lot of work on logging, do you have any existing metrics for logging
overhead via the kernel log buffer?

> Which brings me back to my original question, what does this code do,
> that is not already present in the kernel, and why is a totally new
> interface being proposed for it?

At the least, it supports multiple log channels.  Quite frankly my mind
has been blown a bit by the suggestion to overload the kernel log buffer
with user-space messages.  I would never have gone that route. But I'll have
to find out more about this systemd thing to answer your question.

Secondly, this is not a particularly new or radical interface. It is new
to legacy Linux, but it's been in the Android Linux kernel for some years
now, and it has worked well.
 -- Tim

=============================
Tim Bird
Architecture Group Chair, CE Workgroup of the Linux Foundation
Senior Staff Engineer, Sony Network Entertainment
=============================

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