Date:	Wed, 4 Nov 2009 11:31:03 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mike Travis <travis@....com>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Roland Dreier <rdreier@...co.com>,
	Randy Dunlap <rdunlap@...otime.net>, Tejun Heo <tj@...nel.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Yinghai Lu <yinghai@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	David Rientjes <rientjes@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Jack Steiner <steiner@....com>,
	Frederic Weisbecker <fweisbec@...il.com>, x86@...nel.org,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86_64: Limit the number of processor bootup messages


* Mike Travis <travis@....com> wrote:

>
>
> Ingo Molnar wrote:
>> * Mike Travis <travis@....com> wrote:
>>
>>>
>>> Andi Kleen wrote:
>>>> Mike Travis wrote:
>>>>> This set of patches limits the number of repetitious messages 
>>>>> which  contain
>>>>> no additional information.  Much of this information is 
>>>>> obtainable  from the
>>>>> /proc and /sysfs.   Most of the messages are also sent to the kernel log
>>>>> buffer as KERN_DEBUG messages so it can be used to examine more   
>>>>> closely any
>>>>> details specific to a processor.
>>>> What would be good is to put the information from the booting CPUs
>>>> into some buffer and print it visibly if there's a timeout detected 
>>>> on  the BP.
>>> What do you think of this idea....  Add a "mark kernel log buffer"  
>>> function, and then if any KERN_NOTE or above happens, it sends the  
>>> marked info from the kernel log buffer to the console before the  
>>> current message.  Set the marker to '0' to clear.
>>
>> That's _way_ too complex really, for little benefit. (If there's a boot 
>> hang people will re-try anyway (and this time with a serial console  
>> attached or so), and they can add various boot options to increase  
>> verbosity - depending in which phase the bootup hung.)
>
> I'm ok with this, though generally speaking large server systems have 
> serial consoles attached, and save the output into admin logs. [...]

Typically yes, but not necessarily during basic system bringup, which is 
when most of the hangs/problems are found.

> [...]  One problem with just setting the loglevel high enough to 
> output debug messages, is you get literally 100's of thousands of 
> lines of meaningless information.  We waited over 8 hours for a system 
> with 2k cpus to boot in debug mode, and it never made it all the way 
> up.
>
> My intention for the above was to attempt to print debug information 
> that pertains to the failure, and not everything else.

We want a noise-free default bootup, and printks (on the boot cpu) in 
case of failures.

_that_ abnormal-event printout can then be sufficiently verbose.

>> So please go with the simple solution i suggested days ago: print 
>> stuff on the boot CPU but after that only a single line per AP CPU.
>
> So you think printing 4096 lines provides meaningful additional 
> information?  I would think at least compress it so you only print 
> each new processor socket boots and not the 16 threads each of them 
> have?
>
> I should have timing information soon for 512 cores/1024 threads and 
> printing a single line for each of those will significantly increase 
> the time it takes to boot.

Feel free to compress it further. What i was objecting to was the 
increased complexity of 'buffering' messages somehow and printing them 
conditionally.

	Ingo
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