linux-kernel - Re: [PATCH] printk: rebalance printk

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Message-ID: <55CAAD54.4060109@intel.com>
Date:	Wed, 12 Aug 2015 10:20:04 +0800
From:	Pan Xinhui <xinhuix.pan@...el.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, pmladek@...e.cz,
	rostedt@...dmis.org, Tejun Heo <tj@...nel.org>,
	Peter Hurley <peter@...leysoftware.com>,
	Joe Perches <joe@...ches.com>, hch@....de,
	viro@...iv.linux.org.uk, Vasily Averin <vvs@...tuozzo.com>
Subject: Re: [PATCH] printk: rebalance printk



On 2015年08月12日 10:04, Greg Kroah-Hartman wrote:
> On Wed, Aug 12, 2015 at 09:53:58AM +0800, Pan Xinhui wrote:
>> Hi, Greg
>> 	thanks for your reply!
>> On 2015年08月12日 02:16, Greg Kroah-Hartman wrote:
>>> On Tue, Aug 11, 2015 at 07:23:01PM +0800, Pan Xinhui wrote:
>>>> From: Pan Xinhui <xinhuix.pan@...el.com>
>>>>
>>>> printk can be called in any context, It's very useful to output debug
>>>> info.
>>>>
>>>> But it might cause very bad issues on some special cases. For example,
>>>> some driver hit errors, and it dumps many messages like reg values, etc. 
>>>>
>>>> Sometimes, printk is called when irqs disabled. This is OKay if there is
>>>> a few messages. But What would happen if many messages outputted by other
>>>> drivers at same time.
>>>>
>>>> Here is the scenario.
>>>> CPUA					CPUB
>>>> 					local_irq_save(flags);
>>>> 					printk()
>>>> while(..) {					--> console_unlock
>>>> 	printk(...);			
>>>> 	//hundreds or thousands loops
>>>> }				//all messages flushed out to consoles
>>>> 					local_irq_restore(flags);
>>>>
>>>
>>> Where are you seeing this type of scenario "in the wild"?  Or is this
>>> just a "debug/bringup hardware" issue?  We shouldn't be ever stuck in a
>>> printk that prints hundreds or thousands of loops, if so, we need to fix
>>> the kernel code that does that, as we do have control over this.
>>>
>>
>> Sorry for misleading. these loops are just an example. In real world,
>> there are many pr_info/pr_err when some drivers hit device errors to
>> dump debug information.
> 
> That's not good, they should use dev_dbg() for that instead.
> 
Agree, We changed some codes to use dev_dbg(). But some errors still need be outputted immediately.
But when we enabled dynamic debug. there is not much difference... printk issue still hit.

>> There is camera and sound test(multimedia test) in our side, we enable
>> dynamic_debug to collect logs to fix some weird issues.  sound driver
>> will dump many reg/memory values, and camera will output a few logs
>> with irqs disabled. So we hit this printk issues very frequently, and
>> it blocks our tests indeed.
> 
> Maybe those memory value dumps should be done through debugfs instead of
> printk?
> 
These reg/memory runtime values must be outputted in time, they have changed at any time..

>> As printk is announced that it could be used in most context, So I
>> prefer to enhance the stability of printk to handle such case.
> 
> Stability is nice, but using printk for things it was never ment to be
> used for, isn't good.  Especially if you are forced to add additional
> complexity to the printk logic just for these corner cases that no one
> uses :)
> 

printk is very good, it does work in very complex context, including in interrupt, etc.
However if we could make it better, then why not :)

My patch is a little complex. Sorry for that.
I would try to work out V2.

thanks
xinhui

> thanks,
> 
> greg k-h
> 
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