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

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Date:	Wed, 12 Aug 2015 09:35:59 +0800
From:	Pan Xinhui <xinhuix.pan@...el.com>
To:	Peter Hurley <peter@...leysoftware.com>
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>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Joe Perches <joe@...ches.com>, hch@....de,
	viro@...iv.linux.org.uk, Vasily Averin <vvs@...tuozzo.com>
Subject: Re: [PATCH] printk: rebalance printk

hi, Peter
	thanks for your reply!
On 2015年08月11日 20:00, Peter Hurley wrote:
> On 08/11/2015 07:23 AM, 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);
>>
>> printk runs on CPUA just store the messages in the buf and return.
>> printk runs on CPUB(who owns the console_sem lock) would take the duty
>> to flush all messages to consoles. It would take a long time to flush
>> messages out, IOW, irq would be disabled for a long time. Such case is
>> too bad.  We hit many interrupt related panics, for example, cpu did not
>> respond to IPI.
>>
>> Here is the solution, if we detect such case above, try to rebalance it.
>> Let CPUA take the duty to flush messages to consoles.
>>
>> The idea is simple, but the implementation is a little difficult.
>> Introduce many help functions to fix it.
> 
> Please describe the rebalance state machine/message passing in detail.
> 
> Regards,
> Peter Hurley
> 
yes, I should describe it more detailedly. Sorry for that.
I will send out V2.

thanks
xinhui
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