Re: [PATCH] printk: rebalance printk

From: Pan Xinhui
Date: Tue Aug 11 2015 - 21:38:28 EST

hi, Peter
thanks for your reply!
On 2015å08æ11æ 20:00, Peter Hurley wrote:
> On 08/11/2015 07:23 AM, Pan Xinhui wrote:
>> From: Pan Xinhui <xinhuix.pan@xxxxxxxxx>
>> 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.
>> 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.

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