[Ksummit-discuss] [TECH TOPIC] asynchronous printk

[Andy Lutomirski]

On Tue, Jul 19, 2016 at 8:35 PM, Wangnan (F) <wangnan0 at huawei.com> wrote:
>
>
> On 2016/7/19 14:17, Hannes Reinecke wrote:
>>
>> On 07/19/2016 05:47 AM, Sergey Senozhatsky wrote:
>>>
>>> Hello,
>>>
>>> Wondering if anyone will be interested in printk-related topics
>>> (or we can handle it in the mailing list).
>>>
>>> What I have on my list is:
>>>
>>>
>>> - synchronous printk()
>>>
>>> printk() prints messages from kernel printk buffer until the buffer
>>> is empty. When serial console is attached, printing is slow and thus
>>> other CPUs in the system have plenty of time to append new messages to
>>> the buffer while one CPU is printing. Thus the CPU can spend unbounded
>>> amount of time doing printing in console_unlock().  This is especially
>>> serious problem if the printk() calling console_unlock() was called with
>>> interrupts disabled, or from IRQ, or from spin_lock protected section
>>> (if the spinlock is contended), etc. etc. IOW, printk() is quite
>>> dangerous
>>> function to call in some cases, it can cause different types of lockups
>>> (soft, hard, spinlock), stalls and so on.
>>>
>>> we have some progress on this side. printk() can offload printing from
>>> sensitive and unsafe contexts to a schedulable printk_kthread context (a
>>> special purpose printing kthread).
>>> but "The whole idea remains worrisome", per Andrew :)
>>>
>> Yes. The main problem stems from the fact that printk has two different
>> and conflicting use-cases:
>> - Really urgent, 'I am about to die' messages. Which obviously need to
>>    be printed out as fast as possible.
>> - Rather largish, information/logging 'what I always wanted to tell you'
>>    type of messages. These messages tend to be very large, but at the end
>>    it doesn't really matter _when_ they'll be printed as they are
>>    time-stamped anyway.
>>
>
> Actually, there are 3 types of messages:
>
>  1. Urgent: I'm going to die.
>  2. information/logging.
>  3. Trace.
>

If we do all this stuff, can we also try to clean up earlyprintk a
bit?  The whole earlyconsole mechanism is a mess, and switching over
to the non-early console is only somewhat functional.  I'd love to see
this all simplified: before there's any console at all available, just
buffer messages.  Then, when a console shows up, write the buffer out.
Then earlyprintk can work just like regular printk.

--Andy