Re: [Ksummit-discuss] [TECH TOPIC] asynchronous printk

[Hannes Reinecke <hare@suse.com>]

On 07/19/2016 09:33 AM, Sergey Senozhatsky wrote:
> On (07/19/16 08:17), Hannes Reinecke wrote:
> [..]
>> 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.
>>
>> For the first use-case you absolutely need a synchronous printk, but
>> this is a complete killer for the second case.
>> And OTOH having a separate thread is really the way to go for the second
>> case, but an absolute no-go for the first.
>>
>> So I really wonder if it does make sense to lump both use-cases into one
>> call, or whether it wouldn't be better to have two distinct calls
>> for that (or, for the sake of argument, use KERN_EMERG to trigger
>> synchronous printks).
> 
> yes, panic() must be in sync printk mode. but we do it a
> bit differently: console_verbose() forces printk to switch
> to sync mode.
> 
> so panic() goes like this:
> 
> 	panic()
> 	{
> 		console_verbose(); # switch to sync printk. forever.
> 		bust_spinlocks();
> 		pr_emerg("Kernel panic - not syncing...");
> 
> 		....
> 		debug_locks_off();
> 		console_flush_on_panic();  # flushes kernel log_bug in
> 					   # sync mode
> 
> 		// the rest of panic()
> 
> 	}
> 
> 	-ss
> 
but this is precisely what I meant by priority inversion:
If there are lots of messages in the printk buffer we might not be
_able_ to print out everything as the machine died before the entire
printk buffer could be printed.
Which means we will never see the panic message, at which point we could
as well drop the whole exercise.

Cheers,

Hannes
-- 
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