Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

[Andreas Mohr]

On Wed, May 31, 2017 at 04:30:59PM +0900, Sergey Senozhatsky wrote:
> Hello Jan,
> 
> On (05/29/17 14:12), Jan Kara wrote:
> [..]
> > Actually I had something very similar in old versions of my patch set. And
> > it didn't work very well. The problem was that e.g. sometimes scheduler
> > decided that printk kthread should run on the same CPU as the process
> > currently doing printing and in such case printk kthread never took over
> > printing and the machine locked up due to heavy printing.
> 
> hm, interesting.

Not too knowledgeable, but just my thoughts:
was this on a non-preemption kernel (!CONFIG_PREEMPT).
If so, perhaps we are missing some non-preempt-case yield somewhere.
I'd think that it really *cannot* be that there are relevant processes
(printk kthread) which functionality invoked by a user of that process *does*
know about (read: printk() APIs), yet somehow those printk() APIs then
do not properly cause a yield
[thus possibly and hopefully: to the printk kernel thread]
(quite possibly after some certain amount of printk resource use exceeded).
--> implementation bug??
(due to staying at execution of a client process, despite that one making
massive calls to printk() APIs, where we then ought to be able to
properly set up a preemption point)

Pseudo code:

void printk_use_annotate()
{
  if (count_printk_since_last_servicing > 30)
    yield();
}

Note that a preemption yield action obviously will not reliably guarantee
hitting printk thread
(unless there's an API for directed yield),
thus this mechanism has to be qualified as *unreliable*,
thus this has to be taken into account by implementation code.

But all this reasoning is a strong indication that
there might be no
properly precisely handshaked producer/consumer protocol -
this would e.g. be the case for a proper select() loop
with event handling where the partner would be woken up *precisely*
(via POSIX 3-way handshake!)
when the condition for wakeup is fulfilled
(read: too many pending printk()s or some such),
rather than having some imprecise (sleep-type / "oh I do not have much to
do any more now") "handwaving" scheduling.


> yeah, I can easily make it a normal prio task. at the same time
> printk_kthread has 'soft' limits on its execution. it's under the
> same constraints as the rest of the processes that do printing.
> there can be a random RT task doing console_trylock()->console_unlock(),
> so we still can hog CPUs. but, yeah, I don't want printk_kthread to be
> special.

[printk_thread not special] Indeed. Think clean dependency abstraction.
printk kthread (i.e., that "more special" thread worker context)
should merely be *one* user of these printk queue servicing APIs.
And those servicing APIs should simply be invocable by anyone
who would want to take part in
getting the printk queue load properly serviced,
without any special handwaving.

[I can easily make all those "easy" wishlist requests -
I did not have to painfully design all that stuff ;-)]

Andreas Mohr