On (01/12/17 15:18), Petr Mladek wrote:

On Mon 2016-12-26 20:34:07, Sergey Senozhatsky wrote:

quoted

console_trylock() used to always forbid rescheduling; but it got changed
like a yaer ago.

the other thing is... do we really need to console_conditional_schedule()
from fbcon_*()? console_unlock() does cond_resched() after every line it
prints. wouldn't that be enough?

so may be we can drop some of console_conditional_schedule()
call sites in fbcon. or update console_conditional_schedule()
function to always return the current preemption value, not the
one we saw in console_trylock().

I was curious if it makes sense to remove
console_conditional_schedule() completely.

I was looking at this option at some point as well.

In practice, it never allows rescheduling when the console driver
is called via console_unlock(). It is since 2006 and the commit
78944e549d36673eb62 ("vt: printk: Fix framebuffer console
triggering might_sleep assertion"). This commit added
that

	console_may_schedule = 0;

into console_unlock() before the console drivers are called.


On the other hand, it seems that the rescheduling was always
enabled when some console operations were called via
tty_operations. For example:

struct tty_operations con_ops

  con_ops->con_write()
  -> do_con_write()  #calls console_lock()
   -> do_con_trol()
    -> fbcon_scroll()
     -> fbcon_redraw_move()
      -> console_conditional_schedule()

, where console_lock() sets console_may_schedule = 1;


A complete console scroll/redraw might take a while. The rescheduling
would make sense => IMHO, we should keep console_conditional_schedule()
or some alternative in the console drivers as well.

But I am afraid that we could not use the automatic detection.
We are not able to detect preemption when CONFIG_PREEMPT_COUNT

can one actually have a preemptible kernel with !CONFIG_PREEMPT_COUNT?
how? it's not even possible to change CONFIG_PREEMPT_COUNT in menuconfig.
the option is automatically selected by PREEMPT. and if PREEMPT is not
selected then _cond_resched() is just "{ rcu_all_qs(); return 0; }"

...

We cannot put the automatic detection into console_conditional_schedule().

why can't we?

I am going to prepare a patch for this.

I'm on it.

	-ss