linux-kernel - Re: [PATCH] mm/page_alloc: Wait for oom

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Date:   Thu, 12 Jan 2017 15:18:44 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>, mhocko@...e.com,
        linux-mm@...ck.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.cz>, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, sergey.senozhatsky.work@...il.com
Subject: Re: [PATCH] mm/page_alloc: Wait for oom_lock before retrying.

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

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
is disabled. But we still would like to enable rescheduling
when called from the tty code (guarded by console_lock()).

As a result. We should keep console_may_schedule as a global
variable. We cannot put the automatic detection into
console_conditional_schedule(). Instead, we need to
fix handling of the global variable in console_unlock().

I am going to prepare a patch for this.

Best Regards,
Petr