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

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Date:   Fri, 13 Jan 2017 11:28:43 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        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 (01/12/17 15:18), Petr Mladek wrote:
> 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.

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