linux-kernel - Re: [PATCH 1/7] printk: Hand over printing to console if printing too long

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Date:	Wed, 6 Jan 2016 15:48:36 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.cz>,
	KY Sri nivasan <kys@...rosoft.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] printk: Hand over printing to console if printing
 too long

On (01/05/16 15:37), Jan Kara wrote:
> > How about setting 'sync_print' to 'true' in...
> >   bust_spinlocks() /* only set to true */
> > or
> >   console_verbose() /* um... may be... */
> > or
> >   having a separate one-liner for that
> > 
> > void console_panic_mode(void)
> > {
> > 	sync_print = true;
> > }
> > 
> > and call it early in panic(), before we send out IPI_STOP.
> 
> I like using console_verbose() for setting sync_print to true. That will
> likely be more reliable than using oops in progress. After all
> console_verbose() is used like console_panic_mode() anyway and in quite a
> few places so it is a reasonable match.

another corner case.

a quote from -mm a74b6533ead8 http://www.spinics.net/lists/linux-mm/msg98990.html

:    This patch reduces the probability of such a lockup by introducing a
:    specialized kernel thread (oom_reaper) which tries to reclaim additional
:    memory by preemptively reaping the anonymous or swapped out memory owned
:    by the oom victim under an assumption that such a memory won't be needed
:    when its owner is killed and kicked from the userspace anyway.  There is
:    one notable exception to this, though, if the OOM victim was in the
:    process of coredumping the result would be incomplete.  This is considered
:    a reasonable constrain because the overall system health is more important
:    than debugability of a particular application.
:
:    A kernel thread has been chosen because we need a reliable way of
:    invocation so workqueue context is not appropriate because all the workers
:    might be busy (e.g.  allocating memory).  Kswapd which sounds like another
:    good fit is not appropriate as well because it might get blocked on locks
:    during reclaim as well.

particularly this "workqueue context is not appropriate because all the workers
might be busy (e.g.  allocating memory)" part. I think printk should switch to
sync mode in this case, since printk now does queue_work(system_wq, work).
um... console_verbose() call from oom kill? but it'll be nice to return back
to async mode once (if) memory pressure goes away.

	-ss
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