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 13:25:17 +0100
From:	Jan Kara <jack@...e.cz>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Jan Kara <jack@...e.cz>,
	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 Wed 06-01-16 15:48:36, Sergey Senozhatsky wrote:
> 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.

Hum, yes, some mechanism to switch to sync printing in case work cannot be
executed for a long time is probably needed. I'll think about it.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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