linux-kernel - Re: [PATCH 10/11] sched: Debug nested sleeps

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Message-ID: <20141001161058.GE2843@worktop.programming.kicks-ass.net>
Date:	Wed, 1 Oct 2014 18:10:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	mingo@...nel.org, torvalds@...ux-foundation.org,
	tglx@...utronix.de, ilya.dryomov@...tank.com,
	umgwanakikbuti@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/11] sched: Debug nested sleeps

On Tue, Sep 30, 2014 at 11:47:32PM +0200, Oleg Nesterov wrote:

> > > This is minor, but this way CONFIG_DEBUG_ATOMIC_SLEEP will not imply
> > > a subtle behavioural change.
> >
> > You mean the __set_current_state() that's extra?
> 
> Yes, and note that it only does __set_current_state(RUNNING) if
> CONFIG_DEBUG_ATOMIC_SLEEP. This means that disabling/enabling this
> option can, silently hide/uncover a bug.
> 
> > I would actually argue
> > to keep that since it makes the 'problem' much worse.
> 
> OK, I won't insist, but could you explain why the suggested change can
> make the problem (and which problem ;) worse?

Sure, so the trivial problem is not actually going to sleep in the outer
wait primitive because the inner wait primitive reset ->state to
TASK_RUNNING.

So by always setting the ->state to TASK_RUNNING it never goes to sleep
and it'll revert to spinning, causing spikes in CPU usage that should
hopefully be far easier to notice than the occasional funny.
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