linux-kernel - Re: Warning in worker_enter

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Message-ID: <20120507194042.GG19417@google.com>
Date:	Mon, 7 May 2012 12:40:42 -0700
From:	Tejun Heo <tj@...nel.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Warning in worker_enter_idle()

Hello, Paul.

On Sun, May 06, 2012 at 08:38:14AM -0700, Paul E. McKenney wrote:
> Hello!
> 
> The worker_enter_idle() is complaining that there all workers are idle,
> but that there is work remaining:
> 
> 	/* sanity check nr_running */
> 	WARN_ON_ONCE(gcwq->nr_workers == gcwq->nr_idle &&
> 		     atomic_read(get_gcwq_nr_running(gcwq->cpu)));
> 
> This is running on Power, .config attached.  I must confess that I don't
> see any sort of synchronization or memory barriers that would keep the
> counts straight on a weakly ordered system.  Or is there some clever
> design constraint that prevents worker_enter_idle() from accessing other
> CPUs' gcwq_nr_running variables?

Workers are tied to global cpu workqueues (gcwqs).  There's one gcwq
per cpu and one unbound one, so yeah, workers access these counters
under gcwq->lock.  Atomic accesses to nr_running is depended on only
while nr_idle is adjusted under gcwq->lock, so there shouldn't be a
discrepancy there.  Can you reproduce the problem?  What was going on
the system?  Was CPU being brought up or down?

Thanks.

-- 
tejun
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