linux-kernel - Re: [PATCH] cgroup/cpuset: fix circular locking dependency

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Date:   Tue, 9 Jan 2018 07:21:12 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Prateek Sood <prsood@...eaurora.org>,
        Peter Zijlstra <peterz@...radead.org>, avagin@...il.com,
        mingo@...nel.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, sramana@...eaurora.org
Subject: Re: [PATCH] cgroup/cpuset: fix circular locking dependency

On Tue, Jan 09, 2018 at 05:44:48AM -0800, Tejun Heo wrote:
> Hello, Paul.
> 
> On Mon, Jan 08, 2018 at 08:20:16PM -0800, Paul E. McKenney wrote:
> > OK, so I can put WQ_MEM_RECLAIM on the early boot creation of RCU's
> > workqueue_struct as shown below, right?
> 
> Yes, this looks good to me.  Just one question.
> 
> > +struct workqueue_struct *rcu_gp_workqueue;
> > +
> >  void __init rcu_init(void)
> >  {
> >  	int cpu;
> > @@ -4298,6 +4300,10 @@ void __init rcu_init(void)
> >  		rcu_cpu_starting(cpu);
> >  		rcutree_online_cpu(cpu);
> >  	}
> > +
> > +	/* Create workqueue for expedited GPs and for Tree SRCU. */
> > +	rcu_gp_workqueue = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);
> > +	WARN_ON(!rcu_gp_workqueue);
> 
> The code was previously using both system_power_efficient_wq and
> system_workqueue (for the expedited path).  I guess the options were
> either using two workqueues or dropping POWER_EFFICIENT.  I have no
> idea how big an impact this will make or whether it'd even be
> noticeable but maybe it'd be worthwhile to mention that in the
> description?

Good point!  How about if I change the last paragraph of the commit
log to read as follows?

							Thanx, Paul

------------------------------------------------------------------------

This commit also causes SRCU to use this new RCU-specific
workqueue_struct.  Note that SRCU's use of workqueues never blocks them
waiting for readers, so this should be safe from a forward-progress
viewpoint.  Note that this moves SRCU from system_power_efficient_wq
to a normal workqueue.  In the unlikely event that this results in 
measurable degradation, a separate power-efficient workqueue will be
creates for SRCU.