linux-kernel - Re: [PATCH 5/8] rcu: eliminate deadlock for rcu read site

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Date:	Mon, 12 Aug 2013 09:44:36 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH 5/8] rcu: eliminate deadlock for rcu read site

On Mon, Aug 12, 2013 at 06:21:26PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 12, 2013 at 08:16:18AM -0700, Paul E. McKenney wrote:
> > On Mon, Aug 12, 2013 at 03:55:44PM +0200, Peter Zijlstra wrote:
> > > On Fri, Aug 09, 2013 at 05:31:27PM +0800, Lai Jiangshan wrote:
> > > > On 08/09/2013 04:40 AM, Paul E. McKenney wrote:
> > > > > One problem here -- it may take quite some time for a set_need_resched()
> > > > > to take effect.  This is especially a problem for RCU priority boosting,
> > > > > but can also needlessly delay preemptible-RCU grace periods because
> > > > > local_irq_restore() and friends don't check the TIF_NEED_RESCHED bit.
> > > > 
> > > > 
> > > > The final effect of deboosting(rt_mutex_unlock()) is also accomplished
> > > > via set_need_resched()/set_tsk_need_resched().
> > > > set_need_resched() is enough for RCU priority boosting issue here.
> > > 
> > > But there's a huge difference between the boosting and deboosting side
> > > of things. rcu_read_unlock_special() starts the boost, the deboosting
> > > only matters if/when you reschedule. 
> > 
> > Or if there is a pre-existing runnable task whose priority is such that
> > deboosting makes it the highest-priority task.
> 
> Right, I got horribly lost in rt_mutex, but I suspect we deal with that
> case the right way. -rt people would've noticed us screwing that up ;-)
> 
> But there too, we're fully limited to how fast we can get a
> reschedule(). Deboosting sooner than we can reschedule to run the other
> task is effectively pointless. The converse is obviously not true; we
> must not be able to reschedule sooner than we can deboost ;-)

In addition, the proposed change was to defer the deboost based on
a set_need_resched(), which would provide additional opportunity for
delay -- the running task would retain its high priority until the
scheduler acted on the set_need_resched().

							Thanx, Paul

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