linux-kernel - Re: [PATCH tip/core/rcu 44/55] rcu: wire up RCU_BOOST

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Message-ID: <20110913205055.GF3301@linux.vnet.ibm.com>
Date:	Tue, 13 Sep 2011 13:50:55 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mike Galbraith <efault@....de>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 44/55] rcu: wire up RCU_BOOST_PRIO for
 rcutree

On Tue, Sep 13, 2011 at 06:04:18PM +0200, Mike Galbraith wrote:
> On Tue, 2011-09-13 at 08:34 -0700, Paul E. McKenney wrote:
> > On Tue, Sep 13, 2011 at 02:02:14PM +0200, Mike Galbraith wrote:
> 
> > > @@ -1608,7 +1618,7 @@ static int rcu_node_kthread(void *arg)
> > >  				continue;
> > >  			}
> > >  			per_cpu(rcu_cpu_has_work, cpu) = 1;
> > > -			sp.sched_priority = RCU_KTHREAD_PRIO;
> > > +			sp.sched_priority = current->rt_priority;
> > 
> > This is broken -- the per-node kthread runs at RT prio 99, but we usually
> > would not want to boost that high.
> 
> Ouch, right.  My userland sets things on boot, so it works.

;-)

> > Seems like we should have a global variable that tracks the current
> > priority.  This global variable could then be set in a manner similar
> > to the softirq priorities -- or, perhaps better, simply set whenever
> > the softirq priority is changed.
> > 
> > Thoughts?
> 
> RCU threads would have to constantly watch for user priority changes on
> their own, and update private data methinks.

I believe that we are going to need some sort of -rt-specific handling
of the RCU boost priority in the short term.  Though maybe I could think
about getting runtime modification into mainline as well -- but it would
be different that -rt for a bit.

							Thanx, Paul
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