linux-kernel - Re: [PATCH 1/2] rcu: Stop chasing QS if another CPU did it for us

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Date:	Sat, 11 Dec 2010 00:33:32 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>, laijs@...fujitsu.com
Subject: Re: [PATCH 1/2] rcu: Stop chasing QS if another CPU did it for us

On Fri, Dec 10, 2010 at 02:58:55PM -0800, Paul E. McKenney wrote:
> On Fri, Dec 10, 2010 at 10:11:10PM +0100, Frederic Weisbecker wrote:
> > When a CPU is idle and others CPUs handled its extended
> > quiescent state to complete grace periods on its behalf,
> > it will catch up with completed grace periods numbers
> > when it wakes up.
> > 
> > But at this point there might be no more grace period to
> > complete, but still the woken CPU always keeps its stale
> > qs_pending value and will then continue to chase quiescent
> > states even if its not needed anymore.
> > 
> > This results in clusters of spurious softirqs until a new
> > real grace period is started. Because if we continue to
> > chase quiescent states but we have completed every grace
> > periods, rcu_report_qs_rdp() is puzzled and makes that
> > state run into infinite loops.
> > 
> > As suggested by Lai Jiangshan, just reset qs_pending if
> > someone completed every grace periods on our behalf.
> 
> Nice!!!
> 
> I have queued this patch, and followed it up with a patch that changes
> the condition to "rnp->qsmask & rdp->grpmask", which indicates that RCU
> needs a quiescent state from the CPU, and is valid regardless of how
> messed up the CPU is about which grace period is which.
> 
> I am making a similar change to the check in __note_new_gpnum().
> 
> Seem reasonable?

Look good yeah.

Thanks!

> 
> 							Thanx, Paul
> 
> > Suggested-by: Lai Jiangshan <laijs@...fujitsu.com>
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: Ingo Molnar <mingo@...e.hu>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > ---
> >  kernel/rcutree.c |    8 ++++++++
> >  1 files changed, 8 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index ccdc04c..8c4ed60 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -681,6 +681,14 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
> > 
> >  		/* Remember that we saw this grace-period completion. */
> >  		rdp->completed = rnp->completed;
> > +
> > +		/*
> > +		 * If another CPU handled our extended quiescent states and
> > +		 * we have no more grace period to complete yet, then stop
> > +		 * chasing quiescent states.
> > +		 */
> > +		if (rdp->completed == rnp->gpnum)
> > +			rdp->qs_pending = 0;
> >  	}
> >  }
> > 
> > -- 
> > 1.7.3.2
> > 
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