linux-kernel - Re: [PATCH 17/41] rcu: Restart tick if we enqueue a callback in a nohz/cpuset CPU

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Message-ID: <20120523140010.GD1663@somewhere>
Date:	Wed, 23 May 2012 16:00:15 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	linaro-sched-sig@...ts.linaro.org,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>, Kevin Hilman <khilman@...com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephen Hemminger <shemminger@...tta.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 17/41] rcu: Restart tick if we enqueue a callback in a
 nohz/cpuset CPU

On Tue, May 22, 2012 at 10:27:14AM -0700, Paul E. McKenney wrote:
> On Tue, May 01, 2012 at 01:54:51AM +0200, Frederic Weisbecker wrote:
> > If we enqueue an rcu callback, we need the CPU tick to stay
> > alive until we take care of those by completing the appropriate
> > grace period.
> > 
> > Thus, when we call_rcu(), send a self IPI that checks rcu_needs_cpu()
> > so that we restore a periodic tick behaviour that can take care of
> > everything.
> 
> Ouch, I hadn't considered RCU callbacks being posted from within an
> extended quiescent state.  I guess I need to make __call_rcu() either
> complain about this or handle it correctly...  It would -usually- be
> harmless, but there is getting to be quite a bit of active machinery
> in the various idle loops, so just assuming that it cannot happen is
> probably getting to be an obsolete assumption.

May be first provide some detection to warn in such case. And if it happens
to warn too much perhaps you can allow that?

> 
> 							Thanx, Paul
> 
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Alessio Igor Bogani <abogani@...nel.org>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Avi Kivity <avi@...hat.com>
> > Cc: Chris Metcalf <cmetcalf@...era.com>
> > Cc: Christoph Lameter <cl@...ux.com>
> > Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> > Cc: Geoff Levand <geoff@...radead.org>
> > Cc: Gilad Ben Yossef <gilad@...yossef.com>
> > Cc: Hakan Akkan <hakanakkan@...il.com>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Kevin Hilman <khilman@...com>
> > Cc: Max Krasnyansky <maxk@...lcomm.com>
> > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Stephen Hemminger <shemminger@...tta.com>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Sven-Thorsten Dietrich <thebigcorporation@...il.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > ---
> >  kernel/rcutree.c |    7 +++++++
> >  1 files changed, 7 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 3fffc26..b8d300c 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -1749,6 +1749,13 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
> >  	else
> >  		trace_rcu_callback(rsp->name, head, rdp->qlen);
> > 
> > +	/* Restart the timer if needed to handle the callbacks */
> > +	if (cpuset_adaptive_nohz()) {
> > +		/* Make updates on nxtlist visible to self IPI */
> > +		barrier();
> > +		smp_cpuset_update_nohz(smp_processor_id());
> > +	}
> > +
> >  	/* If interrupts were disabled, don't dive into RCU core. */
> >  	if (irqs_disabled_flags(flags)) {
> >  		local_irq_restore(flags);
> > -- 
> > 1.7.5.4
> > 
> 
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