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

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Date:	Wed, 23 May 2012 16:03:36 +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 10:30:47AM -0700, Paul E. McKenney wrote:
> 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.
> 
> Adaptive ticks does restart the tick upon entering the kernel, correct?

No, it keeps the tick down. The tick is restarted only if it's needed:
when more than one task are on the runqueue, a posix cpu timer is running,
a CPU needs the current one to report a quiescent state, etc...

> If so, wouldn't the return to userspace cause adaptive tick to automatically
> handle a callback posted from within the kernel?
> 
> (And yes, I still need to handle the possibility of callbacks being posted
> from the idle loop, but that is a different extended quiescent state.)
> 
> 							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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