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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, May 23, 2012 at 04:00:15PM +0200, Frederic Weisbecker 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.
> 
> May be first provide some detection to warn in such case. And if it happens
> to warn too much perhaps you can allow that?

Heh.  It is just as simple to allow it as it is to warn about it, so
I am just transitioning immediately to allowing it.

							Thanx, Paul

> > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > > Cc: Alessio Igor Bogani <abogani@kernel.org>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Avi Kivity <avi@redhat.com>
> > > Cc: Chris Metcalf <cmetcalf@tilera.com>
> > > Cc: Christoph Lameter <cl@linux.com>
> > > Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
> > > Cc: Geoff Levand <geoff@infradead.org>
> > > Cc: Gilad Ben Yossef <gilad@benyossef.com>
> > > Cc: Hakan Akkan <hakanakkan@gmail.com>
> > > Cc: Ingo Molnar <mingo@kernel.org>
> > > Cc: Kevin Hilman <khilman@ti.com>
> > > Cc: Max Krasnyansky <maxk@qualcomm.com>
> > > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Stephen Hemminger <shemminger@vyatta.com>
> > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > Cc: Sven-Thorsten Dietrich <thebigcorporation@gmail.com>
> > > Cc: Thomas Gleixner <tglx@linutronix.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
> > > 
> > 
>