Re: [PATCH 16/41] rcu: Restart the tick on non-responding adaptive nohz CPUs

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

On Wed, May 23, 2012 at 03:57:24PM +0200, Frederic Weisbecker wrote:
> On Tue, May 22, 2012 at 10:20:50AM -0700, Paul E. McKenney wrote:
> > On Tue, May 01, 2012 at 01:54:50AM +0200, Frederic Weisbecker wrote:
> > > When a CPU in adaptive nohz mode doesn't respond to complete
> > > a grace period, issue it a specific IPI so that it restarts
> > > the tick and chases a quiescent state.
> > 
> > Hello, Frederic,
> > 
> > I don't understand the need for this patch.  If the CPU is in
> > adaptive-tick mode, RCU should see it as being in dyntick-idle mode,
> > right?  If so, shouldn't RCU have already recognized the CPU as being
> > in an extended quiescent state?
> > 
> > Or is this a belt-and-suspenders situation?
> > 
> > 							Thanx, Paul
> 
> If the tickless CPU is in userspace, it is in extended quiescent state. But
> not if it runs tickless in the kernel. In this case we need to send it an IPI
> so that it restarts the tick after checking rcu_pending().

But if it has registered itself with RCU as idle, for example, by calling
rcu_user_enter(), then RCU will be ignoring that CPU, posting quiescent
states as needed on its behalf.  So I still don't understand the need
for this patch.

							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 |   17 +++++++++++++++++
> > >  1 files changed, 17 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > > index e141c7e..3fffc26 100644
> > > --- a/kernel/rcutree.c
> > > +++ b/kernel/rcutree.c
> > > @@ -50,6 +50,7 @@
> > >  #include <linux/wait.h>
> > >  #include <linux/kthread.h>
> > >  #include <linux/prefetch.h>
> > > +#include <linux/cpuset.h>
> > > 
> > >  #include "rcutree.h"
> > >  #include <trace/events/rcu.h>
> > > @@ -302,6 +303,20 @@ static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
> > > 
> > >  #ifdef CONFIG_SMP
> > > 
> > > +static void cpuset_update_rcu_cpu(int cpu)
> > > +{
> > > +#ifdef CONFIG_CPUSETS_NO_HZ
> > > +	unsigned long flags;
> > > +
> > > +	local_irq_save(flags);
> > > +
> > > +	if (cpuset_cpu_adaptive_nohz(cpu))
> > > +		smp_cpuset_update_nohz(cpu);
> > > +
> > > +	local_irq_restore(flags);
> > > +#endif
> > > +}
> > > +
> > >  /*
> > >   * If the specified CPU is offline, tell the caller that it is in
> > >   * a quiescent state.  Otherwise, whack it with a reschedule IPI.
> > > @@ -325,6 +340,8 @@ static int rcu_implicit_offline_qs(struct rcu_data *rdp)
> > >  		return 1;
> > >  	}
> > > 
> > > +	cpuset_update_rcu_cpu(rdp->cpu);
> > > +
> > >  	/*
> > >  	 * The CPU is online, so send it a reschedule IPI.  This forces
> > >  	 * it through the scheduler, and (inefficiently) also handles cases
> > > -- 
> > > 1.7.5.4
> > > 
> > 
>