Re: [PATCH tip/core/rcu 1/2] rcu: Parallelize and economize NOCB kthread wakeups

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

On Tue, Aug 12, 2014 at 02:39:36PM -0700, Paul E. McKenney wrote:
> On Tue, Aug 12, 2014 at 09:06:21AM -0700, Paul E. McKenney wrote:
> > On Tue, Aug 12, 2014 at 11:03:21AM +0530, Amit Shah wrote:
> 
> [ . . . ]
> 
> > > I know of only virtio-console doing this (via userspace only,
> > > though).
> > 
> > As in userspace within the guest?  That would not work.  The userspace
> > that the qemu is running in might.  There is a way to extract ftrace info
> > from crash dumps, so one approach would be "sendkey alt-sysrq-c", then
> > pull the buffer from the resulting dump.  For all I know, there might also
> > be some script that uses the qemu "x" command to get at the ftrace buffer.
> > 
> > Again, I cannot reproduce this, and I have been through the code several
> > times over the past few days, and am not seeing it.  I could start
> > sending you random diagnostic patches, but it would be much better if
> > we could get the trace data from the failure.
> 
> Hearing no objections, random patch #1.  The compiler could in theory
> cause trouble without this patch, so there is some possibility that
> it is a fix.

#2...  This would have been a problem without the earlier patch, but
who knows?  (#1 moved from theoretically possible but not on x86 to
maybe on x86 given a sufficiently malevolent compiler with the
patch that you located with bisection.)

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 1dc72f523c4a..1da605740e8d 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2137,6 +2137,17 @@ static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
 		trace_rcu_callback(rdp->rsp->name, rhp,
 				   -atomic_long_read(&rdp->nocb_q_count_lazy),
 				   -atomic_long_read(&rdp->nocb_q_count));
+
+	/*
+	 * If called from an extended quiescent state with interrupts
+	 * disabled, invoke the RCU core in order to allow the idle-entry
+	 * deferred-wakeup check to function.
+	 */
+	if (irqs_disabled_flags(flags) &&
+	    !rcu_is_watching() &&
+	    cpu_online(smp_processor_id()))
+		invoke_rcu_core();
+
 	return true;
 }