[PATCH/RFC] synchronize_rcu(): high latency on idle system

[Benjamin LaHaise <bcrl@kvack.org>]

Hello folks,

I'd like to put the patch below out for comments to see if folks think the 
approach is a valid fix to reduce the latency of synchronize_rcu().  The 
motivation is that an otherwise idle system takes about 3 ticks per network 
interface in unregister_netdev() due to multiple calls to synchronize_rcu(), 
which adds up to quite a few seconds for tearing down thousands of 
interfaces.  By flushing pending rcu callbacks in the idle loop, the system 
makes progress hundreds of times faster.  If this is indeed a sane thing to, 
it probably needs to be done for other architectures than x86.  And yes, the 
network stack shouldn't call synchronize_rcu() quite so much, but fixing that 
is a little more involved.

		-ben

diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 9663c2a..592f6e4 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -188,6 +188,9 @@ void cpu_idle(void)
 			rmb();
 			idle = pm_idle;
 
+			if (rcu_pending(cpu))
+				rcu_check_callbacks(cpu, 0);
+
 			if (!idle)
 				idle = default_idle;