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

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Date:	Fri, 11 Jan 2008 20:26:26 -0500
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	dipankar@...ibm.com, Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: [PATCH/RFC] synchronize_rcu(): high latency on idle system

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;

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