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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2009-02-17 06:01:30 -0800
committerIngo Molnar <mingo@elte.hu>2009-02-17 22:47:45 +0100
commitbf51935f3e988e0ed6f34b55593e5912f990750a (patch)
tree7fc82ce1c63eef89ce9a086859d2dc0eaedde363 /arch/x86/kernel
parentbe716615fe596ee117292dc615e95f707fb67fd1 (diff)
x86, rcu: fix strange load average and ksoftirqd behavior
Damien Wyart reported high ksoftirqd CPU usage (20%) on an otherwise idle system. The function-graph trace Damien provided: > 799.521187 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.521371 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.521555 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.521738 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.521934 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.522068 | 1) ksoftir-2324 | | rcu_check_callbacks() { > 799.522208 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.522392 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.522575 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.522759 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.522956 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.523074 | 1) ksoftir-2324 | | rcu_check_callbacks() { > 799.523214 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.523397 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.523579 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.523762 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.523960 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.524079 | 1) ksoftir-2324 | | rcu_check_callbacks() { > 799.524220 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.524403 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.524587 | 1) <idle>-0 | | rcu_check_callbacks() { > 799.524770 | 1) <idle>-0 | | rcu_check_callbacks() { > [ . . . ] Shows rcu_check_callbacks() being invoked way too often. It should be called once per jiffy, and here it is called no less than 22 times in about 3.5 milliseconds, meaning one call every 160 microseconds or so. Why do we need to call rcu_pending() and rcu_check_callbacks() from the idle loop of 32-bit x86, especially given that no other architecture does this? The following patch removes the call to rcu_pending() and rcu_check_callbacks() from the x86 32-bit idle loop in order to reduce the softirq load on idle systems. Reported-by: Damien Wyart <damien.wyart@free.fr> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'arch/x86/kernel')
-rw-r--r--arch/x86/kernel/process_32.c3
1 files changed, 0 insertions, 3 deletions
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index a546f55c77b..bd4da2af08a 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -104,9 +104,6 @@ void cpu_idle(void)
check_pgt_cache();
rmb();
- if (rcu_pending(cpu))
- rcu_check_callbacks(cpu, 0);
-
if (cpu_is_offline(cpu))
play_dead();