The NOHZ patch contains a check for softirqs pending when a CPU goes
idle. The BUG is unrelated to NOHZ, it just was made visible by the NOHZ
patch. The BUG showed up mainly on P4 / hyperthreading enabled machines
which lead the investigations into the wrong direction in the first
place. The real cause is in cond_resched_softirq():
cond_resched_softirq() is enabling softirqs without invoking the softirq
daemon when softirqs are pending. This leads to the warning message in
the NOHZ idle code:
t1 runs softirq disabled code on CPU#0
interrupt happens, softirq is raised, but deferred (softirqs disabled)
t1 calls cond_resched_softirq()
enables softirqs via _local_bh_enable()
calls schedule()
t2 runs
t1 is migrated to CPU#1
t2 is done and invokes idle()
NOHZ detects the pending softirq
Fix: change _local_bh_enable() to local_bh_enable() so the softirq
daemon is invoked.
Thanks to Anant Nitya for debugging this with great patience !
Signed-off-by: Thomas Gleixner <[email protected]>
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4776,7 +4776,7 @@ int __sched cond_resched_softirq(void)
if (need_resched() && system_state == SYSTEM_RUNNING) {
raw_local_irq_disable();
- _local_bh_enable();
+ local_bh_enable();
raw_local_irq_enable();
__cond_resched();
local_bh_disable();
-
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