Commit 98d82567 authored May 23, 2007 by Thomas Gleixner Committed by Linus Torvalds May 23, 2007

Prevent going idle with softirq pending



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 <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 40acc095

kernel/sched.c

+1 −3

Original line number	Diff line number	Diff line
		@@ -4775,9 +4775,7 @@ int __sched cond_resched_softirq(void)
		BUG_ON(!in_softirq());

		if (need_resched() && system_state == SYSTEM_RUNNING) {
		raw_local_irq_disable();
		_local_bh_enable();
		raw_local_irq_enable();
		local_bh_enable();
		__cond_resched();
		local_bh_disable();
		return 1;

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