Commit 9bcbdd9c authored Oct 08, 2009 by Arjan van de Ven Committed by Ingo Molnar Oct 08, 2009

x86, timers: Check for pending timers after (device) interrupts



Now that range timers and deferred timers are common, I found a
problem with these using the "perf timechart" tool. Frans Pop also
reported high scheduler latencies via LatencyTop, when using
iwlagn.

It turns out that on x86, these two 'opportunistic' timers only get
checked when another "real" timer happens. These opportunistic
timers have the objective to save power by hitchhiking on other
wakeups, as to avoid CPU wakeups by themselves as much as possible.

The change in this patch runs this check not only at timer
interrupts, but at all (device) interrupts. The effect is that:

 1) the deferred timers/range timers get delayed less

 2) the range timers cause less wakeups by themselves because
    the percentage of hitchhiking on existing wakeup events goes up.

I've verified the working of the patch using "perf timechart", the
original exposed bug is gone with this patch. Frans also reported
success - the latencies are now down in the expected ~10 msec
range.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Tested-by: Frans Pop <elendil@planet.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <20091008064041.67219b13@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent fdc6f192

arch/x86/kernel/irq.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -244,6 +244,7 @@ unsigned int __irq_entry do_IRQ(struct pt_regs *regs)
		__func__, smp_processor_id(), vector, irq);
		}

		run_local_timers();
		irq_exit();

		set_irq_regs(old_regs);
		@@ -268,6 +269,7 @@ void smp_generic_interrupt(struct pt_regs *regs)
		if (generic_interrupt_extension)
		generic_interrupt_extension();

		run_local_timers();
		irq_exit();

		set_irq_regs(old_regs);

arch/x86/kernel/smp.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -198,6 +198,7 @@ void smp_reschedule_interrupt(struct pt_regs *regs)
		{
		ack_APIC_irq();
		inc_irq_stat(irq_resched_count);
		run_local_timers();
		/*
		* KVM uses this interrupt to force a cpu out of guest mode
		*/

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