(from a bug report submitted by Stuart Hayes)
The purpose of the acpi_pad driver is to implement the "processor power
aggregator" device as described in the ACPI 4.0 spec section 8.5. It
takes requests from the BIOS (via ACPI) to put a specified number of
CPUs into idle, in order to save power, until further notice.
It does this by creating high-priority threads that try to keep the CPUs
in a high C-state (using the monitor/mwait CPU instructions). The
mwait() call is in a loop that checks periodically if the thread should
end and a few other things.
It was discovered through testing that the power_saving threads were
causing the system to consume more power than the system was consuming
before the threads were created. A counter in the main loop of
power_saving_thread() revealed that it was spinning. The mwait()
instruction was not keeping the CPU in a high C state very much if at
all.
Here is a simplification of the loop in function power_saving_thread() in
drivers/acpi/acpi_pad.c
while (!kthread_should_stop()) {
:
try_to_freeze()
:
while (!need_resched()) {
:
if (!need_resched())
__mwait(power_saving_mwait_eax, 1);
:
if (jiffies > expire_time) {
do_sleep = 1;
break;
}
}
}
If need_resched() returns true, then mwait() is not called. It was
returning true because of things like timer interrupts, as in the
following sequence.
hrtimer_interrupt->__run_hrtimer->tick_sched_timer-> update_process_times->
rcu_check_callbacks->rcu_pending->__rcu_pending->set_need_resched
Kernels 3.5.0-rc2+ do not exhibit this problem, because a patch to
try_to_freeze() in include/linux/freezer.h introduces a call to
might_sleep(), which ultimately calls schedule() to clear the reschedule
flag and allows the the loop to execute the call to mwait().
However, the changes to try_to_freeze are unrelated to acpi_pad, and it
does not seem like a good idea to rely on an unrelated patch in a
function that could later be changed and reintroduce this bug.
Therefore, it seems better to make an explicit call to schedule() in the
outer loop when the need_resched flag is set.
Reported-by: Stuart Hayes <stuart_hayes@xxxxxxxx>
Tested-by: Stuart Hayes <stuart_hayes@xxxxxxxx>
Signed-off-by: Tony Camuso <tcamuso@xxxxxxxxxx>
---
drivers/acpi/acpi_pad.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/drivers/acpi/acpi_pad.c b/drivers/acpi/acpi_pad.c
index 27bb6a9..447c55b 100644
--- a/drivers/acpi/acpi_pad.c
+++ b/drivers/acpi/acpi_pad.c
@@ -219,8 +219,15 @@ static int power_saving_thread(void *data)
* borrow CPU time from this CPU and cause RT task use > 95%
* CPU time. To make 'avoid starvation' work, takes a nap here.
*/
- if (do_sleep)
+ if (unlikely(do_sleep))
schedule_timeout_killable(HZ * idle_pct / 100);
+
+ /* If an external event has set the need_resched flag, then
+ * we need to deal with it, or this loop will continue to
+ * spin without calling __mwait().
+ */
+ if (unlikely(need_resched()))
+ schedule();
}
exit_round_robin(tsk_index);
--
1.8.3.1
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