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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-and-tested-by: Stuart Hayes <stuart_hayes@dell.com> Signed-off-by: Tony Camuso <tcamuso@redhat.com> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> |
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.. | ||
acpica | ||
apei | ||
ac.c | ||
acpi_cmos_rtc.c | ||
acpi_extlog.c | ||
acpi_ipmi.c | ||
acpi_lpss.c | ||
acpi_memhotplug.c | ||
acpi_pad.c | ||
acpi_platform.c | ||
acpi_processor.c | ||
battery.c | ||
battery.h | ||
bgrt.c | ||
blacklist.c | ||
bus.c | ||
button.c | ||
container.c | ||
custom_method.c | ||
debugfs.c | ||
device_pm.c | ||
dock.c | ||
ec_sys.c | ||
ec.c | ||
event.c | ||
fan.c | ||
glue.c | ||
hed.c | ||
internal.h | ||
Kconfig | ||
Makefile | ||
numa.c | ||
nvs.c | ||
osl.c | ||
pci_irq.c | ||
pci_link.c | ||
pci_root.c | ||
pci_slot.c | ||
power.c | ||
proc.c | ||
processor_core.c | ||
processor_driver.c | ||
processor_idle.c | ||
processor_perflib.c | ||
processor_thermal.c | ||
processor_throttling.c | ||
reboot.c | ||
resource.c | ||
sbs.c | ||
sbshc.c | ||
sbshc.h | ||
scan.c | ||
sleep.c | ||
sleep.h | ||
sysfs.c | ||
tables.c | ||
thermal.c | ||
utils.c | ||
video_detect.c | ||
video.c | ||
wakeup.c |