Re: A few questions and issues with dynticks, NOHZ and powertop

From: Alan Stern
Date: Sun Apr 04 2010 - 11:18:02 EST

On Sun, 4 Apr 2010, Dominik Brodowski wrote:

> Booting a SMP-capable kernel with "nosmp", or manually offlining one CPU
> (or -- though I haven't tested it -- booting a SMP-capable kernel on a
> system with merely one CPU) means that in up to about half of the calls to
> tick_nohz_stop_sched_tick() are aborted due to rcu_needs_cpu(). This is
> quite strange to me: AFAIK, RCU is an excellent tool for SMP, but not really
> needed for UP?

I can't answer the real question here, not knowing enough about the RCU
implementation. However, your impression is wrong: RCU very definitely
_is_ useful and needed on UP systems. It coordinates among processes
(and interrupt handlers) as well as among processors.

> 3) USB: built-in UHCI and a built-in 0a5c:2101 Broadcom Corp. A-Link
> BlueUsbA2 Bluetooth module; built-in EHCI and a built-in 0ac8:c302 Z-Star
> Microelectronics Corp. Vega USB 2.0 Camera.
> usbcore.autosuspend is enabled (= 2), of course.
> Recent USB suspend statistics
> Active Device name
> 100.0% USB device 7-1 : BCM92045NMD (Broadcom Corp)
> 100.0% USB device 1-2 : Vega USB 2.0 Camera. (Vimicro Corp.)
> 100.0% USB device usb7 : UHCI Host Controller (Linux 2.6.34-rc3 uhci_hcd)
> 100.0% USB device usb1 : EHCI Host Controller (Linux 2.6.34-rc3 ehci_hcd)
> Booting into /bin/bash on a SMP kernel booted with "nosmp" leads to ~ 10
> wakeups per second; disabling the cursor helps halfway (~ 5 wakeups); and
> manually unbinding the USB host drivers from the USB host devices finally
> lead to ~ 1.1 wakeups per second. What's keeping USB from suspending these
> unused devices here?

Either the drivers don't support autosuspend or the devices aren't
enabled for autosuspend. By default, autosuspend is disabled for
(almost) all non-hub devices. You or your distribution must enable
it manually by doing:

echo auto >/sys/bus/usb/devices/.../power/level

If the driver doesn't support autosuspend then enabling it won't be
enough; you'll also have to unbind the driver from the device. The
easiest way to do this is to unconfigure the device:

echo 0 >/sys/bus/usb/devices/.../bConfigurationValue

Alan Stern

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