Messages in this thread | | | Date | Sun, 4 Apr 2010 11:17:37 -0400 (EDT) | From | Alan Stern <> | Subject | Re: A few questions and issues with dynticks, NOHZ and powertop |
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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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