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SubjectRe: A few questions and issues with dynticks, NOHZ and powertop
On Sun, Apr 04, 2010 at 06:39:24PM +0200, Dominik Brodowski wrote:
> On Sun, Apr 04, 2010 at 11:17:37AM -0400, Alan Stern wrote:
> > 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.
>
> Okay, but still: can't this be sped up by much on UP (especially if
> CONFIG_RCU_FAST_NO_HZ is set), so that we can go to sleep right away?

One situation that will prevent CONFIG_RCU_FAST_NO_HZ from putting the
machine to sleep right away is if there is an RCU callback posted that
spawns another RCU callback, and so on. CONFIG_RCU_FAST_NO_HZ will handle
one callback that spawns another, but it gives up if the second callback
spawns a third.

Might this be what is happening to you?

If so, would you be willing to patch your kernel? RCU_NEEDS_CPU_FLUSHES
is currently set to 5, and might be set to (say) 8. This is defined
in kernel/rcutree_plugin.h, near line 990.

Another thing to try would be running with TINY_RCU, at least if it is
OK that RCU be non-preemptible.

Thanx, Paul

> > > 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
>
> Thanks! This way, it works, even without manually unbinding the host
> drivers.
>
> Best,
> Dominik
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