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Date: Sun, 4 Apr 2010 13:47:25 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Dominik Brodowski <linux@...inikbrodowski.net>, Alan Stern <stern@...land.harvard.edu>, linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>, Arjan van de Ven <arjan@...ux.intel.com>, Dmitry Torokhov <dtor@...l.ru> Subject: Re: 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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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