Re: PROBLEM: tickless scheduling

From: Donald Allen
Date: Mon May 17 2010 - 10:02:23 EST

On Mon, May 17, 2010 at 9:44 AM, Donald Allen <donaldcallen@xxxxxxxxx> wrote:
> On Sun, May 16, 2010 at 7:36 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> Donald,
>> On Sat, 15 May 2010, Donald Allen wrote:
>>> Attached. This is from the 2.6.30 kernel on the Arch Linux install cd.
>>> Here's another bit of data. As I've said previously, the problems I'm
>>> reporting were observed on a Toshiba NB310-305 netbook with a
>>> single-core Atom 450 processor. I just built myself a mini-ITX system
>>> using the Intel D510MO motherboard, which provides a dual-core D510
>>> Atom processor. The other hardware on the board is similar to the
>>> Toshiba. I installed the same Slackware snapshot I used on the
>>> Toshiba, and did the home directory transfer without any problem at
>>> all with the default tickless kernel. The hardware isn't identical,
>>> and while I don't know the internals of the Linux kernel at all, my
>>> gut, backed up by many years of OS development work in scheduling and
>>> memory management, is telling me that the key difference is dual- vs.
>>> single-core. Just a guess.
>> I fear you are wrong.
> Please don't be afraid.
>> The key difference is almost certainly that the BIOS of your netbook
>> tries to be overly clever vs. power management and is not aware of the
>> fact that the Linux kernel uses timer hardware in a very different way
>> than the other OS which comes preinstalled on that machine.
>> The overly clever BIOS power management which works nicely with the
>> vendor provided "drivers" for the other OS is just interfering with
>> the kernels way of dealing with the problem.
>> Can you please boot with "hpet=disable" on the kernel command line ?
> I did, and it made no difference.
> To be specific, the test I am doing involves booting with the Arch
> Linux 2009.08 install/live cd. I then run
> fsck.ext2 -f -r /dev/sda3
> to do a read-only check of my root filesystem. I watch the
> disk-activity light, and it reliably goes out and then you've got a
> long wait if you do nothing. Tickling the touchpad gets things moving
> again. This happens reliably with or without the boot-time option you
> requested above.
> I just noticed something else, however, that may lend credence to the
> opinion expressed by Arjan van de Ven that this has nothing to do with
> tickless. I originally noticed this problem on the Toshiba netbook
> when I installed the Slackware 13.1 x86_64 beta on this machine, which
> comes with a tickless kernel. The first symptom I observed
> was attempting to rsync my home directory from another machine to this
> new install and, as previously described, I had to help things along
> by activating the touchpad, or pressing the ctrl key (any kind of
> external stimulus that would generate an interrupt seemed to work).
> Anyway, after some discussion with Patrick Volkerding, I decided to
> build a custom kernel for the netbook and disabled tickless in that
> kernel. After getting that kernel working, I re-did the tests that
> failed with the tickless kernel and they all worked fine, so I thought
> we had our culprit. But just now, after doing the test you requested
> above, I rebooted the system from its installed kernel (the tickful
> kernel I built), and it hung during booting. At first I thought it was
> taking awhile to do the dance with the dhcp server, but after waiting
> longer than I thought this should take, I touched the touchpad and
> forward progress began again immediately (disk light came on, boot
> time chatter proceeded, etc.). So, my current guess, for what it's
> worth, is that there's a race here that causes the system to miss the
> fact that it has a runnable process, and the probability of hitting it
> is reduced, but not to zero, by using tickful scheduling.
> I will do the experiment suggested by Arjan van de Ven and report the
> results of that separately.

I just booted with pci=nomsi and the fsck ran normally without any
help from my finger on the touchpad. So I think Arjan is closing in on
this ...


> /Don
>> Thanks,
>>        tglx
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