Re: CPU scheduler weirdness?

Marton Balint
2009-08-20T16:56:09Z

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On Thu, 20 Aug 2009, Ingo Molnar wrote:

>
> * Marton Balint <cus@fazekas.hu> wrote:
>
>>
>> On Wed, 19 Aug 2009, Peter Zijlstra wrote:
>>
>>> On Wed, 2009-08-19 at 14:34 +0200, Marton Balint wrote:
>>>>
>>>> On Wed, 19 Aug 2009, Peter Zijlstra wrote:
>>>>
>>>>> On Wed, 2009-08-19 at 14:01 +0200, Marton Balint wrote:
>>>>>> On Wed, 19 Aug 2009, Peter Zijlstra wrote:
>>>>>>> On Tue, 2009-08-18 at 21:49 +0200, Marton Balint wrote:
>>>>>>>
>>>>>>>> In the meantime, I was able to create a tiny C program which always
>>>>>>>> succesfully reproduces the bug. It's basically an endless loop which does
>>>>>>>> not stop while the process is running on the last CPU core. The program
>>>>>>>> creates multiple instances of itself, to be able to keep all of the CPU
>>>>>>>> cores busy. After 1 second, the processes running on other than the last
>>>>>>>> CPU core die, the processes running on the last CPU core remain stuck
>>>>>>>> there...
>>>>>>>>
>>>>>>>> I tested it on my dual core system, if someone could test it on a quad
>>>>>>>> core and report back that would probably be useful.
>>>>>>>>
>>>>>>>> Usage: ./schedtest <number of CPU cores>
>>>>>>>>
>>>>>>>> And don't forget to kill the stuck processes after using the program! :)
>>>>>>>
>>>>>>> So what's the bug? Sure one task will stay on the cpu, and because there
>>>>>>> is no contention it doesn't get migrated, and therefore won't quit,
>>>>>>> how's that a problem?
>>>>>>
>>>>>> Problem is that more than one processes remain on that CPU core, and none
>>>>>> of them get migrated to other (idle) cores. I tested it with my E8400
>>>>>> processor and 2.6.31-rc5-git3 kernel.
>>>>>
>>>>> Only one remains here.. on a c2q running 2.6.31-rc6-tip
>>>>>
>>>>> Do you have a .config handy?
>>>>>
>>>>
>>>> Yes it's in my original post:
>>>>
>>>> http://marc.info/?l=linux-kernel&m=125012584709800&w=2
>>>
>>> Right you are,.. so I build a kernel with the cgroup scheduler in and
>>> tested it on a dual-core opteron machine, but I can't seem to reproduce
>>> this.
>>>
>>> Are you using cgroups in any way, or do you simply have it enabled in
>>> your config?
>>
>> No, it's just enabled. Actually the kernel is from the
>> openSUSE build service:
>>
>> http://download.opensuse.org/repositories/Kernel:/HEAD/openSUSE_11.1/x86_64/
>>
>> But the problem is present for both the kernel-default
>> kernel and the kernel-vanilla kernel which does not
>> contain any suse-specific patches.
>>
>> This evening I had a bit more time to test, and I've
>> made a surprising discovery: I can only reproduce the
>> bug if the kernel module of my TV tuner card is loaded.
>> I have a Leadtek Winfast 2000 XP Expert TV card, it
>> uses the cx8800 kernel module. It seems that the
>> problem is somehow related to the infrared sensor of
>> the TV card, because I recompiled the module with the
>> 'case CX88_BOARD_WINFAST2000XP_EXPERT:' line removed
>> from cx88-input.c and I couldn't reproduce the bug with
>> the new kernel module.
>
> Extremely weird. Are timers somehow busted?

How can I check that?

In the meantime, I updated my original C program and also created a kernel
module (schedtest_mod.c) which causes the same scheduling problems as the
kernel module of my TV card. The kernel module is a skeleton of the
infrared sensor polling code in cx88-input.c. It uses
schedule_delayed_work, this seems to cause the problem. The C program
(schedtest.c) is also updated, it now detects the number of CPU cores,
from now, what you can set as a command line parameter is the CPU core
number, on which the schedtest processes will not quit. (previously this
was always the last core).

So to reproduce the bug on a dual core system, compile and insert the
kernel module (schedtest_mod.c). Then check dmesg, it should contain on
which CPU core is the delayed_work running. You should use the CPU core id
of the _other_ CPU core as a command line parameter to the updated
schedtest program.

And by the way, thank you guys for the help so far, hopefully we'll get to
the bottom of this :)

Regards,
Marton

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#include <linux/module.h>
#include <linux/init.h>
#include <linux/workqueue.h>
#include <asm/smp.h>

static int i;
static struct delayed_work d_work;

static void schedtest_work(struct work_struct *work)
{
schedule_delayed_work(&d_work, msecs_to_jiffies(1));
if (i++ % 500 == 0) {
printk(KERN_DEBUG "schedtest: I am on CPU %d.\n", get_cpu());
put_cpu();
}
}

static int __init schedtest_init_module(void)
{
INIT_DELAYED_WORK(&d_work, schedtest_work);
schedule_delayed_work(&d_work, 0);
return 0;
}

static void __exit schedtest_cleanup_module(void)
{
cancel_delayed_work_sync(&d_work);
}

module_init(schedtest_init_module);
module_exit(schedtest_cleanup_module);

MODULE_LICENSE("GPL");

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#define _GNU_SOURCE
#include <utmpx.h>
#include <sys/time.h>
#include <unistd.h>

/* Usage: ./schedtest <cpu core to test> */

int miliseconds() {
struct timeval tv;
gettimeofday(&tv, 0);
return tv.tv_usec/1000;
}

int main(int argc, char *argv[]) {
int lives = 1000, time, lasttime, childs, cores, core_to_test;
cores = sysconf(_SC_NPROCESSORS_ONLN);
childs = cores * 2;
if (argc > 1)
core_to_test = atoi(argv[1]);
else
core_to_test = cores-1;
while (childs-- && !fork());
while (lives) {
time = miliseconds();
if (lasttime != time && sched_getcpu() != core_to_test)
lives--;
lasttime = time;
}
return 0;
}