Re: CPU scheduler weirdness?

Marton Balint
2009-09-04T07:53:51Z

On Fri, 4 Sep 2009, Mike Galbraith wrote:

> On Thu, 2009-09-03 at 23:57 +0200, Marton Balint wrote:
>
>>>> 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 :)
>>>
>>> I reproduced the bug with the previously provided kernel module and C program
>>> on a different computer (it's a laptop with a core2 duo P8400 CPU), and also
>>> bisected the bug to this commit:
>>>
>>> sched: fine-tune SD_MC_INIT:
>>> 14800984706bf6936bbec5187f736e928be5c218
>>>
>>> If I add again the removed SD_BALANCE_NEWIDLE to flags, then everything works
>>> as expected. So what would be the correct fix for this bug? Revert the patch?
>>> Or just add SD_BALANCE_NEWIDLE to flags?
>
> Or, figure out what's going weird with that module loaded.

The problem is most likely caused by scheduled_delayed_work, a work
function is called every time a CPU wakes up.

>> Ingo, Peter, could any of you guys have a look at the commit that caused
>> this bug? Is it OK to revert it? Or a fix somewhere else is necessary? I'm
>> pushing this because I hope that this bug will get fixed in the upcoming
>> stable kernel...
>
> Where does your schedtest.c and schedtest_mod.c live?

They were attached to one of my previous mails, i'm inlining them here to
make the discussion easier. Thanks for looking into this.

Regards,
Marton

schedtest_mod.c
-------------------
#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");

schedtest.c:
--------------------

#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;
}