Re: NOHZ: local_softirq_pending - More info without need for specific code.

[Iratxo Pichel Ortiz <ipichel@albentia.com>]

Thomas,

More info below, I hope it helps.

Iratxo Pichel Ortiz wrote:
> Thomas,
>
> Iratxo Pichel Ortiz wrote:
>>>>  2.6.29-rt16, right ?
>>>>       
>>> I have also tried rt18, but as I had thought seeing change-logs, the
>>> behavior is exactly the same.
>>>     
> You were right here :) It was 29.
>>> Do you know what could be causing this issue. I have managed to 
>>> repeat this
>>> traces (NOHZ...) without using my code, using a workqueue and in the 
>>> work
>>> just by doing something like:
>>>
>>> work_func() {
>>>   mdelay(10);
>>>   msleep(10);
>>>
>>>  queue_work(myqueue, mywork);
>>> }
>>>
>>> And then by heavy loading the box from the outside.
>>>     
> I have written a very small module that causes the
> "local_softirq_pending" under not some load. Please find code at the end
> of this email. Here is pasted some traces of dmesg (I have increased the
> ratelimit of the "NOHZ: local..." trace to 250.
>
> The only strange thing here is that I am calling "set_workqueue_prio" (I
> have hacked source to export this symbol), and I am starting to think
> that this could not be a good idea. Any hints about this?
>
> [  648.954000] NOHZ: local_softirq_pending
> 0e
>
> [  648.955000] NOHZ: local_softirq_pending
> 0e
>
> [  648.956000] NOHZ: local_softirq_pending
> 0e
I have changed the implementation of the module test to use kthreads
instead of workqueues. The behavior is exactly the same. I have tried
with prios from 1 to 99. Please find the code below as before. I have
also atached the differente softirqs codes that had been pending in some
of the tests.
>
>
>>>> Does it work when you disable CONFIG_NOHZ ?
>>>>       
> Still pending to test.
>>> I will try this and let the list know.
>>>    
>>>> Thanks,
>>>>
>>>>        tglx
>>>>     
Thanks a lot again!

    Iratxo.

---- DMESG ----
[4294833.294000] NOHZ: local_softirq_pending
06
[4294833.295000] NOHZ: local_softirq_pending
06
[4294834.246000] NOHZ: local_softirq_pending
02
[4294834.247000] NOHZ: local_softirq_pending
02
[4294834.248000] NOHZ: local_softirq_pending 02

---- Test module code ----

#include <linux/module.h>
#include <linux/kthread.h>
#include <linux/sched.h>
#include <linux/delay.h>

#define LATENCY_PRIO    1

struct task_struct *t;

int LatencyzerWork(void* data)
{
    static int count;

    while(1) {
        /* Busy and normal waiting */
        mdelay(10);
        msleep(1);

    /* Requeue work */
    if(count++ > 400000)
        break;
    }

    printk("LAT: Count reached %d, stopped latencying\n", count);
    return 0;
}

int __init LatencyzerInit(void)
{
    struct sched_param schedp;

    /* Prepare Thread */
    printk("LAT: Initializing latencyzer\n");
    if(!(t = kthread_create(LatencyzerWork, NULL, "LATENCY"))) {
        printk("LAT: Error creating thread\n");
        return -1;
    }

    /* Change task prio */
    schedp.sched_priority = LATENCY_PRIO;
    sched_setscheduler(t, SCHED_FIFO, &schedp);
    printk("LAT: Task prio set to %u\n", LATENCY_PRIO);

    /* Run thread */
    wake_up_process(t);
    return 0;
}

void __exit LatencyzerExit(void)
{
    printk("LAT: Latencyzer exiting...");

    /* Done here */
    kthread_stop(t);
    printk(" DONE\n");

    return;
}

module_init(LatencyzerInit);
module_exit(LatencyzerExit);
MODULE_LICENSE("GPL");


-- 
Iratxo Pichel Ortiz
Software Development Manager

Albentia Systems S.A.
http://www.albentia.com

Tel: +34 914400567
Cel: +34 663808405
Fax: +34 914400569

C\Margarita Salas 22
Parque Tecnológico de Leganés
Leganés (28918)
Madrid
Spain


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