Linux pthread priority real time scheduler (SCHED_RR) doesn't seem work properly

Jia Hong · 04-28-2014, 09:54 PM

Platform: HP dc7900 (intel), Debian distribution (2.6 kernel)
Problem: Create two pthreads with priority 40 and 60. Respective counter (count0 or count 1) gets incrtemented when it is scheduled to run. Main routine prints the two counters every second. Unexpected behaviour is that counter for the first pthread becomes even bigger. Expected behaviour is that the first pthread (pri 40) should be completely blocked by the second pthread (pri 60) since it is lower priority and second thread doesn't block by itself. Need help to understand what is going on.

Code and output are as follows:

===============================

Thread created successfully

First thread processing

Second thread processing

Thread created successfully

223891966 11374955
434026187 22372766
643700229 34908176
783430529 43956430
993360049 55111456
1203710619 65921699

================================

Code:

#include<stdio.h>
#include<string.h>
#include<pthread.h>
#include<stdlib.h>
#include<unistd.h>

pthread_t tid[2];
int pid[ 2 ] = { 1, 2 };
int count0, count1;

void* doSomeThing(void *arg)
{
    unsigned long i = 0;

    if( *((int *)arg) == 1 )
    {
        printf("\n First thread processing\n");
    }
    else
    {
        printf("\n Second thread processing\n");
    }

    while(1)
    {
        switch( *((int *)arg) )
        {
            case 1:
                ++count0;
                break;
            case 2:
                ++count1;
                break;
            default:
                break;
        }
    }
    return NULL;
}

int main(void)
{
    int i = 0;
    int err;
    struct sched_param param;
    pthread_attr_t attr;

    while(i < 2)
    {
        if( i == 0 )
        {
            param.sched_priority = 40;
            err = pthread_attr_init(&attr);
            if( err )
            {
                printf("error\n");
                return 0;
            }
            err = pthread_attr_setschedpolicy(&attr,SCHED_RR);
            if( err )
            {
                printf("error\n");
                return 0;
            }
            err = pthread_attr_setinheritsched(&attr,PTHREAD_EXPLICIT_SCHED);
            if( err )
            {
                printf("error\n");
                return 0;
            }
            err = pthread_attr_setschedparam(&attr, &param);
            if( err )
            {
                printf("error\n");
                return 0;
            }
        }
        else
        {
            param.sched_priority = 60;
            err = pthread_attr_init(&attr);
            if( err )
            {
                printf("error\n");
                return 0;
            }
            err = pthread_attr_setschedpolicy(&attr,SCHED_RR);
            if( err )
            {
                printf("error\n");
                return 0;
            }
            err = pthread_attr_setinheritsched(&attr,PTHREAD_EXPLICIT_SCHED);
            if( err )
            {
                printf("error\n");
                return 0;
            }
            err = pthread_attr_setschedparam(&attr, &param);
            if( err )
            {
                printf("error\n");
                return 0;
            }

        }
        err = pthread_create(&(tid[i]), &attr, doSomeThing, &pid[ i ]);
        if (err != 0)
            printf("\ncan't create thread :[%s]", strerror(err));
        else
            printf("\n Thread created successfully\n");

        i++;
    }
    for(;;)
    {
        printf("%d %d \n", count0, count1);
        sleep(1);
    }
    pthread_join(tid[0], 0);
    pthread_join(tid[1], 0);
    return 0;
}

syg00 · 04-29-2014, 02:14 AM

Try this and see how it might influence your understanding of what's going on

Code:

grep -i ^processor /proc/cpuinfo

(and in future use [code] tags so your posts are at least readable)

Jia Hong · 04-29-2014, 10:06 PM

Thanks for reply

I run the command and it reports two processors (0 and 1 ). Does scheduler place the two pthreads each on one of the two processors? What change should I make to the code so that high priority thread will block lower priority thread until high priority thread completes in multicore environment?

Code is reformatted using code tag. Please see original post above.

bigearsbilly · 04-30-2014, 01:59 AM

you will have to prioritise yourself I should imagine.
if both CPUs are available then obviously both threads can be running.

even better, keep away from threads and use simpler design unless absolutely necessary

sundialsvcs · 04-30-2014, 07:16 AM

Indeed, this is a flawed premise. The mere fact that one thread is running does not mean that another thread is not or should not. If you want to impose any sort of synchronization upon any two threads, then you must impose it, using mutexes or other appropriate synchronization objects. A process or thread may make no assumptions whatsoever as to what else is going on.