Question

我已编写测试程序来测试SCHED_FIFO。我了解到SCHED_FIFO线程无法抢占SCHED_OTHER。但我无法解释同一个程序多次运行时获得的结果。

/* Includes */
#include <unistd.h>     /* Symbolic Constants */
#include <sys/types.h>  /* Primitive System Data Types */ 
#include <errno.h>      /* Errors */
#include <stdio.h>      /* Input/Output */
#include <stdlib.h>     /* General Utilities */
#include <pthread.h>    /* POSIX Threads */
#include <string.h>     /* String handling */
#include <sched.h>
/* prototype for thread routine */
void print_message_function ( void *ptr );
void print_message_function1 ( void *ptr );

/* struct to hold data to be passed to a thread
this shows how multiple data items can be passed to a thread */
typedef struct str_thdata
{
int thread_no;
int thread_value;
char message[100];
  } thdata;

int main()
  {
pthread_t thread1, thread2;  /* thread variables */
thdata data1, data2;         /* structs to be passed to threads */

/* initialize data to pass to thread 1 */
data1.thread_no = 1;
data1.thread_value = 0;
strcpy(data1.message, "Hello!");

/* initialize data to pass to thread 2 */
data2.thread_no = 2;
data2.thread_value = 10000;
strcpy(data2.message, "Hi!");

/* create threads 1 and 2 */    
pthread_create (&thread1, NULL, (void *) &print_message_function, (void *) &data1);
pthread_create (&thread2, NULL, (void *) &print_message_function1, (void *) &data2);

/* Main block now waits for both threads to terminate, before it exits
   If main block exits, both threads exit, even if the threads have not
   finished their work */ 
pthread_join(thread1, NULL);
pthread_join(thread2, NULL);

/* exit */  
exit(0);
} /* main() */

/** 
* print_message_function is used as the start routine for the threads used
* it accepts a void pointer 
**/
void print_message_function ( void *ptr )
{

thdata *data;            
data = (thdata *) ptr;  /* type cast to a pointer to thdata */

struct sched_param param;
//int priority=10;
/* sched_priority will be the priority of the thread */
//param.sched_priority = priority;
/* only supported policy, others will result in ENOTSUP */

int policy = SCHED_OTHER;
/* scheduling parameters of target thread */
pthread_setschedparam(pthread_self(), policy, &param);
printf("Thread %d says sched policy  %d \n", data->thread_no, SCHED_OTHER);
pthread_getschedparam(pthread_self(),&policy,&param);

printf("Thread %d says %s  %d \n", data->thread_no, data->message,policy);

int i=0;
/* do the work */
printf("Thread %d says %s %d \n", data->thread_no, data->message,(int)pthread_self());
for(i=0;i<100;i++)

printf("Thread %d says %d \n", data->thread_no,data->thread_value++);
pthread_exit(0); /* exit */
} /* print_message_function ( void *ptr ) */



void print_message_function1 ( void *ptr )
{

thdata *data;            
data = (thdata *) ptr;  /* type cast to a pointer to thdata */

struct sched_param param;
int priority=10;
/* sched_priority will be the priority of the thread */
param.sched_priority = priority;
/* only supported policy, others will result in ENOTSUP * /

int policy = SCHED_FIFO;
/* scheduling parameters of target thread */
pthread_setschedparam(pthread_self(), policy, &param);
printf("Thread %d says sched policy %d \n", data->thread_no, SCHED_FIFO);

pthread_getschedparam(pthread_self(),&policy,&param);

printf("Thread %d says %s  %d \n", data->thread_no, data->message,policy);

int i=0;
/* do the work */
printf("Thread %d says %s  %d \n", data->thread_no, data->message,(int)pthread_self());
for(i=0;i<100;i++)

printf("Thread %d says %d \n", data->thread_no,data->thread_value++);
pthread_exit(0); /* exit */
} /* print_message_function ( void *ptr ) */

我在多次运行中得到意外的结果，我看到SCHED_FIFO被SCHED_OTHER线程抢占，即根据程序，线程2处于FIFO模式，而线程1是SCHED_OTHER模式。我已经多次看到thread2被thread1抢占了。

有人可以帮助我找到问题吗？

Answer 1

您可能有效的这些sysctl设置是默认值：

kernel.sched_rt_period_us = 1000000
kernel.sched_rt_runtime_us = 950000

这意味着允许实时线程每1秒钟仅占95％。

另见：Can't provoke Priority Inversion in C++

Answer 2

此外，只要您的线程阻止IO，可能来自printf语句，就可以安排另一个线程。

SCHED_FIFO线程被Linux中的SCHED_OTHER线程抢占

2 个答案: