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SubjectScheduling a higher priority thread returning from clock_nanosleep()
Hi all,



The following test program intends to implement periodic threads based on POSIX API.



According to a POSIX recommendation found in clock_nanosleep() man page (3p), this system call is prefered to POSIX timers. I used the absolute time version of clock_nanosleep.



Scenario is the following: a high priority (p=50) thread executes a clock_nanosleep. At the timespec clock_nanosleep() is supposed to return, a low priority (p=1) CPU-intensive thread may be executing (this happens on a regular basis since the low priority periodic thread performs a +1 second busy wait). Unfortunately, although the high priority thread becomes eligible, the low priority CPU-intensive thread is not preemted.



Behavior is the same with both SCHED_FIFO and SCHED_RR.



Kernel is 2.6.12.



Can anyone tell me whether I am misusing the API, or if this is a known pitfall of linux's POSIX port (or - who knows ? - if this is behavior is conformant with POSIX ?)



If my kernel is too old, could you recommend a release / patch ?



I would very much appreciate your help.



Gregory Haik





--



Compile with g++ test.c -lm -lrt

Run as root.



test.c :



#define ONE_MILLION 1000000

#define ONE_BILLION 1000000000



#include <time.h> // clock_nanosleep()

#include <sys/time.h> // gettimeofday() in busy wait implem

#include <pthread.h>

#include <errno.h>

#include <math.h>

#include <iostream>



using namespace std;



int nb_iters_loop = 100000;

float duration_loop;



void calibrate_busy_wait()

{

nb_iters_loop = 100000;

struct timeval begin;

struct timeval end;



gettimeofday( &begin, 0 );



double value;



for( int ct = 0; ct < nb_iters_loop; ct++ )

{

value +=

value + log( 10 + value + log( 10 + value + log( 10 + value ) ) );

}



gettimeofday( &end, 0 );



duration_loop =

( end.tv_sec - begin.tv_sec ) + 1e-6 * ( end.tv_usec - begin.tv_usec );

}



void do_busy_wait( float seconds )

{



int nb_iterations_to_do = nb_iters_loop * ( seconds / duration_loop );



float value;



for( int ct = 0; ct < nb_iterations_to_do; ct++ )

{

value +=

value + log( 10 + value + log( 10 + value + log( 10 + value ) ) );

}

}



void * main_thread_high (void * arg)

{

int c = 0;

struct timespec ts;

int period = 10; // ms



int status = clock_gettime(CLOCK_REALTIME, &ts);

if (status != 0)

{

perror("clock_gettime");

exit(123);

}





unsigned long period_ms_ulong = (unsigned long) period;

unsigned long one_million_ulong = (unsigned long) ONE_MILLION;

unsigned long period_ns_ulong = (unsigned long) (period_ms_ulong * one_million_ulong);

unsigned long one_billion_ulong = (unsigned long) ONE_BILLION;

unsigned long additional_nanoseconds = (unsigned long) (period_ns_ulong % one_billion_ulong);



time_t additional_seconds = period / 1000;



while (true)

{

int status = clock_nanosleep(CLOCK_REALTIME, TIMER_ABSTIME, &ts, NULL);

if (status != 0)

{

switch (status)

{

case EINTR:

perror("clock_nanosleep() returned EINTR");

break;

case EINVAL:

perror("clock_nanosleep() returned EINVAL");

break;

case ENOTSUP:

perror("clock_nanosleep() returned ENOTSUP");

}

exit(1243);

}



ts.tv_nsec += additional_nanoseconds;

ts.tv_sec += additional_seconds;

if (ts.tv_nsec >= one_billion_ulong)

{

ts.tv_sec++;

ts.tv_nsec -= one_billion_ulong;

}





do_busy_wait((float) (period / 2000));

{ int sched_policy;

sched_param sp;

pthread_getschedparam(pthread_self(), &sched_policy, &sp);



cerr << __FILE__ << " : " << __LINE__ << " busy wait done, prio = " << sp.sched_priority << ", " << c++ << endl;

}



}

}



void * main_thread_low (void * arg)

{

int c = 0;

struct timespec ts;

int period = 2050; // ms



int status = clock_gettime(CLOCK_REALTIME, &ts);

if (status != 0)

{

perror("clock_gettime");

exit(123);

}





unsigned long period_ms_ulong = (unsigned long) period;

unsigned long one_million_ulong = (unsigned long) ONE_MILLION;

unsigned long period_ns_ulong = (unsigned long) (period_ms_ulong * one_million_ulong);

unsigned long one_billion_ulong = (unsigned long) ONE_BILLION;

unsigned long additional_nanoseconds = (unsigned long) (period_ns_ulong % one_billion_ulong);



time_t additional_seconds = period / 1000;



while (true)

{



int status = clock_nanosleep(CLOCK_REALTIME, TIMER_ABSTIME, &ts, NULL);

if (status != 0)

{

switch (status)

{

case EINTR:

perror("clock_nanosleep() returned EINTR");

break;

case EINVAL:

perror("clock_nanosleep() returned EINVAL");

break;

case ENOTSUP:

perror("clock_nanosleep() returned ENOTSUP");

}

exit(1243);

}



ts.tv_nsec += additional_nanoseconds;

ts.tv_sec += additional_seconds;

if (ts.tv_nsec >= one_billion_ulong)

{

ts.tv_sec++;

ts.tv_nsec -= one_billion_ulong;

}



do_busy_wait((float) (period / 2000));

{ int sched_policy;

sched_param sp;

pthread_getschedparam(pthread_self(), &sched_policy, &sp);



cerr << __FILE__ << " : " << __LINE__ << " busy wait done, prio = " << sp.sched_priority << ", " << c++ << endl;

}

}

}



int main()

{



calibrate_busy_wait();



pthread_t periodic_thread_high;

{

pthread_attr_t thread_attributes;

int status;



pthread_attr_init(&thread_attributes);

if (status != 0) perror("pthread_attr_init failed ");



status = pthread_attr_setschedpolicy(&thread_attributes, SCHED_FIFO);

if (status != 0) perror("pthread_set_schedpolicy failed ");



struct sched_param sched_parameter;

sched_parameter.sched_priority = 50;

status = pthread_attr_setschedparam(&thread_attributes, &sched_parameter);

if (status != 0) perror("pthread_setschedparam falied ");



status = pthread_attr_setinheritsched(&thread_attributes, PTHREAD_EXPLICIT_SCHED);

if (status != 0) perror("pthread_attr_setinheritsched failed");



status = pthread_attr_setscope( &thread_attributes, PTHREAD_SCOPE_SYSTEM );

if (status != 0) perror("pthread_attr_setscope failed ");



status = pthread_create(&periodic_thread_high, &thread_attributes, main_thread_high, NULL);

if (status != 0) perror("pthread_create failed ");

}



pthread_t periodic_thread_low;

{

pthread_attr_t thread_attributes;

int status;



pthread_attr_init(&thread_attributes);

if (status != 0) perror("pthread_attr_init failed ");



status = pthread_attr_setschedpolicy(&thread_attributes, SCHED_FIFO);

if (status != 0) perror("pthread_set_schedpolicy failed ");



struct sched_param sched_parameter;

sched_parameter.sched_priority = 1;

status = pthread_attr_setschedparam(&thread_attributes, &sched_parameter);

if (status != 0) perror("pthread_setschedparam falied ");



status = pthread_attr_setinheritsched(&thread_attributes, PTHREAD_EXPLICIT_SCHED);

if (status != 0) perror("pthread_attr_setinheritsched failed");



status = pthread_attr_setscope( &thread_attributes, PTHREAD_SCOPE_SYSTEM );

if (status != 0) perror("pthread_attr_setscope failed ");



status = pthread_create(&periodic_thread_low, &thread_attributes, main_thread_low, NULL);

if (status != 0) perror("pthread_create failed ");



}



pthread_join(periodic_thread_high, NULL);

pthread_join(periodic_thread_low, NULL);



cout << "Hopefully this message will never be issued." << endl;

}







/// sorry for the Yoohoo ad below...








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