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--- realtime:documentation:howto:howto_rt_application_base [2016/09/27 16:58]
anna-maria adapt link to sched_policy_prio
+++ — (current)
@@ Line 1: / Line 1: @@
-====== HOWTO build a simple RT application ======
-The POSIX API forms the basis of real-time applications running under
-PREEMPT_RT.  For the real-time thread a POSIX thread is used
-(pthread).  Every real-time application needs proper handling in
-several basic areas like scheduling, priority, memory locking and
-stack prefaulting.
-===== Basic prerequisites =====
-Three basic prerequisites are introduced in the next subsections,
-followed by a short example illustrating those aspects.
-==== Scheduling and priority ====
-The [[realtime:documentation:technical_basics:sched_policy_prio|scheduling policy]] as well as the priority
-must be set by the application explicitly. There are two possibilities
-for this:
-  - **Using ''sched_setscheduler()''** \\  <wrap>
-This funcion needs to be called in the start routine of the pthread
-before calculating RT specific stuff.
-</wrap>
-  - **Using pthread attributes** \\ <wrap>
-The functions ''pthread_attr_setschedpolicy()'' and
-''pthread_attr_setschedparam()''offer the interfaces to set policy and
-priority.  Furthermore scheduler inheritance needs to be set properly
-to PTHREAD_EXPLICIT_SCHED by using ''pthread_attr_setinheritsched()''.
-This forces the new thread to use the policy and priority specified by
-the pthread attributes and not to use the inherit scheduling of the
-thread which created the real-time thread.
-</wrap>
-==== Memory locking ====
-In real-time applications it is important to avoid non-deterministic
-behavior.  If the memory that is needed by the real-time application
-is not locked in the RAM, this memory could be paged out.  If the
-memory is not paged in when the application tries to access the
-memory, a page fault occurs causing non-deterministic high latency.
-For this reason memory should be locked in real-time applications.
-The memory lock persists until the process owning it terminates or
-explicitly unlocks it by calling ''munlock()'' or ''munlockall()''.
-Be aware that page faults due to paged out memory occur in systems
-with swap as well as in systems without swap.  In addition, the binary
-of the executed application itself could be paged out.
-The following call of ''mlockall()'' locks all current pages mapped
-into the address space of the process as well as all pages that will
-be mapped in the future.
-  mlockall(MCL_CURRENT|MCL_FUTURE);
-==== Stack prefaulting ====
-Since page faults cause non-deterministic behavior, the stack should
-be prefaulted before the real-time critical section starts.
-In case several real-time threads are used, it should be done for each
-thread individually.  In the following example, a memory block of a
-certain size is allocated.  All of its pages are touched to get them
-mapped into RAM ensuring that no page faults occur later.
-  void *buffer;
-  buffer = mmap(NULL, MSIZE, PROT_READ | PROT_WRITE,
-                MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
-  memset(&buffer, 0, MSIZE);
-The prefaulted stack can be assigned to a thread.  (''&attr'' is a
-''pthread_attr_t'' pointer; the pthread attribute needs to have been
-previously initialized):
-  pthread_attr_setstack(&attr, buffer, PTHREAD_STACK_MIN);
-===== Example =====
-<code c>
-/*
- * POSIX Real Time Example
- * using a single pthread as RT thread
- */
-#include <stdlib.h>
-#include <stdio.h>
-#include <time.h>
-#include <sched.h>
-#include <sys/mman.h>
-#include <string.h>
-#include <pthread.h>
-#include <limits.h>
-void *thread_func(void *data)
-{
-        /* Do RT specific stuff here */
-        return NULL;
-}
-int main(int argc, char* argv[])
-{
-        struct sched_param param;
-        void *stack_buf;
-        pthread_t thread;
-        pthread_attr_t attr;
-        int ret;
-        /* Lock memory */
-        if(mlockall(MCL_CURRENT|MCL_FUTURE) == -1) {
-                printf("mlockall failed: %m\n");
-                exit(-2);
-        }
-        /* Pre-fault stack for the thread */
-        stack_buf = mmap(NULL, PTHREAD_STACK_MIN, PROT_READ | PROT_WRITE,
-                         MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
-        if (stack_buf == MAP_FAILED) {
-                printf("mmap failed: %m\n");
-                exit(-1);
-        }
-        memset(stack_buf, 0, PTHREAD_STACK_MIN);
-        /* Initialize pthread attributes (default values) */
-        ret = pthread_attr_init(&attr);
-        if (ret) {
-                printf("init pthread attributes failed\n");
-                goto out;
-        }
-        /* Set pthread stack to already pre-faulted stack */
-        ret = pthread_attr_setstack(&attr, stack_buf, PTHREAD_STACK_MIN);
-        if (ret) {
-                printf("pthread setstack failed\n");
-                goto out;
-        }
-        /* Set scheduler policy and priority of pthread */
-        ret = pthread_attr_setschedpolicy(&attr, SCHED_FIFO);
-        if (ret) {
-                printf("pthread setschedpolicy failed\n");
-                goto out;
-        }
-        param.sched_priority = 80;
-        ret = pthread_attr_setschedparam(&attr, &param);
-        if (ret) {
-                printf("pthread setschedparam failed\n");
-                goto out;
-        }
-        /* Use scheduling parameters of attr */
-        ret = pthread_attr_setinheritsched(&attr, PTHREAD_EXPLICIT_SCHED);
-        if (ret) {
-                printf("pthread setinheritsched failed\n");
-                goto out;
-        }
-        /* Create a pthread with specified attributes */
-        ret = pthread_create(&thread, &attr, thread_func, NULL);
-        if (ret) {
-                printf("create pthread failed\n");
-                goto out;
-        }
-        /* Join the thread and wait until it is done */
-        ret = pthread_join(thread, NULL);
-        if (ret)
-                printf("join pthread failed: %m\n");
-out:
-        munmap(stack_buf, PTHREAD_STACK_MIN);
-        return ret;
-}
-</code>

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