libCom/posix: add env var to opt out of FIFO scheduling

- add EPICS_ALLOW_POSIX_THREAD_PRIORITY_SCHEDULING
  (default: YES)
- start EPICS threads with default policy
  when set to NO

modules/libcom/src/env/envDefs.h

@@ -77,6 +77,7 @@ LIBCOM_API extern const ENV_PARAM IOCSH_PS1;
 LIBCOM_API extern const ENV_PARAM IOCSH_HISTSIZE;
 LIBCOM_API extern const ENV_PARAM IOCSH_HISTEDIT_DISABLE;
 LIBCOM_API extern const ENV_PARAM EPICS_ABORT_ON_ASSERT;
+LIBCOM_API extern const ENV_PARAM EPICS_ALLOW_POSIX_THREAD_PRIORITY_SCHEDULING;
 /** @brief List of all parameters.
  *
  * A NULL terminated array of all ENV_PARAM known to EPICS Base.

modules/libcom/src/osi/os/posix/osdThread.c

@@ -50,6 +50,7 @@
 #include "epicsAssert.h"
 #include "epicsExit.h"
 #include "epicsAtomic.h"
+#include "envDefs.h"
 
 LIBCOM_API void epicsThreadShowInfo(epicsThreadOSD *pthreadInfo, unsigned int level);
 LIBCOM_API void osdThreadHooksRun(epicsThreadId id);
@@ -93,6 +94,7 @@ static pthread_mutex_t listLock;
 static ELLLIST pthreadList = ELLLIST_INIT;
 static commonAttr *pcommonAttr = 0;
 static int epicsThreadOnceCalled = 0;
+static int wantPrioScheduling = 0;
 
 static epicsThreadOSD *createImplicit(void);
 
@@ -384,6 +386,7 @@ static void once(void)
     checkStatusOnce(status,"pthread_attr_getschedparam");
 
     findPriorityRange(pcommonAttr);
+    envGetBoolConfigParam(&EPICS_ALLOW_POSIX_THREAD_PRIORITY_SCHEDULING, &wantPrioScheduling);
 
     if(pcommonAttr->maxPriority == -1) {
         pcommonAttr->maxPriority = pcommonAttr->schedParam.sched_priority;
@@ -604,8 +607,10 @@ epicsThreadCreateOpt(const char * name,
         return 0;
 
     pthreadInfo->isEpicsThread = 1;
-    setSchedulingPolicy(pthreadInfo, SCHED_FIFO);
-    pthreadInfo->isRealTimeScheduled = 1;
+    if (wantPrioScheduling) {
+        setSchedulingPolicy(pthreadInfo, SCHED_FIFO);
+        pthreadInfo->isRealTimeScheduled = 1;
+    }
 
     if (pthreadInfo->joinable) {
         /* extra ref for epicsThreadMustJoin() */