Child first after fork violates the SCHED_FIFO and SCHED_RR standard.

[george anzinger]

The standard says a SCHED_FIFO task only gives up the processor if it
blocks, yields, or changes its priority.  

The counter is not really used by SCHED_FIFO tasks, however the
update_process_times() code will set the "need_resched" flag on a
SCHED_FIFO task, even though schedule() effectively ignores the entry. 
The attached patch addresses these issues by setting the counter to -100
for SCHED_FIFO tasks and "teaching" update_process_timers() to not count
down negative counters.  This avoids the calling of schedule() every
jiffie while a SCHED_FIFO task is running.

I tried to keep the change to recalculate confined to only the data
elements it was already touching, however, the standard really doesn't
allow recalculate to touch the SCHED_RR counter.  A standard conforming
test would restrict recalculate to only SCHED_OTHER tasks.

Comments?

George
diff -urP -X /usr/src/patch.exclude linux-2.4.4-kb/kernel/fork.c linux/kernel/fork.c
--- linux-2.4.4-kb/kernel/fork.c        Mon May  7 14:46:17 2001
+++ linux/kernel/fork.c Tue May  8 15:17:51 2001
@@ -673,10 +673,14 @@
         * if the child for a fork() just wants to do a few simple things
         * and then exec(). This is only important in the first timeslice.
         * In the long run, the scheduling behavior is unchanged.
+         * SCHED_FIFO tasks don't count down and have a negative counter.
+         * Don't change these, least they all end up at -1.
         */
-       p->counter = current->counter;
-       current->counter = 0;
-       current->need_resched = 1;
+        if (p->policy == SCHED_OTHER){
+                p->counter = current->counter;
+                current->counter = 0;
+                current->need_resched = 1;
+        }
 
        /*
         * Ok, add it to the run-queues and make it
diff -urP -X /usr/src/patch.exclude linux-2.4.4-kb/kernel/sched.c linux/kernel/sched.c
--- linux-2.4.4-kb/kernel/sched.c       Mon May  7 14:46:17 2001
+++ linux/kernel/sched.c        Tue May  8 13:43:54 2001
@@ -682,7 +682,10 @@
                spin_unlock_irq(&runqueue_lock);
                read_lock(&tasklist_lock);
                for_each_task(p)
-                       p->counter = (p->counter >> 1) + NICE_TO_TICKS(p->nice);
+                        if (p->counter >= 0 ){
+                                p->counter = (p->counter >> 1) + 
+                                        NICE_TO_TICKS(p->nice);
+                        }
                read_unlock(&tasklist_lock);
                spin_lock_irq(&runqueue_lock);
        }
@@ -932,6 +935,11 @@
 
        retval = 0;
        p->policy = policy;
+        if ( policy == SCHED_FIFO) {
+                p->counter = -100;        /* we don't count down neg couters */
+        }else{
+                p->counter = NICE_TO_TICKS(p->nice);
+        }
        p->rt_priority = lp.sched_priority;
        if (task_on_runqueue(p))
                move_first_runqueue(p);
diff -urP -X /usr/src/patch.exclude linux-2.4.4-kb/kernel/timer.c linux/kernel/timer.c
--- linux-2.4.4-kb/kernel/timer.c       Sun Dec 10 09:53:19 2000
+++ linux/kernel/timer.c        Tue May  8 15:14:37 2001
@@ -583,7 +583,11 @@
 
        update_one_process(p, user_tick, system, cpu);
        if (p->pid) {
-               if (--p->counter <= 0) {
+                /*
+                 * SCHED_FIFO and the idle(s) have counters set to -100, 
+                 * so we won't count them.
+                 */
+               if (p->counter >= 0 && --p->counter <= 0) {
                        p->counter = 0;
                        p->need_resched = 1;
                }