Child first after fork violates the SCHED_FIFO and SCHED_RR standard.

george anzinger
2001-05-08T22:32:37Z

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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

[-- Attachment #2: fifo-2.4.4.patch --]
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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;
}