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Subject[PATCH] SCHED_FIFO and SCHED_RR scheduler fix, kernel 2.4.18

The 2.4.18 kernel was behaving incorrectly for SCHED_FIFO and SCHED_RR
scheduling.

The correct behaviour for SCHED_FIFO is priority preemption:
run to completion, or system call, or preemption by higher priority
process. The correct behaviour for SCHED_RR is the same as SCHED_FIFO for
the preemption case, or run for a time slice, and go to the back of the
run queue for that priority.

More details can be found at:

http://www.opengroup.org/onlinepubs/7908799/xsh/realtime.html

As a side note, SCHED_RR is completely broken in the 2.2 series kernels.

This is a small patch, but fixes the behaviour for SCHED_FIFO and SCHED_RR
scheduling in the 2.4.18 kernel. It also improves the efficiency of the
kernel by NOT calling schedule() for every tick for a SCHED_FIFO process.

--
Bhavesh P. Davda
Avaya Inc
bhavesh@avaya.com
diff -Naur linux-2.4.18/kernel/sched.c linux-2.4.18-bpd/kernel/sched.c
--- linux-2.4.18/kernel/sched.c Fri Dec 21 10:42:04 2001
+++ linux-2.4.18-bpd/kernel/sched.c Wed Jun 12 09:12:12 2002
@@ -318,13 +318,17 @@
/*
* Careful!
*
- * This has to add the process to the _beginning_ of the
- * run-queue, not the end. See the comment about "This is
- * subtle" in the scheduler proper..
+ * This has to add the process to the _end_ of the
+ * run-queue, not the beginning. The goodness value will
+ * determine whether this process will run next. This is
+ * important to get SCHED_FIFO and SCHED_RR right, where
+ * a process that is either pre-empted or its time slice
+ * has expired, should be moved to the tail of the run
+ * queue for its priority - Bhavesh Davda
*/
static inline void add_to_runqueue(struct task_struct * p)
{
- list_add(&p->run_list, &runqueue_head);
+ list_add_tail(&p->run_list, &runqueue_head);
nr_running++;
}

@@ -334,12 +338,6 @@
list_add_tail(&p->run_list, &runqueue_head);
}

-static inline void move_first_runqueue(struct task_struct * p)
-{
- list_del(&p->run_list);
- list_add(&p->run_list, &runqueue_head);
-}
-
/*
* Wake up a process. Put it on the run-queue if it's not
* already there. The "current" process is always on the
@@ -955,8 +953,6 @@
retval = 0;
p->policy = policy;
p->rt_priority = lp.sched_priority;
- if (task_on_runqueue(p))
- move_first_runqueue(p);

current->need_resched = 1;

diff -Naur linux-2.4.18/kernel/timer.c linux-2.4.18-bpd/kernel/timer.c
--- linux-2.4.18/kernel/timer.c Mon Oct 8 11:41:41 2001
+++ linux-2.4.18-bpd/kernel/timer.c Wed Jun 12 09:13:43 2002
@@ -585,7 +585,14 @@
if (p->pid) {
if (--p->counter <= 0) {
p->counter = 0;
- p->need_resched = 1;
+ /*
+ * SCHED_FIFO is priority preemption, so this is
+ * not the place to decide whether to reschedule a
+ * SCHED_FIFO task or not - Bhavesh Davda
+ */
+ if (p->policy != SCHED_FIFO) {
+ p->need_resched = 1;
+ }
}
if (p->nice > 0)
kstat.per_cpu_nice[cpu] += user_tick;
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