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Subject[patch] SCHED_SOFTRR starve-free linux scheduling policy ...

This should (hopefully) avoid other tasks starvation exploits :

http://www.xmailserver.org/linux-patches/softrr.html

Making SCHED_TS_KSOFTRR a proc tunable might be an option.



- Davide




diff -Nru linux-2.5.74.vanilla/include/linux/sched.h linux-2.5.74.mod/include/linux/sched.h
--- linux-2.5.74.vanilla/include/linux/sched.h Mon Jul 7 16:57:33 2003
+++ linux-2.5.74.mod/include/linux/sched.h Sun Jul 13 14:33:10 2003
@@ -124,6 +124,8 @@
#define SCHED_NORMAL 0
#define SCHED_FIFO 1
#define SCHED_RR 2
+#define SCHED_SOFTRR 3
+

struct sched_param {
int sched_priority;
diff -Nru linux-2.5.74.vanilla/kernel/sched.c linux-2.5.74.mod/kernel/sched.c
--- linux-2.5.74.vanilla/kernel/sched.c Mon Jul 7 16:57:33 2003
+++ linux-2.5.74.mod/kernel/sched.c Sun Jul 13 14:45:42 2003
@@ -76,6 +76,7 @@
#define MAX_SLEEP_AVG (10*HZ)
#define STARVATION_LIMIT (10*HZ)
#define NODE_THRESHOLD 125
+#define SCHED_TS_KSOFTRR 4

/*
* If a task is 'interactive' then we reinsert it in the active
@@ -158,7 +159,7 @@
struct runqueue {
spinlock_t lock;
unsigned long nr_running, nr_switches, expired_timestamp,
- nr_uninterruptible;
+ nr_uninterruptible, ts_timestamp;
task_t *curr, *idle;
struct mm_struct *prev_mm;
prio_array_t *active, *expired, arrays[2];
@@ -1175,6 +1176,7 @@
void scheduler_tick(int user_ticks, int sys_ticks)
{
int cpu = smp_processor_id();
+ unsigned int time_slice;
runqueue_t *rq = this_rq();
task_t *p = current;

@@ -1216,17 +1218,32 @@
p->sleep_avg--;
if (unlikely(rt_task(p))) {
/*
- * RR tasks need a special form of timeslice management.
+ * RR and SOFTRR tasks need a special form of timeslice management.
* FIFO tasks have no timeslices.
*/
- if ((p->policy == SCHED_RR) && !--p->time_slice) {
- p->time_slice = task_timeslice(p);
+ if ((p->policy == SCHED_RR || p->policy == SCHED_SOFTRR) &&
+ !--p->time_slice) {
+ p->time_slice = time_slice = task_timeslice(p);
p->first_time_slice = 0;
set_tsk_need_resched(p);

- /* put it at the end of the queue: */
+ /*
+ * We rotate SCHED_RR like POSIX states. On the
+ * contrary, SCHED_SOFTRR are real-time tasks without
+ * attitude and we do not want them to starve other
+ * tasks while we want them to be able to preempt
+ * SCHED_NORMAL tasks. The rule is that SCHED_SOFTRR
+ * will be expired if they require roughly more then
+ * 1/SCHED_TS_KSOFTRR percent of CPU time.
+ */
dequeue_task(p, rq->active);
- enqueue_task(p, rq->active);
+ if (p->policy == SCHED_RR ||
+ (jiffies - rq->ts_timestamp) > SCHED_TS_KSOFTRR * time_slice)
+ enqueue_task(p, rq->active);
+ else
+ enqueue_task(p, rq->expired);
+
+ rq->ts_timestamp = jiffies;
}
goto out_unlock;
}
@@ -1243,6 +1260,8 @@
enqueue_task(p, rq->expired);
} else
enqueue_task(p, rq->active);
+
+ rq->ts_timestamp = jiffies;
}
out_unlock:
spin_unlock(&rq->lock);
@@ -1740,12 +1759,22 @@
else {
retval = -EINVAL;
if (policy != SCHED_FIFO && policy != SCHED_RR &&
- policy != SCHED_NORMAL)
+ policy != SCHED_NORMAL && policy != SCHED_SOFTRR)
goto out_unlock;
}

/*
- * Valid priorities for SCHED_FIFO and SCHED_RR are
+ * If the caller requested a SCHED_RR policy without having the
+ * necessary rights, we downgrade the policy to SCHED_SOFTRR. This
+ * is currrently here to enable to test the new SOFTRR realtime
+ * policy with existing programs that try to ask for SCHED_RR. Not
+ * sure if this should remain as permanent feature.
+ */
+ if (policy == SCHED_RR && !capable(CAP_SYS_NICE))
+ policy = SCHED_SOFTRR;
+
+ /*
+ * Valid priorities for SCHED_FIFO, SCHED_RR and SCHED_SOFTRR are
* 1..MAX_USER_RT_PRIO-1, valid priority for SCHED_NORMAL is 0.
*/
retval = -EINVAL;
@@ -2069,6 +2098,7 @@
switch (policy) {
case SCHED_FIFO:
case SCHED_RR:
+ case SCHED_SOFTRR:
ret = MAX_USER_RT_PRIO-1;
break;
case SCHED_NORMAL:
@@ -2092,6 +2122,7 @@
switch (policy) {
case SCHED_FIFO:
case SCHED_RR:
+ case SCHED_SOFTRR:
ret = 1;
break;
case SCHED_NORMAL:
-
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