[RFC][PATCH] SCHED_ISO for interactivity

[Con Kolivas <kernel@kolivas.org>]

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After the recent discussions about the limitations on real time tasks and 
normal users...

Wli coined the term "isochronous" (greek for same time) for a real time task 
that was limited in it's timeslice but still guaranteed to run. I've decided 
to abuse this term and use it to name this new policy in this patch. This is 
neither real time, nor guaranteed.

What this patch does is introduce a new scheduler policy, SCHED_ISO. SI tasks 
get the maximum interactive bonus so they always get a dynamic priority of 5 
better than their static priority, but are penalised by having a smaller 
timeslice. Most interactive tasks don't use up their full timeslice anyway.

Since most users and applications are not going to be altering scheduler 
policies, this patch uses the current semantics in patches up to the O4int 
patch I have posted previously (and in 2.5.75-mm1) to dynamically allocate 
tasks as SI. This happens when their sleep avg hits MAX_SLEEP_AVG which is 
extremely unlikely to happen for the wrong task. Unfortunately it also means 
it takes at least 10 seconds. There are semantics in place to stop it 
happening to idle tasks, tasks forking, and new children.

Anyway it's pretty rough around the edges since I slapped it together more to 
prove it works, but appears to work as planned. The O*int patches appear to 
be hitting a ceiling to their ability and this can help them further but I 
need some feedback about it's usefulness, and appropriateness. 

Without further feedback about the O*int patches I can't do much to develop 
them further.

akpm: obviously this one is not to be included in -mm 

Enough talk. The patch is attached below and available at:
http://kernel.kolivas.org/2.5
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--- linux-2.5.74-mm3/include/linux/sched.h	2003-07-10 10:23:14.000000000 +1000
+++ linux-2.5.74-test/include/linux/sched.h	2003-07-11 19:35:37.000000000 +1000
@@ -125,6 +125,7 @@ extern unsigned long nr_iowait(void);
 #define SCHED_NORMAL		0
 #define SCHED_FIFO		1
 #define SCHED_RR		2
+#define SCHED_ISO		3
 
 struct sched_param {
 	int sched_priority;
--- linux-2.5.74-mm3/kernel/exit.c	2003-07-10 10:23:14.000000000 +1000
+++ linux-2.5.74-test/kernel/exit.c	2003-07-11 19:35:37.000000000 +1000
@@ -223,7 +223,7 @@ void reparent_to_init(void)
 	/* Set the exit signal to SIGCHLD so we signal init on exit */
 	current->exit_signal = SIGCHLD;
 
-	if ((current->policy == SCHED_NORMAL) && (task_nice(current) < 0))
+	if ((current->policy == SCHED_NORMAL || current->policy == SCHED_ISO) && (task_nice(current) < 0))
 		set_user_nice(current, 0);
 	/* cpus_allowed? */
 	/* rt_priority? */
--- linux-2.5.74-mm3/kernel/sched.c	2003-07-11 19:43:34.000000000 +1000
+++ linux-2.5.74-test/kernel/sched.c	2003-07-11 19:38:09.000000000 +1000
@@ -76,7 +76,6 @@
 #define MIN_SLEEP_AVG		(HZ)
 #define MAX_SLEEP_AVG		(10*HZ)
 #define STARVATION_LIMIT	(10*HZ)
-#define SLEEP_BUFFER		(HZ/20)
 #define NODE_THRESHOLD		125
 #define MAX_BONUS		((MAX_USER_PRIO - MAX_RT_PRIO) * PRIO_BONUS_RATIO / 100)
 
@@ -118,6 +117,8 @@
 #define TASK_INTERACTIVE(p) \
 	((p)->prio <= (p)->static_prio - DELTA(p))
 
+#define iso_task(p)		((p)->policy == SCHED_ISO)
+
 /*
  * BASE_TIMESLICE scales user-nice values [ -20 ... 19 ]
  * to time slice values.
@@ -134,7 +135,10 @@
 
 static inline unsigned int task_timeslice(task_t *p)
 {
-	return BASE_TIMESLICE(p);
+	if (iso_task(p))
+		return (BASE_TIMESLICE(p) / 5 ? : 1);
+	else
+		return BASE_TIMESLICE(p);
 }
 
 /*
@@ -319,6 +323,10 @@ static inline void normalise_sleep(task_
 
 	p->sleep_avg = p->sleep_avg * MIN_SLEEP_AVG / old_avg_time;
 	p->avg_start = jiffies - MIN_SLEEP_AVG;
+
+	if (iso_task(p))
+		p->policy = SCHED_NORMAL;
+
 }
 
 /*
@@ -343,26 +351,32 @@ static int effective_prio(task_t *p)
 	if (rt_task(p))
 		return p->prio;
 
-	sleep_period = jiffies - p->avg_start;
+	if (!iso_task(p)){
+		sleep_period = jiffies - p->avg_start;
 
-	if (unlikely(!sleep_period))
-		return p->static_prio;
+		if (unlikely(!sleep_period))
+			return p->static_prio;
 
-	if (sleep_period > MAX_SLEEP_AVG)
-		sleep_period = MAX_SLEEP_AVG;
+		if (sleep_period > MAX_SLEEP_AVG)
+			sleep_period = MAX_SLEEP_AVG;
 
-	if (p->sleep_avg > sleep_period)
-		sleep_period = p->sleep_avg;
+		if (p->sleep_avg > sleep_period)
+			sleep_period = p->sleep_avg;
 
-	/*
-	 * The bonus is determined according to the accumulated
-	 * sleep avg over the duration the task has been running
-	 * until it reaches MAX_SLEEP_AVG. -ck
-	 */
-	bonus = MAX_USER_PRIO*PRIO_BONUS_RATIO*p->sleep_avg/sleep_period/100 -
-			MAX_USER_PRIO*PRIO_BONUS_RATIO/100/2;
+		/*
+		 * The bonus is determined according to the accumulated
+		 * sleep avg over the duration the task has been running
+		 * until it reaches MAX_SLEEP_AVG. -ck
+		 */
+		bonus = MAX_USER_PRIO*PRIO_BONUS_RATIO*p->sleep_avg/sleep_period/100 -
+				MAX_USER_PRIO*PRIO_BONUS_RATIO/100/2;
+
+	} else
+		bonus = MAX_USER_PRIO*PRIO_BONUS_RATIO/100 -
+				MAX_USER_PRIO*PRIO_BONUS_RATIO/100/2;
 
 	prio = p->static_prio - bonus;
+
 	if (prio < MAX_RT_PRIO)
 		prio = MAX_RT_PRIO;
 	if (prio > MAX_PRIO-1)
@@ -401,6 +415,8 @@ static inline void activate_task(task_t 
 			p->avg_start = jiffies - MIN_SLEEP_AVG;
 			p->sleep_avg = MIN_SLEEP_AVG * (MAX_BONUS - INTERACTIVE_DELTA - 1) /
 				MAX_BONUS;
+			if (iso_task(p))
+				p->policy = SCHED_NORMAL;
 		} else {
 			/*
 			 * This code gives a bonus to interactive tasks.
@@ -422,13 +438,11 @@ static inline void activate_task(task_t 
 					(MAX_SLEEP_AVG + MIN_SLEEP_AVG - runtime) *
 					(MAX_BONUS - INTERACTIVE_DELTA) / MAX_BONUS / MAX_SLEEP_AVG;
 
-			/*
-			 * Keep a small buffer of SLEEP_BUFFER sleep_avg to
-			 * prevent fully interactive tasks from becoming
-			 * lower priority with small bursts of cpu usage.
-			 */
-			if (p->sleep_avg > (MAX_SLEEP_AVG + SLEEP_BUFFER))
-				p->sleep_avg = MAX_SLEEP_AVG + SLEEP_BUFFER;
+			if (p->sleep_avg > MAX_SLEEP_AVG){
+				if (p->policy == SCHED_NORMAL)
+					p->policy = SCHED_ISO;
+				p->sleep_avg = MAX_SLEEP_AVG;
+			}
 		}
 
 		if (unlikely(p->avg_start > jiffies)){
@@ -1813,7 +1827,7 @@ static int setscheduler(pid_t pid, int p
 	else {
 		retval = -EINVAL;
 		if (policy != SCHED_FIFO && policy != SCHED_RR &&
-				policy != SCHED_NORMAL)
+				policy != SCHED_NORMAL && policy != SCHED_ISO)
 			goto out_unlock;
 	}
 
@@ -1824,7 +1838,7 @@ static int setscheduler(pid_t pid, int p
 	retval = -EINVAL;
 	if (lp.sched_priority < 0 || lp.sched_priority > MAX_USER_RT_PRIO-1)
 		goto out_unlock;
-	if ((policy == SCHED_NORMAL) != (lp.sched_priority == 0))
+	if ((policy == SCHED_NORMAL || policy == SCHED_ISO) != (lp.sched_priority == 0))
 		goto out_unlock;
 
 	retval = -EPERM;
@@ -1846,7 +1860,7 @@ static int setscheduler(pid_t pid, int p
 	p->policy = policy;
 	p->rt_priority = lp.sched_priority;
 	oldprio = p->prio;
-	if (policy != SCHED_NORMAL)
+	if (policy == SCHED_FIFO || policy == SCHED_RR)
 		p->prio = MAX_USER_RT_PRIO-1 - p->rt_priority;
 	else
 		p->prio = p->static_prio;
@@ -2147,6 +2161,9 @@ asmlinkage long sys_sched_get_priority_m
 	case SCHED_NORMAL:
 		ret = 0;
 		break;
+	case SCHED_ISO:
+		ret = 0;
+		break;
 	}
 	return ret;
 }
@@ -2169,6 +2186,8 @@ asmlinkage long sys_sched_get_priority_m
 		break;
 	case SCHED_NORMAL:
 		ret = 0;
+	case SCHED_ISO:
+		ret = 0;
 	}
 	return ret;
 }