sched: speed up and simplify vslice calculations

speed up and simplify vslice calculations.

[ From: Mike Galbraith <efault@gmx.de>: build fix ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/sched.h

@@ -1400,7 +1400,7 @@ extern void sched_idle_next(void);
 
 #ifdef CONFIG_SCHED_DEBUG
 extern unsigned int sysctl_sched_latency;
-extern unsigned int sysctl_sched_min_granularity;
+extern unsigned int sysctl_sched_nr_latency;
 extern unsigned int sysctl_sched_wakeup_granularity;
 extern unsigned int sysctl_sched_batch_wakeup_granularity;
 extern unsigned int sysctl_sched_child_runs_first;

kernel/sched_debug.c

@@ -210,7 +210,7 @@ static int sched_debug_show(struct seq_file *m, void *v)
 #define PN(x) \
 	SEQ_printf(m, "  .%-40s: %Ld.%06ld\n", #x, SPLIT_NS(x))
 	PN(sysctl_sched_latency);
-	PN(sysctl_sched_min_granularity);
+	PN(sysctl_sched_nr_latency);
 	PN(sysctl_sched_wakeup_granularity);
 	PN(sysctl_sched_batch_wakeup_granularity);
 	PN(sysctl_sched_child_runs_first);

kernel/sched_fair.c

@@ -46,7 +46,7 @@ const_debug unsigned int sysctl_sched_child_runs_first = 1;
  * Minimal preemption granularity for CPU-bound tasks:
  * (default: 2 msec, units: nanoseconds)
  */
-unsigned int sysctl_sched_min_granularity __read_mostly = 2000000ULL;
+const_debug unsigned int sysctl_sched_nr_latency = 20;
 
 /*
  * sys_sched_yield() compat mode
@@ -222,8 +222,7 @@ static inline struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq)
 static u64 __sched_period(unsigned long nr_running)
 {
 	u64 period = sysctl_sched_latency;
-	unsigned long nr_latency =
-		sysctl_sched_latency / sysctl_sched_min_granularity;
+	unsigned long nr_latency = sysctl_sched_nr_latency;
 
 	if (unlikely(nr_running > nr_latency)) {
 		period *= nr_running;
@@ -245,11 +244,15 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
 
 static u64 __sched_vslice(unsigned long nr_running)
 {
-	u64 period = __sched_period(nr_running);
+	unsigned long period = sysctl_sched_latency;
+	unsigned long nr_latency = sysctl_sched_nr_latency;
 
-	do_div(period, nr_running);
+	if (unlikely(nr_running > nr_latency))
+		nr_running = nr_latency;
 
-	return period;
+	period /= nr_running;
+
+	return (u64)period;
 }
 
 /*

kernel/sysctl.c

@@ -222,14 +222,11 @@ static ctl_table kern_table[] = {
 #ifdef CONFIG_SCHED_DEBUG
 	{
 		.ctl_name	= CTL_UNNUMBERED,
-		.procname	= "sched_min_granularity_ns",
-		.data		= &sysctl_sched_min_granularity,
+		.procname	= "sched_nr_latency",
+		.data		= &sysctl_sched_nr_latency,
 		.maxlen		= sizeof(unsigned int),
 		.mode		= 0644,
-		.proc_handler	= &proc_dointvec_minmax,
-		.strategy	= &sysctl_intvec,
-		.extra1		= &min_sched_granularity_ns,
-		.extra2		= &max_sched_granularity_ns,
+		.proc_handler	= &proc_dointvec,
 	},
 	{
 		.ctl_name	= CTL_UNNUMBERED,