sched: use a 2-d bitmap for searching lowest-pri CPU

The current code use a linear algorithm which causes scaling issues
on larger SMP machines.  This patch replaces that algorithm with a
2-dimensional bitmap to reduce latencies in the wake-up path.

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
Acked-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

5 files changed, 239 insertions, 77 deletions

diff --git a/kernel/Makefile b/kernel/Makefile
index 1c9938addb9d..ecdd2d335639 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -69,6 +69,7 @@ obj-$(CONFIG_TASK_DELAY_ACCT) += delayacct.o
 obj-$(CONFIG_TASKSTATS) += taskstats.o tsacct.o
 obj-$(CONFIG_MARKERS) += marker.o
 obj-$(CONFIG_LATENCYTOP) += latencytop.o
+obj-$(CONFIG_SMP) += sched_cpupri.o
 
 ifneq ($(CONFIG_SCHED_NO_NO_OMIT_FRAME_POINTER),y)
 # According to Alan Modra <alan@linuxcare.com.au>, the -fno-omit-frame-pointer is
diff --git a/kernel/sched.c b/kernel/sched.c
index aa960b84b881..8a1257b65560 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -74,6 +74,8 @@
 #include <asm/tlb.h>
 #include <asm/irq_regs.h>
 
+#include "sched_cpupri.h"
+
 /*
  * Convert user-nice values [ -20 ... 0 ... 19 ]
  * to static priority [ MAX_RT_PRIO..MAX_PRIO-1 ],
@@ -450,6 +452,9 @@ struct root_domain {
          */
         cpumask_t rto_mask;
         atomic_t rto_count;
+#ifdef CONFIG_SMP
+        struct cpupri cpupri;
+#endif
 };
 
 /*
@@ -6392,6 +6397,8 @@ static void init_rootdomain(struct root_domain *rd)
 
         cpus_clear(rd->span);
         cpus_clear(rd->online);
+
+        cpupri_init(&rd->cpupri);
 }
 
 static void init_defrootdomain(void)
diff --git a/kernel/sched_cpupri.c b/kernel/sched_cpupri.c
new file mode 100644
index 000000000000..52154fefab7e
--- /dev/null
+++ b/kernel/sched_cpupri.c
@@ -0,0 +1,174 @@
+/*
+ *  kernel/sched_cpupri.c
+ *
+ *  CPU priority management
+ *
+ *  Copyright (C) 2007-2008 Novell
+ *
+ *  Author: Gregory Haskins <ghaskins@novell.com>
+ *
+ *  This code tracks the priority of each CPU so that global migration
+ *  decisions are easy to calculate.  Each CPU can be in a state as follows:
+ *
+ *                 (INVALID), IDLE, NORMAL, RT1, ... RT99
+ *
+ *  going from the lowest priority to the highest.  CPUs in the INVALID state
+ *  are not eligible for routing.  The system maintains this state with
+ *  a 2 dimensional bitmap (the first for priority class, the second for cpus
+ *  in that class).  Therefore a typical application without affinity
+ *  restrictions can find a suitable CPU with O(1) complexity (e.g. two bit
+ *  searches).  For tasks with affinity restrictions, the algorithm has a
+ *  worst case complexity of O(min(102, nr_domcpus)), though the scenario that
+ *  yields the worst case search is fairly contrived.
+ *
+ *  This program is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU General Public License
+ *  as published by the Free Software Foundation; version 2
+ *  of the License.
+ */
+
+#include "sched_cpupri.h"
+
+/* Convert between a 140 based task->prio, and our 102 based cpupri */
+static int convert_prio(int prio)
+{
+        int cpupri;
+
+        if (prio == CPUPRI_INVALID)
+                cpupri = CPUPRI_INVALID;
+        else if (prio == MAX_PRIO)
+                cpupri = CPUPRI_IDLE;
+        else if (prio >= MAX_RT_PRIO)
+                cpupri = CPUPRI_NORMAL;
+        else
+                cpupri = MAX_RT_PRIO - prio + 1;
+
+        return cpupri;
+}
+
+#define for_each_cpupri_active(array, idx)                    \
+  for (idx = find_first_bit(array, CPUPRI_NR_PRIORITIES);     \
+       idx < CPUPRI_NR_PRIORITIES;                            \
+       idx = find_next_bit(array, CPUPRI_NR_PRIORITIES, idx+1))
+
+/**
+ * cpupri_find - find the best (lowest-pri) CPU in the system
+ * @cp: The cpupri context
+ * @p: The task
+ * @lowest_mask: A mask to fill in with selected CPUs
+ *
+ * Note: This function returns the recommended CPUs as calculated during the
+ * current invokation.  By the time the call returns, the CPUs may have in
+ * fact changed priorities any number of times.  While not ideal, it is not
+ * an issue of correctness since the normal rebalancer logic will correct
+ * any discrepancies created by racing against the uncertainty of the current
+ * priority configuration.
+ *
+ * Returns: (int)bool - CPUs were found
+ */
+int cpupri_find(struct cpupri *cp, struct task_struct *p,
+                cpumask_t *lowest_mask)
+{
+        int                  idx      = 0;
+        int                  task_pri = convert_prio(p->prio);
+
+        for_each_cpupri_active(cp->pri_active, idx) {
+                struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
+                cpumask_t mask;
+
+                if (idx >= task_pri)
+                        break;
+
+                cpus_and(mask, p->cpus_allowed, vec->mask);
+
+                if (cpus_empty(mask))
+                        continue;
+
+                *lowest_mask = mask;
+                return 1;
+        }
+
+        return 0;
+}
+
+/**
+ * cpupri_set - update the cpu priority setting
+ * @cp: The cpupri context
+ * @cpu: The target cpu
+ * @pri: The priority (INVALID-RT99) to assign to this CPU
+ *
+ * Note: Assumes cpu_rq(cpu)->lock is locked
+ *
+ * Returns: (void)
+ */
+void cpupri_set(struct cpupri *cp, int cpu, int newpri)
+{
+        int                 *currpri = &cp->cpu_to_pri[cpu];
+        int                  oldpri  = *currpri;
+        unsigned long        flags;
+
+        newpri = convert_prio(newpri);
+
+        BUG_ON(newpri >= CPUPRI_NR_PRIORITIES);
+
+        if (newpri == oldpri)
+                return;
+
+        /*
+         * If the cpu was currently mapped to a different value, we
+         * first need to unmap the old value
+         */
+        if (likely(oldpri != CPUPRI_INVALID)) {
+                struct cpupri_vec *vec  = &cp->pri_to_cpu[oldpri];
+
+                spin_lock_irqsave(&vec->lock, flags);
+
+                vec->count--;
+                if (!vec->count)
+                        clear_bit(oldpri, cp->pri_active);
+                cpu_clear(cpu, vec->mask);
+
+                spin_unlock_irqrestore(&vec->lock, flags);
+        }
+
+        if (likely(newpri != CPUPRI_INVALID)) {
+                struct cpupri_vec *vec = &cp->pri_to_cpu[newpri];
+
+                spin_lock_irqsave(&vec->lock, flags);
+
+                cpu_set(cpu, vec->mask);
+                vec->count++;
+                if (vec->count == 1)
+                        set_bit(newpri, cp->pri_active);
+
+                spin_unlock_irqrestore(&vec->lock, flags);
+        }
+
+        *currpri = newpri;
+}
+
+/**
+ * cpupri_init - initialize the cpupri structure
+ * @cp: The cpupri context
+ *
+ * Returns: (void)
+ */
+void cpupri_init(struct cpupri *cp)
+{
+        int i;
+
+        memset(cp, 0, sizeof(*cp));
+
+        for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) {
+                struct cpupri_vec *vec = &cp->pri_to_cpu[i];
+
+                spin_lock_init(&vec->lock);
+                vec->count = 0;
+                cpus_clear(vec->mask);
+        }
+
+        for_each_possible_cpu(i)
+                cp->cpu_to_pri[i] = CPUPRI_INVALID;
+}
+
+
diff --git a/kernel/sched_cpupri.h b/kernel/sched_cpupri.h
new file mode 100644
index 000000000000..0b6a3d110fac
--- /dev/null
+++ b/kernel/sched_cpupri.h
@@ -0,0 +1,36 @@
+#ifndef _LINUX_CPUPRI_H
+#define _LINUX_CPUPRI_H
+
+#include <linux/sched.h>
+
+#define CPUPRI_NR_PRIORITIES 2+MAX_RT_PRIO
+#define CPUPRI_NR_PRI_WORDS CPUPRI_NR_PRIORITIES/BITS_PER_LONG
+
+#define CPUPRI_INVALID -1
+#define CPUPRI_IDLE     0
+#define CPUPRI_NORMAL   1
+/* values 2-101 are RT priorities 0-99 */
+
+struct cpupri_vec {
+        spinlock_t lock;
+        int        count;
+        cpumask_t  mask;
+};
+
+struct cpupri {
+        struct cpupri_vec pri_to_cpu[CPUPRI_NR_PRIORITIES];
+        long              pri_active[CPUPRI_NR_PRI_WORDS];
+        int               cpu_to_pri[NR_CPUS];
+};
+
+#ifdef CONFIG_SMP
+int  cpupri_find(struct cpupri *cp,
+                 struct task_struct *p, cpumask_t *lowest_mask);
+void cpupri_set(struct cpupri *cp, int cpu, int pri);
+void cpupri_init(struct cpupri *cp);
+#else
+#define cpupri_set(cp, cpu, pri) do { } while (0)
+#define cpupri_init() do { } while (0)
+#endif
+
+#endif /* _LINUX_CPUPRI_H */
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index fefed39fafd8..44b06d75416e 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -391,8 +391,11 @@ void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
         WARN_ON(!rt_prio(rt_se_prio(rt_se)));
         rt_rq->rt_nr_running++;
 #if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
-        if (rt_se_prio(rt_se) < rt_rq->highest_prio)
+        if (rt_se_prio(rt_se) < rt_rq->highest_prio) {
+                struct rq *rq = rq_of_rt_rq(rt_rq);
                 rt_rq->highest_prio = rt_se_prio(rt_se);
+                cpupri_set(&rq->rd->cpupri, rq->cpu, rt_se_prio(rt_se));
+        }
 #endif
 #ifdef CONFIG_SMP
         if (rt_se->nr_cpus_allowed > 1) {
@@ -416,6 +419,10 @@ void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
 static inline
 void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
 {
+#ifdef CONFIG_SMP
+        int highest_prio = rt_rq->highest_prio;
+#endif
+
         WARN_ON(!rt_prio(rt_se_prio(rt_se)));
         WARN_ON(!rt_rq->rt_nr_running);
         rt_rq->rt_nr_running--;
@@ -439,6 +446,11 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
                 rq->rt.rt_nr_migratory--;
         }
 
+        if (rt_rq->highest_prio != highest_prio) {
+                struct rq *rq = rq_of_rt_rq(rt_rq);
+                cpupri_set(&rq->rd->cpupri, rq->cpu, rt_rq->highest_prio);
+        }
+
         update_rt_migration(rq_of_rt_rq(rt_rq));
 #endif /* CONFIG_SMP */
 #ifdef CONFIG_RT_GROUP_SCHED
@@ -763,73 +775,6 @@ static struct task_struct *pick_next_highest_task_rt(struct rq *rq, int cpu)
 
 static DEFINE_PER_CPU(cpumask_t, local_cpu_mask);
 
-static int find_lowest_cpus(struct task_struct *task, cpumask_t *lowest_mask)
-{
-        int       lowest_prio = -1;
-        int       lowest_cpu  = -1;
-        int       count       = 0;
-        int       cpu;
-
-        cpus_and(*lowest_mask, task_rq(task)->rd->online, task->cpus_allowed);
-
-        /*
-         * Scan each rq for the lowest prio.
-         */
-        for_each_cpu_mask(cpu, *lowest_mask) {
-                struct rq *rq = cpu_rq(cpu);
-
-                /* We look for lowest RT prio or non-rt CPU */
-                if (rq->rt.highest_prio >= MAX_RT_PRIO) {
-                        /*
-                         * if we already found a low RT queue
-                         * and now we found this non-rt queue
-                         * clear the mask and set our bit.
-                         * Otherwise just return the queue as is
-                         * and the count==1 will cause the algorithm
-                         * to use the first bit found.
-                         */
-                        if (lowest_cpu != -1) {
-                                cpus_clear(*lowest_mask);
-                                cpu_set(rq->cpu, *lowest_mask);
-                        }
-                        return 1;
-                }
-
-                /* no locking for now */
-                if ((rq->rt.highest_prio > task->prio)
-                    && (rq->rt.highest_prio >= lowest_prio)) {
-                        if (rq->rt.highest_prio > lowest_prio) {
-                                /* new low - clear old data */
-                                lowest_prio = rq->rt.highest_prio;
-                                lowest_cpu = cpu;
-                                count = 0;
-                        }
-                        count++;
-                } else
-                        cpu_clear(cpu, *lowest_mask);
-        }
-
-        /*
-         * Clear out all the set bits that represent
-         * runqueues that were of higher prio than
-         * the lowest_prio.
-         */
-        if (lowest_cpu > 0) {
-                /*
-                 * Perhaps we could add another cpumask op to
-                 * zero out bits. Like cpu_zero_bits(cpumask, nrbits);
-                 * Then that could be optimized to use memset and such.
-                 */
-                for_each_cpu_mask(cpu, *lowest_mask) {
-                        if (cpu >= lowest_cpu)
-                                break;
-                        cpu_clear(cpu, *lowest_mask);
-                }
-        }
-
-        return count;
-}
-
 static inline int pick_optimal_cpu(int this_cpu, cpumask_t *mask)
 {
         int first;
@@ -851,17 +796,12 @@ static int find_lowest_rq(struct task_struct *task)
         cpumask_t *lowest_mask = &__get_cpu_var(local_cpu_mask);
         int this_cpu = smp_processor_id();
         int cpu      = task_cpu(task);
-        int count    = find_lowest_cpus(task, lowest_mask);
 
-        if (!count)
-                return -1; /* No targets found */
+        if (task->rt.nr_cpus_allowed == 1)
+                return -1; /* No other targets possible */
 
-        /*
-         * There is no sense in performing an optimal search if only one
-         * target is found.
-         */
-        if (count == 1)
-                return first_cpu(*lowest_mask);
+        if (!cpupri_find(&task_rq(task)->rd->cpupri, task, lowest_mask))
+                return -1; /* No targets found */
 
         /*
          * At this point we have built a mask of cpus representing the
@@ -1218,6 +1158,8 @@ static void join_domain_rt(struct rq *rq)
 {
         if (rq->rt.overloaded)
                 rt_set_overload(rq);
+
+        cpupri_set(&rq->rd->cpupri, rq->cpu, rq->rt.highest_prio);
 }
 
 /* Assumes rq->lock is held */
@@ -1225,6 +1167,8 @@ static void leave_domain_rt(struct rq *rq)
 {
         if (rq->rt.overloaded)
                 rt_clear_overload(rq);
+
+        cpupri_set(&rq->rd->cpupri, rq->cpu, CPUPRI_INVALID);
 }
 
 /*