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>

1 files changed, 174 insertions, 0 deletions

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;
+}
+
+