cpupri.c

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/*
 *  kernel/sched/cpupri.c
 *
 *  CPU priority management
 *
 *  Copyright (C) 2007-2008 Novell
 *
 *  Author: Gregory Haskins <[email protected]>
 *
 *  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 <linux/gfp.h>
#include "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;
}

int cpupri_find(struct cpupri *cp, struct task_struct *p,
        struct cpumask *lowest_mask)
{
    int idx = 0;
    int task_pri = convert_prio(p->prio);

    if (task_pri >= MAX_RT_PRIO)
        return 0;

    for (idx = 0; idx < task_pri; idx++) {
        struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
        int skip = 0;

        if (!atomic_read(&(vec)->count))
            skip = 1;
        /*
         * When looking at the vector, we need to read the counter,
         * do a memory barrier, then read the mask.
         *
         * Note: This is still all racey, but we can deal with it.
         *  Ideally, we only want to look at masks that are set.
         *
         *  If a mask is not set, then the only thing wrong is that we
         *  did a little more work than necessary.
         *
         *  If we read a zero count but the mask is set, because of the
         *  memory barriers, that can only happen when the highest prio
         *  task for a run queue has left the run queue, in which case,
         *  it will be followed by a pull. If the task we are processing
         *  fails to find a proper place to go, that pull request will
         *  pull this task if the run queue is running at a lower
         *  priority.
         */
        smp_rmb();

        /* Need to do the rmb for every iteration */
        if (skip)
            continue;

        if (cpumask_any_and(&p->cpus_allowed, vec->mask) >= nr_cpu_ids)
            continue;

        if (lowest_mask) {
            cpumask_and(lowest_mask, &p->cpus_allowed, vec->mask);

            /*
             * We have to ensure that we have at least one bit
             * still set in the array, since the map could have
             * been concurrently emptied between the first and
             * second reads of vec->mask.  If we hit this
             * condition, simply act as though we never hit this
             * priority level and continue on.
             */
            if (cpumask_any(lowest_mask) >= nr_cpu_ids)
                continue;
        }

        return 1;
    }

    return 0;
}

void cpupri_set(struct cpupri *cp, int cpu, int newpri)
{
    int *currpri = &cp->cpu_to_pri[cpu];
    int oldpri = *currpri;
    int do_mb = 0;

    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
     * need to map it to the new value then remove the old value.
     * Note, we must add the new value first, otherwise we risk the
     * cpu being missed by the priority loop in cpupri_find.
     */
    if (likely(newpri != CPUPRI_INVALID)) {
        struct cpupri_vec *vec = &cp->pri_to_cpu[newpri];

        cpumask_set_cpu(cpu, vec->mask);
        /*
         * When adding a new vector, we update the mask first,
         * do a write memory barrier, and then update the count, to
         * make sure the vector is visible when count is set.
         */
        smp_mb__before_atomic_inc();
        atomic_inc(&(vec)->count);
        do_mb = 1;
    }
    if (likely(oldpri != CPUPRI_INVALID)) {
        struct cpupri_vec *vec  = &cp->pri_to_cpu[oldpri];

        /*
         * Because the order of modification of the vec->count
         * is important, we must make sure that the update
         * of the new prio is seen before we decrement the
         * old prio. This makes sure that the loop sees
         * one or the other when we raise the priority of
         * the run queue. We don't care about when we lower the
         * priority, as that will trigger an rt pull anyway.
         *
         * We only need to do a memory barrier if we updated
         * the new priority vec.
         */
        if (do_mb)
            smp_mb__after_atomic_inc();

        /*
         * When removing from the vector, we decrement the counter first
         * do a memory barrier and then clear the mask.
         */
        atomic_dec(&(vec)->count);
        smp_mb__after_atomic_inc();
        cpumask_clear_cpu(cpu, vec->mask);
    }

    *currpri = newpri;
}

int 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];

        atomic_set(&vec->count, 0);
        if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL))
            goto cleanup;
    }

    for_each_possible_cpu(i)
        cp->cpu_to_pri[i] = CPUPRI_INVALID;
    return 0;

cleanup:
    for (i--; i >= 0; i--)
        free_cpumask_var(cp->pri_to_cpu[i].mask);
    return -ENOMEM;
}

void cpupri_cleanup(struct cpupri *cp)
{
    int i;

    for (i = 0; i < CPUPRI_NR_PRIORITIES; i++)
        free_cpumask_var(cp->pri_to_cpu[i].mask);
}