kernel/sched/cpupri.c

   1 /*
   2  *  kernel/sched/cpupri.c
   3  *
   4  *  CPU priority management
   5  *
   6  *  Copyright (C) 2007-2008 Novell
   7  *
   8  *  Author: Gregory Haskins <ghaskins@novell.com>
   9  *
  10  *  This code tracks the priority of each CPU so that global migration
  11  *  decisions are easy to calculate.  Each CPU can be in a state as follows:
  12  *
  13  *                 (INVALID), IDLE, NORMAL, RT1, ... RT99
  14  *
  15  *  going from the lowest priority to the highest.  CPUs in the INVALID state
  16  *  are not eligible for routing.  The system maintains this state with
  17  *  a 2 dimensional bitmap (the first for priority class, the second for cpus
  18  *  in that class).  Therefore a typical application without affinity
  19  *  restrictions can find a suitable CPU with O(1) complexity (e.g. two bit
  20  *  searches).  For tasks with affinity restrictions, the algorithm has a
  21  *  worst case complexity of O(min(102, nr_domcpus)), though the scenario that
  22  *  yields the worst case search is fairly contrived.
  23  *
  24  *  This program is free software; you can redistribute it and/or
  25  *  modify it under the terms of the GNU General Public License
  26  *  as published by the Free Software Foundation; version 2
  27  *  of the License.
  28  */
  29
  30 #include <linux/gfp.h>
  31 #include "cpupri.h"
  32
  33 /* Convert between a 140 based task->prio, and our 102 based cpupri */
  34 static int convert_prio(int prio)
  35 {
  36         int cpupri;
  37
  38         if (prio == CPUPRI_INVALID)
  39                 cpupri = CPUPRI_INVALID;
  40         else if (prio == MAX_PRIO)
  41                 cpupri = CPUPRI_IDLE;
  42         else if (prio >= MAX_RT_PRIO)
  43                 cpupri = CPUPRI_NORMAL;
  44         else
  45                 cpupri = MAX_RT_PRIO - prio + 1;
  46
  47         return cpupri;
  48 }
  49
  50 /**
  51  * cpupri_find - find the best (lowest-pri) CPU in the system
  52  * @cp: The cpupri context
  53  * @p: The task
  54  * @lowest_mask: A mask to fill in with selected CPUs (or NULL)
  55  *
  56  * Note: This function returns the recommended CPUs as calculated during the
  57  * current invocation.  By the time the call returns, the CPUs may have in
  58  * fact changed priorities any number of times.  While not ideal, it is not
  59  * an issue of correctness since the normal rebalancer logic will correct
  60  * any discrepancies created by racing against the uncertainty of the current
  61  * priority configuration.
  62  *
  63  * Returns: (int)bool - CPUs were found
  64  */
  65 int cpupri_find(struct cpupri *cp, struct task_struct *p,
  66                 struct cpumask *lowest_mask)
  67 {
  68         int idx = 0;
  69         int task_pri = convert_prio(p->prio);
  70
  71         if (task_pri >= MAX_RT_PRIO)
  72                 return 0;
  73
  74         for (idx = 0; idx < task_pri; idx++) {
  75                 struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
  76                 int skip = 0;
  77
  78                 if (!atomic_read(&(vec)->count))
  79                         skip = 1;
  80                 /*
  81                  * When looking at the vector, we need to read the counter,
  82                  * do a memory barrier, then read the mask.
  83                  *
  84                  * Note: This is still all racey, but we can deal with it.
  85                  *  Ideally, we only want to look at masks that are set.
  86                  *
  87                  *  If a mask is not set, then the only thing wrong is that we
  88                  *  did a little more work than necessary.
  89                  *
  90                  *  If we read a zero count but the mask is set, because of the
  91                  *  memory barriers, that can only happen when the highest prio
  92                  *  task for a run queue has left the run queue, in which case,
  93                  *  it will be followed by a pull. If the task we are processing
  94                  *  fails to find a proper place to go, that pull request will
  95                  *  pull this task if the run queue is running at a lower
  96                  *  priority.
  97                  */
  98                 smp_rmb();
  99
 100                 /* Need to do the rmb for every iteration */
 101                 if (skip)
 102                         continue;
 103
 104                 if (cpumask_any_and(&p->cpus_allowed, vec->mask) >= nr_cpu_ids)
 105                         continue;
 106
 107                 if (lowest_mask) {
 108                         cpumask_and(lowest_mask, &p->cpus_allowed, vec->mask);
 109
 110                         /*
 111                          * We have to ensure that we have at least one bit
 112                          * still set in the array, since the map could have
 113                          * been concurrently emptied between the first and
 114                          * second reads of vec->mask.  If we hit this
 115                          * condition, simply act as though we never hit this
 116                          * priority level and continue on.
 117                          */
 118                         if (cpumask_any(lowest_mask) >= nr_cpu_ids)
 119                                 continue;
 120                 }
 121
 122                 return 1;
 123         }
 124
 125         return 0;
 126 }
 127
 128 /**
 129  * cpupri_set - update the cpu priority setting
 130  * @cp: The cpupri context
 131  * @cpu: The target cpu
 132  * @newpri: The priority (INVALID-RT99) to assign to this CPU
 133  *
 134  * Note: Assumes cpu_rq(cpu)->lock is locked
 135  *
 136  * Returns: (void)
 137  */
 138 void cpupri_set(struct cpupri *cp, int cpu, int newpri)
 139 {
 140         int *currpri = &cp->cpu_to_pri[cpu];
 141         int oldpri = *currpri;
 142         int do_mb = 0;
 143
 144         newpri = convert_prio(newpri);
 145
 146         BUG_ON(newpri >= CPUPRI_NR_PRIORITIES);
 147
 148         if (newpri == oldpri)
 149                 return;
 150
 151         /*
 152          * If the cpu was currently mapped to a different value, we
 153          * need to map it to the new value then remove the old value.
 154          * Note, we must add the new value first, otherwise we risk the
 155          * cpu being missed by the priority loop in cpupri_find.
 156          */
 157         if (likely(newpri != CPUPRI_INVALID)) {
 158                 struct cpupri_vec *vec = &cp->pri_to_cpu[newpri];
 159
 160                 cpumask_set_cpu(cpu, vec->mask);
 161                 /*
 162                  * When adding a new vector, we update the mask first,
 163                  * do a write memory barrier, and then update the count, to
 164                  * make sure the vector is visible when count is set.
 165                  */
 166                 smp_mb__before_atomic_inc();
 167                 atomic_inc(&(vec)->count);
 168                 do_mb = 1;
 169         }
 170         if (likely(oldpri != CPUPRI_INVALID)) {
 171                 struct cpupri_vec *vec  = &cp->pri_to_cpu[oldpri];
 172
 173                 /*
 174                  * Because the order of modification of the vec->count
 175                  * is important, we must make sure that the update
 176                  * of the new prio is seen before we decrement the
 177                  * old prio. This makes sure that the loop sees
 178                  * one or the other when we raise the priority of
 179                  * the run queue. We don't care about when we lower the
 180                  * priority, as that will trigger an rt pull anyway.
 181                  *
 182                  * We only need to do a memory barrier if we updated
 183                  * the new priority vec.
 184                  */
 185                 if (do_mb)
 186                         smp_mb__after_atomic_inc();
 187
 188                 /*
 189                  * When removing from the vector, we decrement the counter first
 190                  * do a memory barrier and then clear the mask.
 191                  */
 192                 atomic_dec(&(vec)->count);
 193                 smp_mb__after_atomic_inc();
 194                 cpumask_clear_cpu(cpu, vec->mask);
 195         }
 196
 197         *currpri = newpri;
 198 }
 199
 200 /**
 201  * cpupri_init - initialize the cpupri structure
 202  * @cp: The cpupri context
 203  *
 204  * Returns: -ENOMEM if memory fails.
 205  */
 206 int cpupri_init(struct cpupri *cp)
 207 {
 208         int i;
 209
 210         memset(cp, 0, sizeof(*cp));
 211
 212         for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) {
 213                 struct cpupri_vec *vec = &cp->pri_to_cpu[i];
 214
 215                 atomic_set(&vec->count, 0);
 216                 if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL))
 217                         goto cleanup;
 218         }
 219
 220         for_each_possible_cpu(i)
 221                 cp->cpu_to_pri[i] = CPUPRI_INVALID;
 222         return 0;
 223
 224 cleanup:
 225         for (i--; i >= 0; i--)
 226                 free_cpumask_var(cp->pri_to_cpu[i].mask);
 227         return -ENOMEM;
 228 }
 229
 230 /**
 231  * cpupri_cleanup - clean up the cpupri structure
 232  * @cp: The cpupri context
 233  */
 234 void cpupri_cleanup(struct cpupri *cp)
 235 {
 236         int i;
 237
 238         for (i = 0; i < CPUPRI_NR_PRIORITIES; i++)
 239                 free_cpumask_var(cp->pri_to_cpu[i].mask);
 240 }