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 #include "sched.h"
  30
  31 /* Convert between a 140 based task->prio, and our 102 based cpupri */
  32 static int convert_prio(int prio)
  33 {
  34         int cpupri;
  35
  36         if (prio == CPUPRI_INVALID)
  37                 cpupri = CPUPRI_INVALID;
  38         else if (prio == MAX_PRIO)
  39                 cpupri = CPUPRI_IDLE;
  40         else if (prio >= MAX_RT_PRIO)
  41                 cpupri = CPUPRI_NORMAL;
  42         else
  43                 cpupri = MAX_RT_PRIO - prio + 1;
  44
  45         return cpupri;
  46 }
  47
  48 /**
  49  * cpupri_find - find the best (lowest-pri) CPU in the system
  50  * @cp: The cpupri context
  51  * @p: The task
  52  * @lowest_mask: A mask to fill in with selected CPUs (or NULL)
  53  *
  54  * Note: This function returns the recommended CPUs as calculated during the
  55  * current invocation.  By the time the call returns, the CPUs may have in
  56  * fact changed priorities any number of times.  While not ideal, it is not
  57  * an issue of correctness since the normal rebalancer logic will correct
  58  * any discrepancies created by racing against the uncertainty of the current
  59  * priority configuration.
  60  *
  61  * Return: (int)bool - CPUs were found
  62  */
  63 int cpupri_find(struct cpupri *cp, struct task_struct *p,
  64                 struct cpumask *lowest_mask)
  65 {
  66         int idx = 0;
  67         int task_pri = convert_prio(p->prio);
  68
  69         BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
  70
  71         for (idx = 0; idx < task_pri; idx++) {
  72                 struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
  73                 int skip = 0;
  74
  75                 if (!atomic_read(&(vec)->count))
  76                         skip = 1;
  77                 /*
  78                  * When looking at the vector, we need to read the counter,
  79                  * do a memory barrier, then read the mask.
  80                  *
  81                  * Note: This is still all racey, but we can deal with it.
  82                  *  Ideally, we only want to look at masks that are set.
  83                  *
  84                  *  If a mask is not set, then the only thing wrong is that we
  85                  *  did a little more work than necessary.
  86                  *
  87                  *  If we read a zero count but the mask is set, because of the
  88                  *  memory barriers, that can only happen when the highest prio
  89                  *  task for a run queue has left the run queue, in which case,
  90                  *  it will be followed by a pull. If the task we are processing
  91                  *  fails to find a proper place to go, that pull request will
  92                  *  pull this task if the run queue is running at a lower
  93                  *  priority.
  94                  */
  95                 smp_rmb();
  96
  97                 /* Need to do the rmb for every iteration */
  98                 if (skip)
  99                         continue;
 100
 101                 if (cpumask_any_and(&p->cpus_allowed, vec->mask) >= nr_cpu_ids)
 102                         continue;
 103
 104                 if (lowest_mask) {
 105                         cpumask_and(lowest_mask, &p->cpus_allowed, vec->mask);
 106
 107                         /*
 108                          * We have to ensure that we have at least one bit
 109                          * still set in the array, since the map could have
 110                          * been concurrently emptied between the first and
 111                          * second reads of vec->mask.  If we hit this
 112                          * condition, simply act as though we never hit this
 113                          * priority level and continue on.
 114                          */
 115                         if (cpumask_any(lowest_mask) >= nr_cpu_ids)
 116                                 continue;
 117                 }
 118
 119                 return 1;
 120         }
 121
 122         return 0;
 123 }
 124
 125 /**
 126  * cpupri_set - update the CPU priority setting
 127  * @cp: The cpupri context
 128  * @cpu: The target CPU
 129  * @newpri: The priority (INVALID-RT99) to assign to this CPU
 130  *
 131  * Note: Assumes cpu_rq(cpu)->lock is locked
 132  *
 133  * Returns: (void)
 134  */
 135 void cpupri_set(struct cpupri *cp, int cpu, int newpri)
 136 {
 137         int *currpri = &cp->cpu_to_pri[cpu];
 138         int oldpri = *currpri;
 139         int do_mb = 0;
 140
 141         newpri = convert_prio(newpri);
 142
 143         BUG_ON(newpri >= CPUPRI_NR_PRIORITIES);
 144
 145         if (newpri == oldpri)
 146                 return;
 147
 148         /*
 149          * If the CPU was currently mapped to a different value, we
 150          * need to map it to the new value then remove the old value.
 151          * Note, we must add the new value first, otherwise we risk the
 152          * cpu being missed by the priority loop in cpupri_find.
 153          */
 154         if (likely(newpri != CPUPRI_INVALID)) {
 155                 struct cpupri_vec *vec = &cp->pri_to_cpu[newpri];
 156
 157                 cpumask_set_cpu(cpu, vec->mask);
 158                 /*
 159                  * When adding a new vector, we update the mask first,
 160                  * do a write memory barrier, and then update the count, to
 161                  * make sure the vector is visible when count is set.
 162                  */
 163                 smp_mb__before_atomic();
 164                 atomic_inc(&(vec)->count);
 165                 do_mb = 1;
 166         }
 167         if (likely(oldpri != CPUPRI_INVALID)) {
 168                 struct cpupri_vec *vec  = &cp->pri_to_cpu[oldpri];
 169
 170                 /*
 171                  * Because the order of modification of the vec->count
 172                  * is important, we must make sure that the update
 173                  * of the new prio is seen before we decrement the
 174                  * old prio. This makes sure that the loop sees
 175                  * one or the other when we raise the priority of
 176                  * the run queue. We don't care about when we lower the
 177                  * priority, as that will trigger an rt pull anyway.
 178                  *
 179                  * We only need to do a memory barrier if we updated
 180                  * the new priority vec.
 181                  */
 182                 if (do_mb)
 183                         smp_mb__after_atomic();
 184
 185                 /*
 186                  * When removing from the vector, we decrement the counter first
 187                  * do a memory barrier and then clear the mask.
 188                  */
 189                 atomic_dec(&(vec)->count);
 190                 smp_mb__after_atomic();
 191                 cpumask_clear_cpu(cpu, vec->mask);
 192         }
 193
 194         *currpri = newpri;
 195 }
 196
 197 /**
 198  * cpupri_init - initialize the cpupri structure
 199  * @cp: The cpupri context
 200  *
 201  * Return: -ENOMEM on memory allocation failure.
 202  */
 203 int cpupri_init(struct cpupri *cp)
 204 {
 205         int i;
 206
 207         for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) {
 208                 struct cpupri_vec *vec = &cp->pri_to_cpu[i];
 209
 210                 atomic_set(&vec->count, 0);
 211                 if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL))
 212                         goto cleanup;
 213         }
 214
 215         cp->cpu_to_pri = kcalloc(nr_cpu_ids, sizeof(int), GFP_KERNEL);
 216         if (!cp->cpu_to_pri)
 217                 goto cleanup;
 218
 219         for_each_possible_cpu(i)
 220                 cp->cpu_to_pri[i] = CPUPRI_INVALID;
 221
 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         kfree(cp->cpu_to_pri);
 239         for (i = 0; i < CPUPRI_NR_PRIORITIES; i++)
 240                 free_cpumask_var(cp->pri_to_cpu[i].mask);
 241 }