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 <linux/sched.h>
  32 #include <linux/sched/rt.h>
  33 #include <linux/slab.h>
  34 #include "cpupri.h"
  35
  36 /* Convert between a 140 based task->prio, and our 102 based cpupri */
  37 static int convert_prio(int prio)
  38 {
  39         int cpupri;
  40
  41         if (prio == CPUPRI_INVALID)
  42                 cpupri = CPUPRI_INVALID;
  43         else if (prio == MAX_PRIO)
  44                 cpupri = CPUPRI_IDLE;
  45         else if (prio >= MAX_RT_PRIO)
  46                 cpupri = CPUPRI_NORMAL;
  47         else
  48                 cpupri = MAX_RT_PRIO - prio + 1;
  49
  50         return cpupri;
  51 }
  52
  53 /**
  54  * cpupri_find - find the best (lowest-pri) CPU in the system
  55  * @cp: The cpupri context
  56  * @p: The task
  57  * @lowest_mask: A mask to fill in with selected CPUs (or NULL)
  58  *
  59  * Note: This function returns the recommended CPUs as calculated during the
  60  * current invocation.  By the time the call returns, the CPUs may have in
  61  * fact changed priorities any number of times.  While not ideal, it is not
  62  * an issue of correctness since the normal rebalancer logic will correct
  63  * any discrepancies created by racing against the uncertainty of the current
  64  * priority configuration.
  65  *
  66  * Return: (int)bool - CPUs were found
  67  */
  68 int cpupri_find(struct cpupri *cp, struct task_struct *p,
  69                 struct cpumask *lowest_mask)
  70 {
  71         int idx = 0;
  72         int task_pri = convert_prio(p->prio);
  73
  74         BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
  75
  76         for (idx = 0; idx < task_pri; idx++) {
  77                 struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
  78                 int skip = 0;
  79
  80                 if (!atomic_read(&(vec)->count))
  81                         skip = 1;
  82                 /*
  83                  * When looking at the vector, we need to read the counter,
  84                  * do a memory barrier, then read the mask.
  85                  *
  86                  * Note: This is still all racey, but we can deal with it.
  87                  *  Ideally, we only want to look at masks that are set.
  88                  *
  89                  *  If a mask is not set, then the only thing wrong is that we
  90                  *  did a little more work than necessary.
  91                  *
  92                  *  If we read a zero count but the mask is set, because of the
  93                  *  memory barriers, that can only happen when the highest prio
  94                  *  task for a run queue has left the run queue, in which case,
  95                  *  it will be followed by a pull. If the task we are processing
  96                  *  fails to find a proper place to go, that pull request will
  97                  *  pull this task if the run queue is running at a lower
  98                  *  priority.
  99                  */
 100                 smp_rmb();
 101
 102                 /* Need to do the rmb for every iteration */
 103                 if (skip)
 104                         continue;
 105
 106                 if (cpumask_any_and(tsk_cpus_allowed(p), vec->mask) >= nr_cpu_ids)
 107                         continue;
 108
 109                 if (lowest_mask) {
 110                         cpumask_and(lowest_mask, tsk_cpus_allowed(p), vec->mask);
 111
 112                         /*
 113                          * We have to ensure that we have at least one bit
 114                          * still set in the array, since the map could have
 115                          * been concurrently emptied between the first and
 116                          * second reads of vec->mask.  If we hit this
 117                          * condition, simply act as though we never hit this
 118                          * priority level and continue on.
 119                          */
 120                         if (cpumask_any(lowest_mask) >= nr_cpu_ids)
 121                                 continue;
 122                 }
 123
 124                 return 1;
 125         }
 126
 127         return 0;
 128 }
 129
 130 /**
 131  * cpupri_set - update the cpu priority setting
 132  * @cp: The cpupri context
 133  * @cpu: The target cpu
 134  * @newpri: The priority (INVALID-RT99) to assign to this CPU
 135  *
 136  * Note: Assumes cpu_rq(cpu)->lock is locked
 137  *
 138  * Returns: (void)
 139  */
 140 void cpupri_set(struct cpupri *cp, int cpu, int newpri)
 141 {
 142         int *currpri = &cp->cpu_to_pri[cpu];
 143         int oldpri = *currpri;
 144         int do_mb = 0;
 145
 146         newpri = convert_prio(newpri);
 147
 148         BUG_ON(newpri >= CPUPRI_NR_PRIORITIES);
 149
 150         if (newpri == oldpri)
 151                 return;
 152
 153         /*
 154          * If the cpu was currently mapped to a different value, we
 155          * need to map it to the new value then remove the old value.
 156          * Note, we must add the new value first, otherwise we risk the
 157          * cpu being missed by the priority loop in cpupri_find.
 158          */
 159         if (likely(newpri != CPUPRI_INVALID)) {
 160                 struct cpupri_vec *vec = &cp->pri_to_cpu[newpri];
 161
 162                 cpumask_set_cpu(cpu, vec->mask);
 163                 /*
 164                  * When adding a new vector, we update the mask first,
 165                  * do a write memory barrier, and then update the count, to
 166                  * make sure the vector is visible when count is set.
 167                  */
 168                 smp_mb__before_atomic();
 169                 atomic_inc(&(vec)->count);
 170                 do_mb = 1;
 171         }
 172         if (likely(oldpri != CPUPRI_INVALID)) {
 173                 struct cpupri_vec *vec  = &cp->pri_to_cpu[oldpri];
 174
 175                 /*
 176                  * Because the order of modification of the vec->count
 177                  * is important, we must make sure that the update
 178                  * of the new prio is seen before we decrement the
 179                  * old prio. This makes sure that the loop sees
 180                  * one or the other when we raise the priority of
 181                  * the run queue. We don't care about when we lower the
 182                  * priority, as that will trigger an rt pull anyway.
 183                  *
 184                  * We only need to do a memory barrier if we updated
 185                  * the new priority vec.
 186                  */
 187                 if (do_mb)
 188                         smp_mb__after_atomic();
 189
 190                 /*
 191                  * When removing from the vector, we decrement the counter first
 192                  * do a memory barrier and then clear the mask.
 193                  */
 194                 atomic_dec(&(vec)->count);
 195                 smp_mb__after_atomic();
 196                 cpumask_clear_cpu(cpu, vec->mask);
 197         }
 198
 199         *currpri = newpri;
 200 }
 201
 202 /**
 203  * cpupri_init - initialize the cpupri structure
 204  * @cp: The cpupri context
 205  *
 206  * Return: -ENOMEM on memory allocation failure.
 207  */
 208 int cpupri_init(struct cpupri *cp)
 209 {
 210         int i;
 211
 212         memset(cp, 0, sizeof(*cp));
 213
 214         for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) {
 215                 struct cpupri_vec *vec = &cp->pri_to_cpu[i];
 216
 217                 atomic_set(&vec->count, 0);
 218                 if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL))
 219                         goto cleanup;
 220         }
 221
 222         cp->cpu_to_pri = kcalloc(nr_cpu_ids, sizeof(int), GFP_KERNEL);
 223         if (!cp->cpu_to_pri)
 224                 goto cleanup;
 225
 226         for_each_possible_cpu(i)
 227                 cp->cpu_to_pri[i] = CPUPRI_INVALID;
 228
 229         return 0;
 230
 231 cleanup:
 232         for (i--; i >= 0; i--)
 233                 free_cpumask_var(cp->pri_to_cpu[i].mask);
 234         return -ENOMEM;
 235 }
 236
 237 /**
 238  * cpupri_cleanup - clean up the cpupri structure
 239  * @cp: The cpupri context
 240  */
 241 void cpupri_cleanup(struct cpupri *cp)
 242 {
 243         int i;
 244
 245         kfree(cp->cpu_to_pri);
 246         for (i = 0; i < CPUPRI_NR_PRIORITIES; i++)
 247                 free_cpumask_var(cp->pri_to_cpu[i].mask);
 248 }