kernel/sched/cpupri.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  *  kernel/sched/cpupri.c
   4  *
   5  *  CPU priority management
   6  *
   7  *  Copyright (C) 2007-2008 Novell
   8  *
   9  *  Author: Gregory Haskins <ghaskins@novell.com>
  10  *
  11  *  This code tracks the priority of each CPU so that global migration
  12  *  decisions are easy to calculate.  Each CPU can be in a state as follows:
  13  *
  14  *                 (INVALID), IDLE, NORMAL, RT1, ... RT99
  15  *
  16  *  going from the lowest priority to the highest.  CPUs in the INVALID state
  17  *  are not eligible for routing.  The system maintains this state with
  18  *  a 2 dimensional bitmap (the first for priority class, the second for CPUs
  19  *  in that class).  Therefore a typical application without affinity
  20  *  restrictions can find a suitable CPU with O(1) complexity (e.g. two bit
  21  *  searches).  For tasks with affinity restrictions, the algorithm has a
  22  *  worst case complexity of O(min(102, nr_domcpus)), though the scenario that
  23  *  yields the worst case search is fairly contrived.
  24  */
  25 #include "sched.h"
  26
  27 /* Convert between a 140 based task->prio, and our 102 based cpupri */
  28 static int convert_prio(int prio)
  29 {
  30         int cpupri;
  31
  32         if (prio == CPUPRI_INVALID)
  33                 cpupri = CPUPRI_INVALID;
  34         else if (prio == MAX_PRIO)
  35                 cpupri = CPUPRI_IDLE;
  36         else if (prio >= MAX_RT_PRIO)
  37                 cpupri = CPUPRI_NORMAL;
  38         else
  39                 cpupri = MAX_RT_PRIO - prio + 1;
  40
  41         return cpupri;
  42 }
  43
  44 static inline int __cpupri_find(struct cpupri *cp, struct task_struct *p,
  45                                 struct cpumask *lowest_mask, int idx)
  46 {
  47         struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
  48         int skip = 0;
  49
  50         if (!atomic_read(&(vec)->count))
  51                 skip = 1;
  52         /*
  53          * When looking at the vector, we need to read the counter,
  54          * do a memory barrier, then read the mask.
  55          *
  56          * Note: This is still all racey, but we can deal with it.
  57          *  Ideally, we only want to look at masks that are set.
  58          *
  59          *  If a mask is not set, then the only thing wrong is that we
  60          *  did a little more work than necessary.
  61          *
  62          *  If we read a zero count but the mask is set, because of the
  63          *  memory barriers, that can only happen when the highest prio
  64          *  task for a run queue has left the run queue, in which case,
  65          *  it will be followed by a pull. If the task we are processing
  66          *  fails to find a proper place to go, that pull request will
  67          *  pull this task if the run queue is running at a lower
  68          *  priority.
  69          */
  70         smp_rmb();
  71
  72         /* Need to do the rmb for every iteration */
  73         if (skip)
  74                 return 0;
  75
  76         if (cpumask_any_and(p->cpus_ptr, vec->mask) >= nr_cpu_ids)
  77                 return 0;
  78
  79         if (lowest_mask) {
  80                 cpumask_and(lowest_mask, p->cpus_ptr, vec->mask);
  81
  82                 /*
  83                  * We have to ensure that we have at least one bit
  84                  * still set in the array, since the map could have
  85                  * been concurrently emptied between the first and
  86                  * second reads of vec->mask.  If we hit this
  87                  * condition, simply act as though we never hit this
  88                  * priority level and continue on.
  89                  */
  90                 if (cpumask_empty(lowest_mask))
  91                         return 0;
  92         }
  93
  94         return 1;
  95 }
  96
  97 int cpupri_find(struct cpupri *cp, struct task_struct *p,
  98                 struct cpumask *lowest_mask)
  99 {
 100         return cpupri_find_fitness(cp, p, lowest_mask, NULL);
 101 }
 102
 103 /**
 104  * cpupri_find_fitness - find the best (lowest-pri) CPU in the system
 105  * @cp: The cpupri context
 106  * @p: The task
 107  * @lowest_mask: A mask to fill in with selected CPUs (or NULL)
 108  * @fitness_fn: A pointer to a function to do custom checks whether the CPU
 109  *              fits a specific criteria so that we only return those CPUs.
 110  *
 111  * Note: This function returns the recommended CPUs as calculated during the
 112  * current invocation.  By the time the call returns, the CPUs may have in
 113  * fact changed priorities any number of times.  While not ideal, it is not
 114  * an issue of correctness since the normal rebalancer logic will correct
 115  * any discrepancies created by racing against the uncertainty of the current
 116  * priority configuration.
 117  *
 118  * Return: (int)bool - CPUs were found
 119  */
 120 int cpupri_find_fitness(struct cpupri *cp, struct task_struct *p,
 121                 struct cpumask *lowest_mask,
 122                 bool (*fitness_fn)(struct task_struct *p, int cpu))
 123 {
 124         int task_pri = convert_prio(p->prio);
 125         int idx, cpu;
 126
 127         BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
 128
 129         for (idx = 0; idx < task_pri; idx++) {
 130
 131                 if (!__cpupri_find(cp, p, lowest_mask, idx))
 132                         continue;
 133
 134                 if (!lowest_mask || !fitness_fn)
 135                         return 1;
 136
 137                 /* Ensure the capacity of the CPUs fit the task */
 138                 for_each_cpu(cpu, lowest_mask) {
 139                         if (!fitness_fn(p, cpu))
 140                                 cpumask_clear_cpu(cpu, lowest_mask);
 141                 }
 142
 143                 /*
 144                  * If no CPU at the current priority can fit the task
 145                  * continue looking
 146                  */
 147                 if (cpumask_empty(lowest_mask))
 148                         continue;
 149
 150                 return 1;
 151         }
 152
 153         /*
 154          * If we failed to find a fitting lowest_mask, kick off a new search
 155          * but without taking into account any fitness criteria this time.
 156          *
 157          * This rule favours honouring priority over fitting the task in the
 158          * correct CPU (Capacity Awareness being the only user now).
 159          * The idea is that if a higher priority task can run, then it should
 160          * run even if this ends up being on unfitting CPU.
 161          *
 162          * The cost of this trade-off is not entirely clear and will probably
 163          * be good for some workloads and bad for others.
 164          *
 165          * The main idea here is that if some CPUs were overcommitted, we try
 166          * to spread which is what the scheduler traditionally did. Sys admins
 167          * must do proper RT planning to avoid overloading the system if they
 168          * really care.
 169          */
 170         if (fitness_fn)
 171                 return cpupri_find(cp, p, lowest_mask);
 172
 173         return 0;
 174 }
 175
 176 /**
 177  * cpupri_set - update the CPU priority setting
 178  * @cp: The cpupri context
 179  * @cpu: The target CPU
 180  * @newpri: The priority (INVALID-RT99) to assign to this CPU
 181  *
 182  * Note: Assumes cpu_rq(cpu)->lock is locked
 183  *
 184  * Returns: (void)
 185  */
 186 void cpupri_set(struct cpupri *cp, int cpu, int newpri)
 187 {
 188         int *currpri = &cp->cpu_to_pri[cpu];
 189         int oldpri = *currpri;
 190         int do_mb = 0;
 191
 192         newpri = convert_prio(newpri);
 193
 194         BUG_ON(newpri >= CPUPRI_NR_PRIORITIES);
 195
 196         if (newpri == oldpri)
 197                 return;
 198
 199         /*
 200          * If the CPU was currently mapped to a different value, we
 201          * need to map it to the new value then remove the old value.
 202          * Note, we must add the new value first, otherwise we risk the
 203          * cpu being missed by the priority loop in cpupri_find.
 204          */
 205         if (likely(newpri != CPUPRI_INVALID)) {
 206                 struct cpupri_vec *vec = &cp->pri_to_cpu[newpri];
 207
 208                 cpumask_set_cpu(cpu, vec->mask);
 209                 /*
 210                  * When adding a new vector, we update the mask first,
 211                  * do a write memory barrier, and then update the count, to
 212                  * make sure the vector is visible when count is set.
 213                  */
 214                 smp_mb__before_atomic();
 215                 atomic_inc(&(vec)->count);
 216                 do_mb = 1;
 217         }
 218         if (likely(oldpri != CPUPRI_INVALID)) {
 219                 struct cpupri_vec *vec  = &cp->pri_to_cpu[oldpri];
 220
 221                 /*
 222                  * Because the order of modification of the vec->count
 223                  * is important, we must make sure that the update
 224                  * of the new prio is seen before we decrement the
 225                  * old prio. This makes sure that the loop sees
 226                  * one or the other when we raise the priority of
 227                  * the run queue. We don't care about when we lower the
 228                  * priority, as that will trigger an rt pull anyway.
 229                  *
 230                  * We only need to do a memory barrier if we updated
 231                  * the new priority vec.
 232                  */
 233                 if (do_mb)
 234                         smp_mb__after_atomic();
 235
 236                 /*
 237                  * When removing from the vector, we decrement the counter first
 238                  * do a memory barrier and then clear the mask.
 239                  */
 240                 atomic_dec(&(vec)->count);
 241                 smp_mb__after_atomic();
 242                 cpumask_clear_cpu(cpu, vec->mask);
 243         }
 244
 245         *currpri = newpri;
 246 }
 247
 248 /**
 249  * cpupri_init - initialize the cpupri structure
 250  * @cp: The cpupri context
 251  *
 252  * Return: -ENOMEM on memory allocation failure.
 253  */
 254 int cpupri_init(struct cpupri *cp)
 255 {
 256         int i;
 257
 258         for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) {
 259                 struct cpupri_vec *vec = &cp->pri_to_cpu[i];
 260
 261                 atomic_set(&vec->count, 0);
 262                 if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL))
 263                         goto cleanup;
 264         }
 265
 266         cp->cpu_to_pri = kcalloc(nr_cpu_ids, sizeof(int), GFP_KERNEL);
 267         if (!cp->cpu_to_pri)
 268                 goto cleanup;
 269
 270         for_each_possible_cpu(i)
 271                 cp->cpu_to_pri[i] = CPUPRI_INVALID;
 272
 273         return 0;
 274
 275 cleanup:
 276         for (i--; i >= 0; i--)
 277                 free_cpumask_var(cp->pri_to_cpu[i].mask);
 278         return -ENOMEM;
 279 }
 280
 281 /**
 282  * cpupri_cleanup - clean up the cpupri structure
 283  * @cp: The cpupri context
 284  */
 285 void cpupri_cleanup(struct cpupri *cp)
 286 {
 287         int i;
 288
 289         kfree(cp->cpu_to_pri);
 290         for (i = 0; i < CPUPRI_NR_PRIORITIES; i++)
 291                 free_cpumask_var(cp->pri_to_cpu[i].mask);
 292 }