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 // SPDX-License-Identifier: GPL-2.0-only
 /*
  *  kernel/sched/cpupri.c
  *
  *  CPU priority management
  *
  *  Copyright (C) 2007-2008 Novell
  *
  *  Author: Gregory Haskins <ghaskins@novell.com>
  *
  *  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), NORMAL, RT1, ... RT99, HIGHER
  *
  *  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(101, nr_domcpus)), though the scenario that
  *  yields the worst case search is fairly contrived.
  */
 
 /*
  * p->rt_priority   p->prio   newpri   cpupri
  *
  *                -1       -1 (CPUPRI_INVALID)
  *
  *                99        0 (CPUPRI_NORMAL)
  *
  *      1        98       98        1
  *        ...
  *         49        50       50       49
  *         50        49       49       50
  *        ...
  *         99         0        0       99
  *
  *               100      100 (CPUPRI_HIGHER)
  */
 static int convert_prio(int prio)
 {
     int cpupri;
 
     switch (prio) {
     case CPUPRI_INVALID:
         cpupri = CPUPRI_INVALID;    /* -1 */
         break;
 
     case 0 ... 98:
         cpupri = MAX_RT_PRIO-1 - prio;  /* 1 ... 99 */
         break;
 
     case MAX_RT_PRIO-1:
         cpupri = CPUPRI_NORMAL;     /*  0 */
         break;
 
     case MAX_RT_PRIO:
         cpupri = CPUPRI_HIGHER;     /* 100 */
         break;
     }
 
     return cpupri;
 }
 
 static inline int __cpupri_find(struct cpupri *cp, struct task_struct *p,
                 struct cpumask *lowest_mask, int 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 racy, 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)
         return 0;
 
     if (cpumask_any_and(&p->cpus_mask, vec->mask) >= nr_cpu_ids)
         return 0;
 
     if (lowest_mask) {
         cpumask_and(lowest_mask, &p->cpus_mask, 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_empty(lowest_mask))
             return 0;
     }
 
     return 1;
 }
 
 int cpupri_find(struct cpupri *cp, struct task_struct *p,
         struct cpumask *lowest_mask)
 {
     return cpupri_find_fitness(cp, p, lowest_mask, NULL);
 }
 
 /**
  * cpupri_find_fitness - find the best (lowest-pri) CPU in the system
  * @cp: The cpupri context
  * @p: The task
  * @lowest_mask: A mask to fill in with selected CPUs (or NULL)
  * @fitness_fn: A pointer to a function to do custom checks whether the CPU
  *              fits a specific criteria so that we only return those CPUs.
  *
  * Note: This function returns the recommended CPUs as calculated during the
  * current invocation.  By the time the call returns, the CPUs may have in
  * fact changed priorities any number of times.  While not ideal, it is not
  * an issue of correctness since the normal rebalancer logic will correct
  * any discrepancies created by racing against the uncertainty of the current
  * priority configuration.
  *
  * Return: (int)bool - CPUs were found
  */
 int cpupri_find_fitness(struct cpupri *cp, struct task_struct *p,
         struct cpumask *lowest_mask,
         bool (*fitness_fn)(struct task_struct *p, int cpu))
 {
     int task_pri = convert_prio(p->prio);
     int idx, cpu;
 
     BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
 
     for (idx = 0; idx < task_pri; idx++) {
 
         if (!__cpupri_find(cp, p, lowest_mask, idx))
             continue;
 
         if (!lowest_mask || !fitness_fn)
             return 1;
 
         /* Ensure the capacity of the CPUs fit the task */
         for_each_cpu(cpu, lowest_mask) {
             if (!fitness_fn(p, cpu))
                 cpumask_clear_cpu(cpu, lowest_mask);
         }
 
         /*
          * If no CPU at the current priority can fit the task
          * continue looking
          */
         if (cpumask_empty(lowest_mask))
             continue;
 
         return 1;
     }
 
     /*
      * If we failed to find a fitting lowest_mask, kick off a new search
      * but without taking into account any fitness criteria this time.
      *
      * This rule favours honouring priority over fitting the task in the
      * correct CPU (Capacity Awareness being the only user now).
      * The idea is that if a higher priority task can run, then it should
      * run even if this ends up being on unfitting CPU.
      *
      * The cost of this trade-off is not entirely clear and will probably
      * be good for some workloads and bad for others.
      *
      * The main idea here is that if some CPUs were over-committed, we try
      * to spread which is what the scheduler traditionally did. Sys admins
      * must do proper RT planning to avoid overloading the system if they
      * really care.
      */
     if (fitness_fn)
         return cpupri_find(cp, p, lowest_mask);
 
     return 0;
 }
 
 /**
  * cpupri_set - update the CPU priority setting
  * @cp: The cpupri context
  * @cpu: The target CPU
  * @newpri: The priority (INVALID,NORMAL,RT1-RT99,HIGHER) to assign to this CPU
  *
  * Note: Assumes cpu_rq(cpu)->lock is locked
  *
  * Returns: (void)
  */
 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();
         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();
 
         /*
          * 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();
         cpumask_clear_cpu(cpu, vec->mask);
     }
 
     *currpri = newpri;
 }
 
 /**
  * cpupri_init - initialize the cpupri structure
  * @cp: The cpupri context
  *
  * Return: -ENOMEM on memory allocation failure.
  */
 int cpupri_init(struct cpupri *cp)
 {
     int i;
 
     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;
     }
 
     cp->cpu_to_pri = kcalloc(nr_cpu_ids, sizeof(int), GFP_KERNEL);
     if (!cp->cpu_to_pri)
         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;
 }
 
 /**
  * cpupri_cleanup - clean up the cpupri structure
  * @cp: The cpupri context
  */
 void cpupri_cleanup(struct cpupri *cp)
 {
     int i;
 
     kfree(cp->cpu_to_pri);
     for (i = 0; i < CPUPRI_NR_PRIORITIES; i++)
         free_cpumask_var(cp->pri_to_cpu[i].mask);
 }

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