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Linux 调度器之RT选核




1. 主要调用路径

rt_sched_class.select_task_rq //RT调度类回调 
    select_task_rq_rt //rt.c 前面trace_android_rvh_select_task_rq_rt()若是选到cpu就直接退出了; 若test或cpu算力不满足时调用 
        find_lowest_rq //rt.c 
            trace_android_rvh_find_lowest_rq(task, lowest_mask, ret, &cpu);

二、select_task_rq_rt 函数

1. 三种选核路径传参

try_to_wake_up //core.c 
    select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags); //唤醒选核路径 
wake_up_new_task //core.c 
    select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0); //fork选核路径 
sched_exec //core.c 
    select_task_rq(p, task_cpu(p), SD_BALANCE_EXEC, 0); //exec选核路径

注:传参cpu p->wake_cpu 就是p上次运行的cpu.

static int select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags) //rt.c 
    struct task_struct *curr; 
    struct rq *rq; 
    struct rq *this_cpu_rq; 
    bool test; 
    int target_cpu = -1; 
    bool may_not_preempt; 
    bool sync = !!(flags & WF_SYNC); 
    int this_cpu; 
    trace_android_rvh_select_task_rq_rt(p, cpu, sd_flag, flags, &target_cpu); //mtk_select_task_rq_rt 
    if (target_cpu >= 0) 
        return target_cpu; 
    /* For anything but wake ups, just return the task_cpu */ 
    //也是只对唤醒和fork新任务场景调用, 另一种 SD_BALANCE_EXEC 的不走这里 
    if (sd_flag != SD_BALANCE_WAKE && sd_flag != SD_BALANCE_FORK) 
        goto out; 
    rq = cpu_rq(cpu); //任务上次运行的cpu的rq 
    curr = READ_ONCE(rq->curr); /* unlocked access */ //上次运行的cpu正在执行的任务 
    this_cpu = smp_processor_id(); //当前cpu 
    this_cpu_rq = cpu_rq(this_cpu); //当前cpu的rq 
     * If the current task on @p's runqueue is a softirq task, 
     * it may run without preemption for a time that is 
     * ill-suited for a waiting RT task. Therefore, try to 
     * wake this RT task on another runqueue. 
     * Also, if the current task on @p's runqueue is an RT task, then 
     * try to see if we can wake this RT task up on another 
     * runqueue. Otherwise simply start this RT task 
     * on its current runqueue. 
     * We want to avoid overloading runqueues. If the woken 
     * task is a higher priority, then it will stay on this CPU 
     * and the lower prio task should be moved to another CPU. 
     * Even though this will probably make the lower prio task 
     * lose its cache, we do not want to bounce a higher task 
     * around just because it gave up its CPU, perhaps for a 
     * lock? 
     * For equal prio tasks, we just let the scheduler sort it out. 
     * Otherwise, just let it ride on the affined RQ and the 
     * post-schedule router will push the preempted task away 
     * This test is optimistic, if we get it wrong the load-balancer 
     * will have to sort it out. 
     * We take into account the capacity of the CPU to ensure it fits the 
     * requirement of the task - which is only important on heterogeneous 
     * systems like big.LITTLE. 
    may_not_preempt = task_may_not_preempt(curr, cpu); 
    test = (curr && (may_not_preempt || (unlikely(rt_task(curr)) && (curr->nr_cpus_allowed < 2 || curr->prio <= p->prio)))); 
     * Respect the sync flag as long as the task can run on this CPU. 
    if (should_honor_rt_sync(this_cpu_rq, p, sync) && cpumask_test_cpu(this_cpu, p->cpus_ptr)) { 
        cpu = this_cpu; 
        goto out_unlock; 
     * 若p不能运行在之前运行的cpu上,或p之前运行的cpu算力不满足p的需求了,才进行后续的选核。 
     * 这个条件判断应该很可能为假,也即p可以运行在之前运行的cpu上且之前运行的cpu满足p的算力需求。也就是说 
     * 任务p很可能运行在之前运行过的cpu上,==> RT线程对算力满足需求的之前运行过的cpu有亲和性!一定概率下不 
     * 会走后续的选核流程。 
    if (test || !rt_task_fits_capacity(p, cpu)) { 
        int target = find_lowest_rq(p); 
         * Bail out if we were forcing a migration to find a better 
         * fitting CPU but our search failed. 
         * 若p能运行在之前运行的cpu上,且这里选出的cpu也不满足算力需求,就选任务p之前运行的cpu, 
         * 即使之前运行的cpu的算力也不满足. ==> 对之前运行过的cpu有亲和性 
        if (!test && target != -1 && !rt_task_fits_capacity(p, target)) 
            goto out_unlock; 
         * If cpu is non-preemptible, prefer remote cpu 
         * even if it's running a higher-prio task. 
         * Otherwise: Don't bother moving it if the destination CPU is 
         * not running a lower priority task. 
         * 选出了目标cpu且,且p不能抢占之前运行的cpu或p的优先级高于选出的cpu的rq上最高优任务的先级,就选新 
         * 选出的cpu,否则不赋值,还是选之前cpu。 
        if (target != -1 && (may_not_preempt || p->prio < cpu_rq(target)->rt.highest_prio.curr)) 
            cpu = target; 
    return cpu; 

2. 函数总结:
(1) 若是没有选到目标cpu,就返回任务p上次运行的cpu。
(2) trace_android_rvh_select_task_rq_rt 这个hook中传递了上层的所有参数,vendor可以在这里定制选核逻辑。
(3) 只有唤醒场景和fork新任务场景的才走选核流程,exec执行场景的选核直接返回之前运行的cpu作为目标cpu。
(4) 若是被RT任务sync唤醒且当前cpu上正在运行RT任务的优先级比p低,且当前cpu在任务p的亲和性中,就选当前cpu作为目标cpu。
(5) 若p不能运行在之前运行的cpu上,或p之前运行的cpu算力不满足p的需求了,才会继续选核,否则选p之前运行的cpu。说明RT任务对之前运行的cpu有一定的“亲和性”。
(6) 主要的选核逻辑在 find_lowest_rq() 中。

三、find_lowest_rq 函数

1. select_task_rq_rt 传参为待选核的任务

static int find_lowest_rq(struct task_struct *task) 
    struct sched_domain *sd; 
    struct cpumask *lowest_mask = this_cpu_cpumask_var_ptr(local_cpu_mask); 
    int this_cpu = smp_processor_id(); //当前正在运行的cpu 
    int cpu      = -1; 
    int ret; 
    /* Make sure the mask is initialized first */ 
    if (unlikely(!lowest_mask)) 
        return -1; 
    if (task->nr_cpus_allowed == 1) 
        return -1; /* No other targets possible */ 
     * If we're on asym system ensure we consider the different capacities 
     * of the CPUs when searching for the lowest_mask. 
    if (static_branch_unlikely(&sched_asym_cpucapacity)) { 
        //这个完全执行在前,lowest_mask 里面要么都是满足算力需求的cpu,要么都是不满足算力需求的cpu(之后大概率选之前的cpu) 
        ret = cpupri_find_fitness(&task_rq(task)->rd->cpupri, task, lowest_mask, rt_task_fits_capacity); 
    } else { 
        ret = cpupri_find(&task_rq(task)->rd->cpupri, task, lowest_mask); 
    trace_android_rvh_find_lowest_rq(task, lowest_mask, ret, &cpu); //HOOK 
    if (cpu >= 0) 
        return cpu; 
    if (!ret) 
        return -1; /* No targets found */ 
    cpu = task_cpu(task); //待选核rt任务之前运行的cpu 
     * At this point we have built a mask of CPUs representing the 
     * lowest priority tasks in the system.  Now we want to elect 
     * the best one based on our affinity and topology. 
     * We prioritize the last CPU that the task executed on since 
     * it is most likely cache-hot in that location. 
    if (cpumask_test_cpu(cpu, lowest_mask)) 
        return cpu; 
     * Otherwise, we consult the sched_domains span maps to figure 
     * out which CPU is logically closest to our hot cache data. 
     * 翻译: 
     * 否则,我们会查阅 sched_domains 中的cpu以确定哪个 CPU 在逻辑上最 
     * 接近我们的热缓存数据。 
    //若当前cpu不在候选cpu中就将 this_cpu 设为-1 
    if (!cpumask_test_cpu(this_cpu, lowest_mask)) 
        this_cpu = -1; /* Skip this_cpu opt if not among lowest */ 
    for_each_domain(cpu, sd) { //MC-->DIE 
        if (sd->flags & SD_WAKE_AFFINE) { //MC和DIE都有这个标志 
            int best_cpu; 
            /* "this_cpu" is cheaper to preempt than a remote processor.*/ 
             * 当前cpu在候选cpu中且当前cpu和p之前运行的cpu在同一个cluster内(MC的span为本cluster,DIE的span为所有cpu), 
             * 就返回当前cpu作为目标cpu. 
            if (this_cpu != -1 && cpumask_test_cpu(this_cpu, sched_domain_span(sd))) { 
                return this_cpu; 
            best_cpu = cpumask_first_and(lowest_mask, sched_domain_span(sd)); 
            if (best_cpu < nr_cpu_ids) { 
                return best_cpu; 
     * And finally, if there were no matches within the domains 
     * just give the caller *something* to work with from the compatible 
     * locations. 
    if (this_cpu != -1) 
        return this_cpu; 
    cpu = cpumask_any(lowest_mask); 
    if (cpu < nr_cpu_ids) 
        return cpu; 
    return -1; 

2. 函数总结:
(1) 先调用 cpupri_find_fitness() 候选cpu放到 lowest_mask 中,由于此函数在选不到候选cpu的时候后舍去 fitness_fn 回调重新选择一次。因此lowest_mask 中的候选cpu可能是都是算力满足待选核任务p需求的,或是都不满足p需求的。
(2) trace_android_rvh_find_lowest_rq 允许vendor厂商插入hook来更改候选cpu或指定目标cpu
(3) 确定候选cpu的lowest_mask后,选择优先级为:
a. 若p之前运行的cpu在候选cpu中,那么就选之前运行的cpu,以便利用cache-hot特性。
b. 若当前cpu在候选cpu中,且当前cpu和p之前运行的cpu位于同一cluster,则选当前cpu。
c. 选候选cpu和sd->span交集的第一个cpu做为目标cpu,即选任务p之前运行的cluster的一个cpu。
d. 若当前cpu在候选cpu中,则选当前cpu。
e. 选候选cpu中的第一个cpu。

四、cpupri_find_fitness 函数

1. find_lowest_rq调用传参(&task_rq(task)->rd->cpupri, task, lowest_mask, rt_task_fits_capacity)

cp 是全局唯一的,p 是待选核任务,lowest_mask 是刚初始化还没使用的,fitness_fn 是回调 rt_task_fits_capacity

int cpupri_find_fitness(struct cpupri *cp, struct task_struct *p, 
    struct cpumask *lowest_mask, bool (*fitness_fn)(struct task_struct *p, int cpu)) //cpupri.c 
    int task_pri = convert_prio(p->prio); 
    int idx, cpu; 
    bool drop_nopreempts = task_pri <= MAX_RT_PRIO; //100 只有prio=0的最高优先级的RT任务不满足 
    BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES); //102 convert_prio 转换后最大是101 
    for (idx = 0; idx < task_pri; idx++) { 
        //若选到了cpu,__cpupri_find 返回1 
        if (!__cpupri_find(cp, p, lowest_mask, idx, drop_nopreempts)) 
        if (!lowest_mask || !fitness_fn) 
            return 1; 
        /* Ensure the capacity of the CPUs fit the task */ 
        //对于 lowest_mask 中选出的cpu,剔除算力不满足需求的cpu。 
        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)) 
        return 1; 
     * If we can't find any non-preemptible cpu's, retry so we can 
     * find the lowest priority target and avoid priority inversion. 
    if (drop_nopreempts) { 
        drop_nopreempts = false; 
        goto retry; 
     * 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 overcommitted, 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. 
     * 若还是没有选到核,走这里,其是不再提供过滤回调函数,再重新调用一次 
     * cpupri_find_fitness(), 这次就不考虑RT任务算力需求了,__cpupri_find() 
     * 选到核后就直接返回了。 
     * TODO: 此情况下可以尽量选中核大核。 
    if (fitness_fn) 
        return cpupri_find(cp, p, lowest_mask); 
    return 0; 
// cpupri_find_fitness传参:(cp, p, lowest_mask) 
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 传参:(cp, p, lowest_mask, idx, drop_nopreempts) 
 * drop_nopreempts 只有 p->prio=0 的最高RT优先级才会为真. 
 * 选到了cpu返回真。 
static inline int __cpupri_find(struct cpupri *cp, struct task_struct *p, 
    struct cpumask *lowest_mask, int idx, bool drop_nopreempts) 
    struct cpupri_vec *vec  = &cp->pri_to_cpu[idx]; 
    int skip = 0; 
    if (!atomic_read(&(vec)->count)) 
        skip = 1; 
    /* Need to do the rmb for every iteration */ 
    if (skip) 
        return 0; 
    if (cpumask_any_and(p->cpus_ptr, vec->mask) >= nr_cpu_ids) 
        return 0; 
    if (lowest_mask) { 
        cpumask_and(lowest_mask, p->cpus_ptr, vec->mask); 
        cpumask_and(lowest_mask, lowest_mask, cpu_active_mask); 
        if (drop_nopreempts) 
         * 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; 

2. 函数总结:
(1) 会先带着过滤回调函数fitness_fn选一次候选cpu,若是没有选到,就取消过滤函数回调重新选择一次。