一、select_task_rq_fair()函数
CFS任务选核最终都是要走 select_task_rq_fair() 函数，三种CFS选核路径如下：
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
对比可以看到，只有任务唤醒选核才传了wake_flags，可能选择sync唤醒。sched_domain 的flag中是否包含 SD_BALANCE_XX 可以通过下面方法查看：
/proc/sys/kernel/sched_domain # find ./ -name flags | xargs cat SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES //MC SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_ASYM_CPUCAPACITY SD_PREFER_SIBLING //DIE
可以看出对于嵌入式这种大小核的Arm64架构中，MC和DIE都没有 SD_BALANCE_WAKE 这个标志。
函数解读：
/* * select_task_rq_fair: Select target runqueue for the waking task in domains * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE, * SD_BALANCE_FORK, or SD_BALANCE_EXEC. * * Balances load by selecting the idlest CPU in the idlest group, or under * certain conditions an idle sibling CPU if the domain has SD_WAKE_AFFINE set. * * Returns the target CPU number. * * preempt must be disabled. */ /* * 传参： * p: 待选核任务 * prev_cpu：任务之前运行的cpu * sd_flag: 在包含此标志的sd中为任务选核，可取值 SD_BALANCE_WAKE(唤醒任务的选核)、SD_BALANCE_FORK(fork的新任务选核)、SD_BALANCE_EXEC(exec的任务选核) * wake_flags: 可取值有 WF_SYNC、WF_FORK、WF_MIGRATED、WF_ON_CPU、WF_ANDROID_VENDOR，但是只对WF_SYNC进行了特殊处理，表示同步唤醒，其它标志被此函数忽略。 * * 返回值：选出的目标cpu * * 执行环境：必须关抢占 */ static int select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags) { struct sched_domain *tmp, *sd = NULL; int cpu = smp_processor_id(); int new_cpu = prev_cpu; //目标cpu先默认取prev_cpu int want_affine = 0; int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING); int target_cpu = -1; //注册了这个HOOK且不是对新fork出来的任务进行选核，就更新任务的负载 if (trace_android_rvh_select_task_rq_fair_enabled() && !(sd_flag & SD_BALANCE_FORK)) sync_entity_load_avg(&p->se); //Vendor厂商可以更改选核路径，若是选到了核，就直接退出了 trace_android_rvh_select_task_rq_fair(p, prev_cpu, sd_flag, wake_flags, &target_cpu); probe_android_rvh_select_task_rq_fair if (target_cpu >= 0) return target_cpu; //只有对唤醒任务的选核才有可能走EAS选核路径 if (sd_flag & SD_BALANCE_WAKE) { //记录当前任何和被唤醒任务p之间是否有固定的唤醒关系 record_wakee(p); //全局使能控制/proc/sys/kernel/sched_energy_aware是否开启EAS if (sched_energy_enabled()) { //进行EAS选核，EAS选到核了，就直接退出。里面判断了rd->overutilized，若overutilized则直接失败退出 new_cpu = find_energy_efficient_cpu(p, prev_cpu, sync); if (new_cpu >= 0) return new_cpu; //若EAS没有选到核，就回退为初始状态，继续走传统路径进行选核 new_cpu = prev_cpu; } //若p和current之间比较有相关性且任务p允许运行在当前cpu上，就认为是want_affine want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, p->cpus_ptr); } rcu_read_lock(); for_each_domain(cpu, tmp) { //MC-->DIE /* If both 'cpu' and 'prev_cpu' are part of this domain, cpu is a valid SD_WAKE_AFFINE target.*/ /* SD_WAKE_AFFINE 标志MC和DIE都定义了，恒成立, 因此可以解释为只要满足want_affine, 就肯定会进入 */ if (want_affine && (tmp->flags & SD_WAKE_AFFINE) && cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) { //若当前cpu就是任务p之前运行的prev_cpu, 不用调用wake_affine()选核了，若是唤醒选核直接选idle的兄弟cpu,非唤醒选核直接选prev_cpu if (cpu != prev_cpu) new_cpu = wake_affine(tmp, p, cpu, prev_cpu, sync); //若是既不走快速路径，又不走慢速路径，就返回它。 sd = NULL; /* Prefer wake_affine over balance flags */ break; } /* * 注意 tmp->flags 不会包含 SD_BALANCE_WAKE, 因为MC和DIE都没设置这个标志。只有 SD_BALANCE_FORK、SD_BALANCE_EXEC 选核才有可能赋值 * 对于want_affine=0的情况，这个循环退出后，sd就是当前cpu对应的DIE层级的sd. 这里的作用是找到支持sd_flag的最高层级的sd */ if (tmp->flags & sd_flag) sd = tmp; else if (!want_affine) break; } if (unlikely(sd)) { //SD_BALANCE_WAKE这类休眠唤醒的任务选核不可能走慢速路径，因为sd恒为NULL /* Slow path */ new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag); } else if (sd_flag & SD_BALANCE_WAKE) { //对唤醒的任务进行选核 /* Fast path */ new_cpu = select_idle_sibling(p, prev_cpu, new_cpu); if (want_affine) current->recent_used_cpu = cpu; } rcu_read_unlock(); return new_cpu; }
1. 主要逻辑
(1) Vendor可以注册HOOK来决定是否执行这个原生的选核流程，若注册了且选到了核，则原生选核流程不再执行。否则执行原生选核流程。
(2) 判断是否进行EAS选核，若EAS选到和核则直接退出，否则继续往下走选核流程。EAS选核的条件为：
a. 必须是唤醒场景的选核，fork和exec场景的选核都不会走EAS。
b. EAS全局控制开关/proc/sys/kernel/sched_energy_aware必须是开启的。
c. 系统没有overutilized，若是overutilized则直接退出EAS选核。
(3) 满足wake_affine选出的核会作为一个候选结果，若是唤醒场景，还有走快速路径进行选核。
(4) 慢速路径选核只有fork和exec两类选核流程会走，唤醒选核是不会走慢速路径的。且慢速路径传参sd只可能是当前cpu对应的DIE层级的sd。
(5) 快速路径选核只有唤醒选核流程会走，fork和exec两类选核流程不会走快速选核流程。虽然名字叫select_idle_sibling，但是在同cluster中选不到idle核时，还是会在所有cluster中进行选核。
二、wake affine 属性
wake affine 只支持唤醒场景的选核。主要判断当前任务与被唤醒任务之间是否有比较固定的唤醒关系，来帮助选出一个候选cpu。
1. task_struct 中有三个成员用来线程唤醒信息
struct task_struct { ... unsigned int wakee_flips; //该线程唤醒不同wakee的次数，值越大说明越偏向于唤醒不同的线程且唤醒越频繁 unsigned long wakee_flip_decay_ts; //记录上次wakee_flips衰减的时间点，其每秒衰减为原来的50% struct task_struct *last_wakee; //该线程上次唤醒的线程 ... }
2. record_wakee()
/* * 若current对p有比较固定的唤醒关系的话，current->wakee_flips将非常的小。 * 若经常唤醒其它不同的线程，其值将比较大 */ static void record_wakee(struct task_struct *p) { /* * Only decay a single time; tasks that have less then 1 wakeup per * jiffy will not have built up many flips. */ //超过1秒衰减为原来的1/2 if (time_after(jiffies, current->wakee_flip_decay_ts + HZ)) { current->wakee_flips >>= 1; current->wakee_flip_decay_ts = jiffies; } //当前任务上下文中，若上次唤醒的不是任务p, 当前任务的wakee_flips加1 if (current->last_wakee != p) { current->last_wakee = p; current->wakee_flips++; } }
Waker唤醒wakee的场景中，有两种选核思路：一种是聚合的思路，即让waker和wakee尽量的close，从而提高cache hit。另外一种思考是分散，即让load尽量平均分配在多个cpu上。不同的唤醒模型使用不同的放置策略，两种简单的唤醒模型：
(1) 在1:N模型中，一个server会不断的唤醒多个不同的client。
(2) 在1:1模型中，线程A和线程B不断的唤醒对方。
在1:N模型中，如果N是一个较大的数值，那么让waker和wakee尽量的close会导致负荷的极度不平均，这会waker所在的sd会承担太多的task，从而引起性能下降。
在1:1模型中，让waker和wakee尽量的close不存在这样的问题，还能提高性能。
实际的程序中，唤醒关系可能没有那么简单，一个wakee可能是另外一个关系中的waker，交互可能是M:N的形式。考虑这样一个场景：waker把wakee拉近，而wakee自身的wakee flips比较大，由于wakee也会做waker，那么更多的线程也会拉近waker所在的sd，从而进一步加剧CPU资源的竞争。因此waker和wakee的wakee flips的数值都不能太大，太大的时候应该禁止wake affine。内核中通过 wake_wide() 来判断是否使能wake affine。
3. wake_wide()
/* * 通过开关频率启发式检测 M:N 唤醒者/被唤醒者关系。 * 许多唤醒者唤醒的是与上次不同的任务，其频率大约是唤醒某一任务的 N 倍。 * 为了确定我们是否应该让负载分散与合并到共享缓存，我们在一对伙伴的一个中寻找最小为 llc_size 的“翻转”频率，在另一个中寻找大于 lls_size 频率的因子。 * 满足这两个条件，我们可以相对确定这种关系是非一夫一妻制的，伙伴数量超过套接字大小。 * Waker/wakee 是client/server、worker/dispatcher、中断源或任何无关紧要的东西，传播标准是明显的合作伙伴数量超过套接字大小。 * * 对于有比较固定唤醒关系的，返回0，否则返回1 */ static int wake_wide(struct task_struct *p) { unsigned int master = current->wakee_flips; unsigned int slave = p->wakee_flips; int factor = __this_cpu_read(sd_llc_size); //表示此cpu所在cluster中cpu的个数 if (master < slave) swap(master, slave); //这里是用的是或 if (slave < factor || master < slave * factor) return 0; return 1; }
这个函数返回0，并且当前cpu在任务p的亲和性里面，就判断为want_affine。这里的or逻辑是存疑的，因为master和slave其一的wakee_flips比较小就满足wake affine，这会使得任务太容易在LLC domain堆积了。在1:N模型中（例如手机转屏的时候，一个线程会唤醒非常非常多的线程来处理configChange消息），master的wakee_flips巨大无比，slave的wakee_flips非常小，如果仍然wake affine是不合理的。
4. wake_affine()
/* * select_task_rq_fair传参：sd为当前cpu对应的MC或DIE层级的sd; p为待选核任务; this_cpu为当前正在执行的cpu; * prev_cpu为任务之前运行的cpu; sync表示选核时是否指定了同步WF_SYNC标志 */ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int this_cpu, int prev_cpu, int sync) { int target = nr_cpumask_bits; if (sched_feat(WA_IDLE)) target = wake_affine_idle(this_cpu, prev_cpu, sync); if (sched_feat(WA_WEIGHT) && target == nr_cpumask_bits) //若上面没有选到cpu target = wake_affine_weight(sd, p, this_cpu, prev_cpu, sync); schedstat_inc(p->se.statistics.nr_wakeups_affine_attempts); //增加统计计数 if (target == nr_cpumask_bits) return prev_cpu; //若是还是没选到核，就返回prev_cpu，否则返回选到的cpu schedstat_inc(sd->ttwu_move_affine); schedstat_inc(p->se.statistics.nr_wakeups_affine); return target; } /* * The purpose of wake_affine() is to quickly determine on which CPU we can run * soonest. For the purpose of speed we only consider the waking and previous CPU. * * wake_affine_idle() - only considers 'now', it check if the waking CPU is * cache-affine and is (or will be) idle. * * wake_affine_weight() - considers the weight to reflect the average * scheduling latency of the CPUs. This seems to work * for the overloaded case. * * wake_affine传参：this_cpu是当前正在执行的cpu; prev_cpu是任务上次运行的cpu; sync是是 * 否指定了AF_SYNC同步唤醒标志 */ static int wake_affine_idle(int this_cpu, int prev_cpu, int sync) { /* * If this_cpu is idle, it implies the wakeup is from interrupt * context. Only allow the move if cache is shared. Otherwise an * interrupt intensive(密集型) workload could force all tasks onto one * node depending on the IO topology or IRQ affinity settings. * * If the prev_cpu is idle and cache affine then avoid a migration. * There is no guarantee that the cache hot data from an interrupt * is more important than cache hot data on the prev_cpu and from * a cpufreq perspective, it's better to have higher utilisation * on one CPU. */ //若正在执行的cpu是idle的(中断唤醒)，且当前cpu和任务上次运行的cpu属于同一个cluster if (available_idle_cpu(this_cpu) && cpus_share_cache(this_cpu, prev_cpu)) //若prev_cpu也idle就返回prev_cpu,否则返回当前正在运行的idle cpu return available_idle_cpu(prev_cpu) ? prev_cpu : this_cpu; /* * 若同步唤醒且当前cpu上只有一个任务(也就是waker)在运行，就返回当前cpu，因为sync唤醒， * 当前waker马上就要睡眠了。 */ if (sync && cpu_rq(this_cpu)->nr_running == 1) return this_cpu; return nr_cpumask_bits; //没选到cpu返回它 } /* * wake_affine传参：sd是当前正在运行的cpu对应的MC或DIE层级的sd; p是待选核的任务; this_cpu是当前正在执行的cpu; prev_cpu是任务上次运行的cpu; * sync是是否指定了AF_SYNC同步唤醒标志。 */ static int wake_affine_weight(struct sched_domain *sd, struct task_struct *p, int this_cpu, int prev_cpu, int sync) { s64 this_eff_load, prev_eff_load; unsigned long task_load; //当前正在运行cpu上的CFS任务的负载 this_eff_load = cpu_load(cpu_rq(this_cpu)); //return cfs_rq->avg.load_avg; if (sync) { unsigned long current_load = task_h_load(current); //约为 current->se.avg.load_avg; //当前任务的负载比当前cpu上cfs任务的负载之和还大？应该是安全的容错处理 if (current_load > this_eff_load) return this_cpu; this_eff_load -= current_load; } task_load = task_h_load(p); //约为 p->se.avg.load_avg; this_eff_load += task_load; //计算若p把current从当前cpu上挤下去后，cpu上CFS任务的总负载 if (sched_feat(WA_BIAS)) this_eff_load *= 100; //乘以100 this_eff_load *= capacity_of(prev_cpu); //再乘以cpu算力中可用于cfs任务的算力(出去温限/irq/rt/dl占用后的算力)，乘以的竟然是prev_cpu的！ prev_eff_load = cpu_load(cpu_rq(prev_cpu)); //prev_cpu::rq.cfs_rq->avg.load_avg prev_eff_load -= task_load; //这里为什么要减去？ if (sched_feat(WA_BIAS)) prev_eff_load *= 100 + (sd->imbalance_pct - 100) / 2; //MC和DIE都是乘以 100+(117-100)/2 prev_eff_load *= capacity_of(this_cpu); //乘以的竟然是this_cpu可用于cfs任务的算力！ /* * If sync, adjust the weight of prev_eff_load such that if * prev_eff == this_eff that select_idle_sibling() will consider * stacking the wakee on top of the waker if no other CPU is * idle. * 如果同步唤醒，则调整 prev_eff_load 的权重，以便如果 prev_eff == this_eff 则 select_idle_sibling() 将考虑在没有其它空闲CPU的情况下将 * 被唤醒的任务放在唤醒它的CPU上。 */ if (sync) prev_eff_load += 1; //前面乘以100又乘以CPU可用于CFS任务的算力了，这里加1有啥用？ /*公式整理一下为：this_cpu_load/this_cpu_cap < 108.5% * prev_cpu_load/prev_cpu_cap 成立就返回this_cpu*/ return this_eff_load < prev_eff_load ? this_cpu : nr_cpumask_bits; }
三、慢速路径
只有fork和exec类型的选核才会走慢速路径，慢速路径选核函数为 find_idlest_cpu()，走慢速路径选核时传参sd只可能是DIE层级的。
/* * 只有fork或exec的选核才可能走这个函数。 * * 传参：sd当前cpu对应的DIE层级的sd(只可能是DIE层级); p是待选核任务; cpu为当前cpu; prev_cpu为任务p之前运行的cpu; sd_flag表示哪种选核类型。 * * 作用：在最idle的sg中找出最idle的cpu, 若是没找到就返回当前cpu. */ static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p, int cpu, int prev_cpu, int sd_flag) { int new_cpu = cpu; //这个sd中的所有cpu都不在任务p的cpu亲和性里面 if (!cpumask_intersects(sched_domain_span(sd), p->cpus_ptr)) return prev_cpu; /* * We need task's util for cpu_util_without, sync it up to prev_cpu's last_update_time. */ //仅对于exec类型的选核更新任务的负载 if (!(sd_flag & SD_BALANCE_FORK)) sync_entity_load_avg(&p->se); while (sd) { struct sched_group *group; struct sched_domain *tmp; int weight; if (!(sd->flags & sd_flag)) { //恒不成立,因为fork或exec类型的选核MC/DIE都支持 sd = sd->child; continue; } //若是返回NULL,直接选的是当前cpu group = find_idlest_group(sd, p, cpu); if (!group) { sd = sd->child; continue; } //若是在d中找到了最忙的sg，就在其中选退出延迟最小且最后进idle的cpu,若是没有idle的cpu就选load最小的cpu new_cpu = find_idlest_group_cpu(group, p, cpu); if (new_cpu == cpu) { /* Now try balancing at a lower domain level of 'cpu': */ sd = sd->child; continue; } /* Now try balancing at a lower domain level of 'new_cpu': */ cpu = new_cpu; weight = sd->span_weight; sd = NULL; for_each_domain(cpu, tmp) { if (weight <= tmp->span_weight) //一进来就退出了，相当于没有执行 break; if (tmp->flags & sd_flag) sd = tmp; } } return new_cpu; }
1. find_idlest_group()
在当前cpu对应的sd中找出最空闲的sg
/* * find_idlest_group() finds and returns the least busy CPU group within the domain. * * Assumes p is allowed on at least one CPU in sd. * * find_idlest_cpu传参：sd为当前cpu DIE层级对应的sd; p为待选核任务; cpu为当前cpu; */ static struct sched_group *find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu) { struct sched_group *idlest = NULL, *local = NULL, *group = sd->groups; struct sg_lb_stats local_sgs, tmp_sgs; struct sg_lb_stats *sgs; unsigned long imbalance; struct sg_lb_stats idlest_sgs = { .avg_load = UINT_MAX, .group_type = group_overloaded, //初始化为最大的不均衡 }; //1024 * (sd->imbalance_pct-100) / 100 imbalance = scale_load_down(NICE_0_LOAD) * (sd->imbalance_pct-100) / 100; //遍历MC层级先的每个cpu do { int local_group; /* Skip over this group if it has no CPUs allowed */ //跳过任务p亲和性不允许的cpu if (!cpumask_intersects(sched_group_span(group), p->cpus_ptr)) continue; //当前正在运行的cpu是否在这个group中 local_group = cpumask_test_cpu(this_cpu, sched_group_span(group)); if (local_group) { sgs = &local_sgs; local = group; } else { sgs = &tmp_sgs; } update_sg_wakeup_stats(sd, group, sgs, p); //group和idlest比较，看group是否比idlest更闲 if (!local_group && update_pick_idlest(idlest, &idlest_sgs, group, sgs)) { idlest = group; idlest_sgs = *sgs; } } while (group = group->next, group != sd->groups); /* There is no idlest group to push tasks to */ //若没找到idlest sg，选当前cpu if (!idlest) return NULL; /* The local group has been skipped because of CPU affinity */ //此sd中若没有local group就直接返回idlest，否则还要和local group再PK一下，此例中，local group恒存在的 if (!local) return idlest; /* * If the local group is idler than the selected idlest group * don't try and push the task. */ //若local sg更闲，选当前cpu if (local_sgs.group_type < idlest_sgs.group_type) return NULL; /* * If the local group is busier than the selected idlest group * try and push the task. */ if (local_sgs.group_type > idlest_sgs.group_type) return idlest; //local group和idlest group一样闲的情况了： switch (local_sgs.group_type) { case group_overloaded: case group_fully_busy: /* * When comparing groups across NUMA domains, it's possible for * the local domain to be very lightly loaded relative to the * remote domains but "imbalance" skews the comparison making * remote CPUs look much more favourable. When considering * cross-domain, add imbalance to the load on the remote node * and consider staying local. */ //大小核架构没有SD_NUMA标志，恒不执行 if ((sd->flags & SD_NUMA) && ((idlest_sgs.avg_load + imbalance) >= local_sgs.avg_load)) return NULL; /* * If the local group is less loaded than the selected * idlest group don't try and push any tasks. */ //最闲的sg和local sg差值大于阈值17*1024，选当前cpu if (idlest_sgs.avg_load >= (local_sgs.avg_load + imbalance)) return NULL; //local_avg_load/idlest_avg_load <= sd->imbalance_pct%，选当前cpu if (100 * local_sgs.avg_load <= sd->imbalance_pct * idlest_sgs.avg_load) return NULL; break; case group_imbalanced: case group_asym_packing: /* Those type are not used in the slow wakeup path */ return NULL; //选当前cpu case group_misfit_task: /* Select group with the highest max capacity */ if (local->sgc->max_capacity >= idlest->sgc->max_capacity) return NULL; //选当前cpu break; case group_has_spare: if (sd->flags & SD_NUMA) { #ifdef CONFIG_NUMA_BALANCING //没使能，不执行 int idlest_cpu; /* * If there is spare capacity at NUMA, try to select * the preferred node */ if (cpu_to_node(this_cpu) == p->numa_preferred_nid) return NULL; idlest_cpu = cpumask_first(sched_group_span(idlest)); if (cpu_to_node(idlest_cpu) == p->numa_preferred_nid) return idlest; #endif /* * Otherwise, keep the task on this node to stay close * its wakeup source and improve locality. If there is * a real need of migration, periodic load balance will * take care of it. */ if (local_sgs.idle_cpus) return NULL; //若local sg有idle cpu存在，选当前cpu } /* * Select group with highest number of idle CPUs. We could also * compare the utilization which is more stable but it can end * up that the group has less spare capacity but finally more * idle CPUs which means more opportunity to run task. */ //若local sg的idle cpu个数比idlest sg更多，选当前cpu if (local_sgs.idle_cpus >= idlest_sgs.idle_cpus) return NULL; break; } return idlest; }
update_sg_wakeup_stats 更新 sgs：
/* * update_sg_wakeup_stats - Update sched_group's statistics for wakeup. * @sd: The sched_domain level to look for idlest group. * @group: sched_group whose statistics are to be updated. * @sgs: variable to hold the statistics for this group. * @p: The task for which we look for the idlest group/CPU. * * find_idlest_group传参：sd为当前cpu对应的MC层级的sd; group为MC层级的一个sg; sgs为未初始化的参数; p为待选核任务 * 作用：更新 sgs */ static inline void update_sg_wakeup_stats(struct sched_domain *sd, struct sched_group *group, struct sg_lb_stats *sgs, struct task_struct *p) { int i, nr_running; memset(sgs, 0, sizeof(*sgs)); //MC层级一个sg中只有一个cpu for_each_cpu(i, sched_group_span(group)) { struct rq *rq = cpu_rq(i); unsigned int local; sgs->group_load += cpu_load_without(rq, p); sgs->group_util += cpu_util_without(i, p); sgs->group_runnable += cpu_runnable_without(rq, p); local = task_running_on_cpu(i, p); //判断p是否queue在这个cpu的rq上，此case下应该return 0 sgs->sum_h_nr_running += rq->cfs.h_nr_running - local; nr_running = rq->nr_running - local; sgs->sum_nr_running += nr_running; /* * No need to call idle_cpu_without() if nr_running is not 0 */ if (!nr_running && idle_cpu_without(i, p)) sgs->idle_cpus++; } /* Check if task fits in the group */ //只有DIE层级有这个标志,恒不成立 if (sd->flags & SD_ASYM_CPUCAPACITY && !task_fits_capacity(p, group->sgc->max_capacity)) { sgs->group_misfit_task_load = 1; } sgs->group_capacity = group->sgc->capacity; sgs->group_weight = group->group_weight; sgs->group_type = group_classify(sd->imbalance_pct, group, sgs); /* * Computing avg_load makes sense only when group is fully busy or overloaded */ if (sgs->group_type == group_fully_busy || sgs->group_type == group_overloaded) sgs->avg_load = (sgs->group_load * SCHED_CAPACITY_SCALE) / sgs->group_capacity; } //根据imbalance_pct和sgs来确认group_type static inline enum group_type group_classify(unsigned int imbalance_pct, struct sched_group *group, struct sg_lb_stats *sgs) { /* * sgs->sum_nr_running <= sgs->group_weight 返回fase, 只要cpu个数大于任务个数，就不认为group_overloaded * group_util/group_capacity > imbalance_pct/100=117% 或 group_runnable/group_capacity > imbalance_pct/100=117% 返回true */ if (group_is_overloaded(imbalance_pct, sgs)) return group_overloaded; if (sg_imbalanced(group)) //return group->sgc->imbalance; return group_imbalanced; if (sgs->group_asym_packing) return group_asym_packing; if (sgs->group_misfit_task_load) return group_misfit_task; if (!group_has_capacity(imbalance_pct, sgs)) return group_fully_busy; return group_has_spare; }
后续判断的group和idlest比较，看group是否比idlest更闲：
static bool update_pick_idlest(struct sched_group *idlest, struct sg_lb_stats *idlest_sgs, struct sched_group *group, struct sg_lb_stats *sgs) { if (sgs->group_type < idlest_sgs->group_type) return true; if (sgs->group_type > idlest_sgs->group_type) return false; /* * The candidate and the current idlest group are the same type of * group. Let check which one is the idlest according to the type. */ //group_type相等的情况下比较谁更闲 switch (sgs->group_type) { case group_overloaded: case group_fully_busy: //哪个sg的avg_load，哪个sg更闲 /* Select the group with lowest avg_load. */ if (idlest_sgs->avg_load <= sgs->avg_load) return false; break; case group_imbalanced: case group_asym_packing: /* Those types are not used in the slow wakeup path */ return false; case group_misfit_task: //哪个sg的最大算力大，哪个sg更闲 /* Select group with the highest max capacity */ if (idlest->sgc->max_capacity >= group->sgc->max_capacity) return false; break; case group_has_spare: //哪个sg的idle cpu个数多，哪个sg更闲 /* Select group with most idle CPUs */ if (idlest_sgs->idle_cpus > sgs->idle_cpus) return false; /* Select group with lowest group_util */ //若idle cpu个数相等，哪个group_util小哪个更闲 if (idlest_sgs->idle_cpus == sgs->idle_cpus && idlest_sgs->group_util <= sgs->group_util) return false; break; } return true; }
2. find_idlest_group_cpu()
/* * find_idlest_group_cpu - find the idlest CPU among the CPUs in the group. * * find_idlest_cpu传参：group为在当前cpu对应的MC层级sd中找出的最闲的sg; p为待选核任务; this_cpu为当前正在执行的cpu */ static int find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this_cpu) { unsigned long load, min_load = ULONG_MAX; unsigned int min_exit_latency = UINT_MAX; u64 latest_idle_timestamp = 0; int least_loaded_cpu = this_cpu; int shallowest_idle_cpu = -1; int i; /* Check if we have any choice: */ //对于MC层级的sg这里就直接就返回了，因为sg中只有一个cpu，DIE层级继续往下走。 if (group->group_weight == 1) return cpumask_first(sched_group_span(group)); /* Traverse only the allowed CPUs */ //DIE层级的sg,遍历这个cluster中任务p亲和性允许的cpu for_each_cpu_and(i, sched_group_span(group), p->cpus_ptr) { if (sched_idle_cpu(i)) //若此cpu rq中只有SCHED_IDLE类型的任务，就选此cpu return i; //判断此cpu是否是idle cpu: rq->curr==rq->idle && rq->nr_running==0 && rq->ttwu_pending==0 if (available_idle_cpu(i)) { struct rq *rq = cpu_rq(i); struct cpuidle_state *idle = idle_get_state(rq); //return rq->idle_state; if (idle && idle->exit_latency < min_exit_latency) { /* * We give priority to a CPU whose idle state has the smallest exit latency irrespective * of any idle timestamp. */ min_exit_latency = idle->exit_latency; //记录最小退出延迟 latest_idle_timestamp = rq->idle_stamp; shallowest_idle_cpu = i; //记录最小的退出延迟的cpu } else if ((!idle || idle->exit_latency == min_exit_latency) && rq->idle_stamp > latest_idle_timestamp) { //idle退出延迟相等，且进入idle的时间更晚 /* * If equal or no active idle state, then the most recently idled CPU might have * a warmer cache. */ latest_idle_timestamp = rq->idle_stamp; shallowest_idle_cpu = i; } } else if (shallowest_idle_cpu == -1) {//对于非idle类型的cpu,并且之前也没有遍历到idle cpu load = cpu_load(cpu_rq(i)); if (load < min_load) { min_load = load; //记录load最小的cpu least_loaded_cpu = i; } } } //若是有idle cpu就选idle cpu,若是没有idle cpu就选load最小的cpu return shallowest_idle_cpu != -1 ? shallowest_idle_cpu : least_loaded_cpu; }
同 load_balance()寻找最忙的cpu类似。这里的慢速路径，选择最空闲的cpu，先在sd中找最空闲的sg，然后再sg中找最空闲的cpu。这里最空闲指的是当前cpu所在cluster内，有idle的cpu就算idle的cpu，若是没有idle的cpu就选负载最小的cpu。
四、快速路径
快速选核路径只有唤醒选核流程才会走。虽然函数名中是选兄弟cpu，但是在选不到idle cpu的时候，也会在其它Cluster进行选核
/* * Try and locate an idle core/thread in the LLC cache domain. * * select_task_rq_fair传参：p为待选核任务; prev是任务上次运行的cpu; target是备选目标cpu，可能是prev_cpu也可能是wake_affine选出的cpu */ static int select_idle_sibling(struct task_struct *p, int prev, int target) { struct sched_domain *sd; unsigned long task_util; int i, recent_used_cpu; /* * On asymmetric system, update task utilization because we will check * that the task fits with cpu's capacity. */ if (static_branch_unlikely(&sched_asym_cpucapacity)) { sync_entity_load_avg(&p->se); task_util = uclamp_task_util(p); //将util_est clamp 在MIN和MAX之间 } /* * 传入的target是一个idle cpu且可以容纳的了uclamp后的任务的util，就直接选之前选的cpu。 * Wake affine的场景下，target cpu是通过wake_affine函数寻找的target cpu，其他场景下， * target CPU其实等于prev CPU。 */ if ((available_idle_cpu(target) || sched_idle_cpu(target)) && asym_fits_capacity(task_util, target)) return target; /* * If the previous CPU is cache affine and idle, don't be stupid: * * 若传入的参数是由wake_affine选出的cpu，且和prev_cpu属于同一个cluster(共享L2-Cache), 且prev_cpu是idle的, 且prev_cpu能容纳的下uclamp后的任务p的util */ if (prev != target && cpus_share_cache(prev, target) && (available_idle_cpu(prev) || sched_idle_cpu(prev)) && asym_fits_capacity(task_util, prev)) return prev; /* * Allow a per-cpu kthread to stack with the wakee if the kworker thread and the tasks previous CPUs are the same. * The assumption is that the wakee queued work for the per-cpu kthread that is now complete and the wakeup is * essentially a sync wakeup. An obvious example of this pattern is IO completions. * 翻译： * 如果 kworker 线程和任务prev_cpu是同一个，则允许 per-cpu kthread 与被唤醒线程堆叠。假设是被唤醒任务往per-cpu的kthread * 上queue work, 唤醒本质上是同步唤醒。这种模式的一个明显例子是 IO 完成。 * * 当前任务是一个per-cpu的内核线程，且任务的prev_cpu就是当前cpu，且当前cpu上最多只有一个任务在运行，就选prev_cpu. */ if (is_per_cpu_kthread(current) && prev == smp_processor_id() && this_rq()->nr_running <= 1) { return prev; } /* Check a recently used CPU as a potential idle candidate: */ /* * recent_used_cpu的更新位置： * select_task_rq_fair: 更新 current->recent_used_cpu 为当前正在运行的cpu * select_idle_sibling: 将待选核任务 p->recent_used_cpu 设置为 prev_cpu * * 若 p->recent_used_cpu 既不是prev_cpu也不是target_cpu, 但是是和target_cpu共cluster的，且是idle的, 且任务的亲和性允许， * 且算力能够容纳任务，那么就选 p->recent_used_cpu。 */ recent_used_cpu = p->recent_used_cpu; if (recent_used_cpu != prev && recent_used_cpu != target && cpus_share_cache(recent_used_cpu, target) && (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) && cpumask_test_cpu(p->recent_used_cpu, p->cpus_ptr) && asym_fits_capacity(task_util, recent_used_cpu)) { /* * Replace recent_used_cpu with prev as it is a potential candidate for the next wake: */ p->recent_used_cpu = prev; return recent_used_cpu; //还将p放在它最近使用过的cpu上 } /* * For asymmetric CPU capacity systems, our domain of interest is sd_asym_cpucapacity rather than sd_llc. */ if (static_branch_unlikely(&sched_asym_cpucapacity)) { sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, target)); //target_cpu对应的DIE层级的sd /* * On an asymmetric CPU capacity system where an exclusive cpuset defines a symmetric island (i.e. one unique * capacity_orig value through the cpuset), the key will be set but the CPUs within that cpuset will not have * a domain with SD_ASYM_CPUCAPACITY. These should follow the usual symmetric capacity path. */ if (sd) { //这个函数还可能返回-1,但是并没有兼容这个错误的结果！后续的使用也没有做判断，是个BUG!! i = select_idle_capacity(p, sd, target); return ((unsigned)i < nr_cpumask_bits) ? i : target; //这里会返回，下面的部分不会被执行到 } } /* 下面就是非大小核架构会执行的路径了 */ sd = rcu_dereference(per_cpu(sd_llc, target)); //target_cpu对应的MC层级的sd if (!sd) return target; i = select_idle_core(p, sd, target); //CONFIG_SCHED_SMT没定义为空函数 if ((unsigned)i < nr_cpumask_bits) return i; i = select_idle_cpu(p, sd, target); //CONFIG_SCHED_SMT没定义为空函数 if ((unsigned)i < nr_cpumask_bits) return i; i = select_idle_smt(p, sd, target); //CONFIG_SCHED_SMT没定义为空函数 if ((unsigned)i < nr_cpumask_bits) return i; return target; } /* * select_idle_sibling传参：p为待选核任务; sd为参数target_cpu对应的DIE层级的sd; target为候选的target_cpu * * 作用：从target_cpu开始遍历，找一个能容纳下任务p的idle cpu, 若所有idle cpu都不能容纳下任务，就返回算力最大的idle cpu, * 否则返回-1; */ static int select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target) { unsigned long task_util, best_cap = 0; int cpu, best_cpu = -1; struct cpumask *cpus; cpus = this_cpu_cpumask_var_ptr(select_idle_mask); //先赋值后使用 cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr); task_util = uclamp_task_util(p); //这里从target_cpu开始遍历，利用了之前判断的结果target_cpu的算力是能容纳任务p的 for_each_cpu_wrap(cpu, cpus, target) { unsigned long cpu_cap = capacity_of(cpu); if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu)) //若此cpu非idle则跳过这个cpu continue; if (fits_capacity(task_util, cpu_cap)) //返回一个能容纳任务的idle cpu return cpu; if (cpu_cap > best_cap) { //若是没有能容纳的，则返回一个算力最大的 best_cap = cpu_cap; best_cpu = cpu; } } return best_cpu; }
1. 此函数中要使用到p的util, 所以需要先更新其负载。
2. 快速路径选核依次判断：
(1) 传入的target是一个idle cpu, 且可以容纳的了uclamp后的任务的util，就直接选之前选的这个 target_cpu
(2) 若传入的参数target是由wake_affine选出的cpu，且和prev_cpu属于同一个cluster(共享L2-Cache), 且prev_cpu是idle的, 且prev_cpu能容纳的下uclamp后的任务p的util, 那么选择 prev_cpu。
(3) 当前任务是一个per-cpu的内核线程，且任务的prev_cpu就是当前cpu，且当前cpu上最多只有一个任务在运行，就选 prev_cpu。一个典型的例子是IO操作场景。
(4) 若 p->recent_used_cpu 既不是prev_cpu也不是target_cpu, 但是是和target_cpu共cluster的，且是idle的, 且任务的亲和性允许，且算力能够容纳任务，那么就选 p->recent_used_cpu。
(5) 从target_cpu开始遍历，找一个能容纳下任务p的idle cpu, 若所有idle cpu都不能容纳下任务，就返回算力最大的idle cpu, 否则返回-1;
3. 注意有个BUG: select_idle_capacity()返回-1的话没有做容错处理。
五、总结
1. 有三种任务选核路径，sd_flag分别对应为 SD_BALANCE_WAKE(唤醒场景)、SD_BALANCE_FORK(fork新任务场景)、SD_BALANCE_EXEC(exec场景)。其中EAS选核和wake-affine选核只适用于唤醒场景，EAS选到核后直接退出，wake-affine选到核后还需要走快速路径继续选核，希望在当前cpu的同cluster中选一个空闲cpu或prev_cpu，若选不到，最终也会尝试在所有cluster中选核。fork新任务场景和exec场景走慢速选核路径，找出系统中最闲的cpu作为目标cpu。
参考：https://blog.csdn.net/feelabclihu/article/details/122007603?spm=1001.2014.3001.5501