QTI EAS学习之find_energy_efficient_cpu
Energy Awareness Scheduler是由ARM和Linaro开发的新的linux kernel调度器。
原先CFS调度器是基于policy进行调度，并有不同的吞吐量。例如，有一个新的task创建，同时也有一个idle cpu时，CFS始终会把新的task放到这个idle cpu上运行。但是，这样对节省功耗来说，并不是一个最好的决定。而EAS就是为了解决这样的问题。在不影响性能的前提下，EAS会在调度时实现节省功耗。
从SDM845开始，QTI在EAS基础上进行了一些修改，以满足移动市场的需要。所以QTI在EAS基础上添加了一些feature，来获得更好的性能和功耗。
Energy model
在dts中，针对不同的cpu平台，已定义好不同的energy model。模型主要是由【频率，能量】的数组构成，对应了CPU和cluster不同的OOP（Operating Performance Point）；同时也提供了不同idle state的能量消耗：idle cost。
CPU0: cpu@ {
device_type = "cpu";
compatible = "arm,armv8";
reg = <0x0 0x0>;
enable-method = "psci";
efficiency = <>;
cache-size = <0x8000>;
cpu-release-addr = <0x0 0x90000000>;
qcom,lmh-dcvs = <&lmh_dcvs0>;
#cooling-cells = <>;
next-level-cache = <&L2_0>;
sched-energy-costs = <&CPU_COST_0 &CLUSTER_COST_0>; //小核都用CPU_COST_0 CLUSTER_COST_0
。。。。。。
CPU4: cpu@ {
device_type = "cpu";
compatible = "arm,armv8";
reg = <0x0 0x400>;
enable-method = "psci";
efficiency = <>;
cache-size = <0x20000>;
cpu-release-addr = <0x0 0x90000000>;
qcom,lmh-dcvs = <&lmh_dcvs1>;
#cooling-cells = <>;
next-level-cache = <&L2_400>;
sched-energy-costs = <&CPU_COST_1 &CLUSTER_COST_1>; //大核都用CPU_COST_1 CLUSTER_COST_1
。。。。。。。
对应的数组如下，
energy_costs: energy-costs {
compatible = "sched-energy";
CPU_COST_0: core-cost0 {
busy-cost-data = <
/* speedbin 0,1 */
/* speedbin 2 */
/* speedbin 0,1 */
/* speedbin 2 */
>;
idle-cost-data = <
>;
};
CPU_COST_1: core-cost1 {
busy-cost-data = <
/* speedbin 1,2 */
/* speedbin 1 */
/* speedbin 2 */
/* speedbin 2 */
>;
idle-cost-data = <
>;
};
CLUSTER_COST_0: cluster-cost0 {
busy-cost-data = <
/* speedbin 0,1 */
/* speedbin 2 */
/* speedbin 0,1 */
/* speedbin 2 */
>;
idle-cost-data = <
>;
};
CLUSTER_COST_1: cluster-cost1 {
busy-cost-data = <
/* speedbin 1,2 */
/* speedbin 1 */
/* speedbin 2 */
/* speedbin 2 */
>;
idle-cost-data = <
>;
};
}; /* energy-costs */
在代码kernel/sched/energy.c中遍历所有cpu，并读取dts中的数据
for_each_possible_cpu(cpu) {
cn = of_get_cpu_node(cpu, NULL);
if (!cn) {
pr_warn("CPU device node missing for CPU %d\n", cpu);
return;
}
if (!of_find_property(cn, "sched-energy-costs", NULL)) {
pr_warn("CPU device node has no sched-energy-costs\n");
return;
}
for_each_possible_sd_level(sd_level) {
cp = of_parse_phandle(cn, "sched-energy-costs", sd_level);
if (!cp)
break;
prop = of_find_property(cp, "busy-cost-data", NULL);
if (!prop || !prop->value) {
pr_warn("No busy-cost data, skipping sched_energy init\n");
goto out;
}
sge = kcalloc(, sizeof(struct sched_group_energy),
GFP_NOWAIT);
if (!sge)
goto out;
nstates = (prop->length / sizeof(u32)) / ;
cap_states = kcalloc(nstates,
sizeof(struct capacity_state),
GFP_NOWAIT);
if (!cap_states) {
kfree(sge);
goto out;
}
for (i = , val = prop->value; i < nstates; i++) { //将读取的[freq,energy]数组存放起来
cap_states[i].cap = SCHED_CAPACITY_SCALE;
cap_states[i].frequency = be32_to_cpup(val++);
cap_states[i].power = be32_to_cpup(val++);
}
sge->nr_cap_states = nstates; //state为[freq,energy]组合个数，就是支持多少个状态：将所有数据flatten之后，再处以2
sge->cap_states = cap_states;
prop = of_find_property(cp, "idle-cost-data", NULL);
if (!prop || !prop->value) {
pr_warn("No idle-cost data, skipping sched_energy init\n");
kfree(sge);
kfree(cap_states);
goto out;
}
nstates = (prop->length / sizeof(u32));
idle_states = kcalloc(nstates,
sizeof(struct idle_state),
GFP_NOWAIT);
if (!idle_states) {
kfree(sge);
kfree(cap_states);
goto out;
}
for (i = , val = prop->value; i < nstates; i++)
idle_states[i].power = be32_to_cpup(val++); //将读取的idle cost data存放起来
sge->nr_idle_states = nstates; //idle state的个数，就是idle cost data的长度
sge->idle_states = idle_states;
sge_array[cpu][sd_level] = sge; //将当前cpu获取的energy模型存放再sge_array[cpu][sd_level]中。其中cpu就是对应哪个cpu，sd_level则对应是哪个sched_domain，也就是是cpu level还是cluster level
}
}
Load Tracking
QTI EAS使用的负载计算是WALT，是基于时间窗口的load统计方法，具体参考之前文章：https://www.cnblogs.com/lingjiajun/p/12317090.html
其中会跟踪计算出2个比较关键的数据，就是task_util和cpu_util
当执行wakeup task placement，scheduler就会使用task utilization和CPU utilization
可以理解为将load的情况转化为Utilization，并且将其标准化为1024的值。
Task utilization boosted = Task utilization + (1024-task_util) x boost_percent -----boost percent是使用schedtune boost时，所需要乘上的百分比
CPU utilization = 1024 x (累计的runnable均值 / window size)--------累计的runnable均值，个人理解就是rq上所有task util的总和
Task placement的主要概念：
EAS是Task placement 是EAS影响调度的主要模块。 其主要keypoint如下：
1、EAS依靠energy model来进行精确地进行选择CPU运行
2、使用energy model估算：把一个任务安排在一个CPU上，或者将任务从一个CPU迁移到另一个CPU上，所发生的能量变化
3、EAS会在不影响performance情况下（比如满足满足最低的latency），趋向于选择消耗能量最小的CPU，去运行当前的task
4、EAS仅发生在system没有overutilized的情况下
5、EAS的概念与QTI EAS的一样
6、一旦系统处于overutilized，QTI EAS仍然在wake up的path下进行energy aware。不会考虑系统overutilized的情形。
EAS核心调度算法
不同版本的EAS在不同版本下的主要task placement实现函数（针对CFS task）：
Zone scheduler： select_best_cpu()
QTI EAS r1.： energy_aware_wake_cpu()
QTI EAS r1.： find_energy_efficienct_cpu()
task placement调用路径：
QTI EAS r1. (Kernel 4.14) Task wake-up： try_to_wake_up() →select_task_rq_fair() →invokes find_energy_efficient_cpu() Scheduler tick occurs： scheduler_tick() →check_for_migration() →invokes find_energy_efficient_cpu() New task arrives： do_fork() →wake_up_new_task() →select_task_rq_fair() →invokes find_energy_efficient_cpu()
EAS的task placement代码流程，主要目标是找到一个合适的cpu来运行当前这个task p。
主要代码就是find_energy_efficient_cpu（）这个函数里面，如下：
/*
* find_energy_efficient_cpu(): Find most energy-efficient target CPU for the
* waking task. find_energy_efficient_cpu() looks for the CPU with maximum
* spare capacity in each performance domain and uses it as a potential
* candidate to execute the task. Then, it uses the Energy Model to figure
* out which of the CPU candidates is the most energy-efficient.
*
* The rationale for this heuristic is as follows. In a performance domain,
* all the most energy efficient CPU candidates (according to the Energy
* Model) are those for which we'll request a low frequency. When there are
* several CPUs for which the frequency request will be the same, we don't
* have enough data to break the tie between them, because the Energy Model
* only includes active power costs. With this model, if we assume that
* frequency requests follow utilization (e.g. using schedutil), the CPU with
* the maximum spare capacity in a performance domain is guaranteed to be among
* the best candidates of the performance domain.
*
* In practice, it could be preferable from an energy standpoint to pack
* small tasks on a CPU in order to let other CPUs go in deeper idle states,
* but that could also hurt our chances to go cluster idle, and we have no
* ways to tell with the current Energy Model if this is actually a good
* idea or not. So, find_energy_efficient_cpu() basically favors
* cluster-packing, and spreading inside a cluster. That should at least be
* a good thing for latency, and this is consistent with the idea that most
* of the energy savings of EAS come from the asymmetry of the system, and
* not so much from breaking the tie between identical CPUs. That's also the
* reason why EAS is enabled in the topology code only for systems where
* SD_ASYM_CPUCAPACITY is set.
*
* NOTE: Forkees are not accepted in the energy-aware wake-up path because
* they don't have any useful utilization data yet and it's not possible to
* forecast their impact on energy consumption. Consequently, they will be
* placed by find_idlest_cpu() on the least loaded CPU, which might turn out
* to be energy-inefficient in some use-cases. The alternative would be to
* bias new tasks towards specific types of CPUs first, or to try to infer
* their util_avg from the parent task, but those heuristics could hurt
* other use-cases too. So, until someone finds a better way to solve this,
* let's keep things simple by re-using the existing slow path.
*/ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu,
int sync, int sibling_count_hint)
{
unsigned long prev_energy = ULONG_MAX, best_energy = ULONG_MAX;
struct root_domain *rd = cpu_rq(smp_processor_id())->rd;
int weight, cpu = smp_processor_id(), best_energy_cpu = prev_cpu; //cpu：当前执行的cpu
unsigned long cur_energy;
struct perf_domain *pd;
struct sched_domain *sd;
cpumask_t *candidates;
bool is_rtg;
struct find_best_target_env fbt_env;
bool need_idle = wake_to_idle(p); //是否set flag PF_WAKE_UP_IDLE
int placement_boost = task_boost_policy(p); //获取task sched boost policy：none/on_big/on_all 与sched_boost、schedtune设置也有关
u64 start_t = ;
int delta = ;
int task_boost = per_task_boost(p); //仅网络有打开该boost，这里可以认为没有boost
int boosted = (schedtune_task_boost(p) > ) || (task_boost > ); //查看task的schedtune有没有打开boost
int start_cpu = get_start_cpu(p); //获取从哪个cpu core开始，尝试作为target cpu if (start_cpu < )
goto eas_not_ready; is_rtg = task_in_related_thread_group(p); //判断task是否在一个group内 fbt_env.fastpath = ; if (trace_sched_task_util_enabled())
start_t = sched_clock(); //trace log /* Pre-select a set of candidate CPUs. */
candidates = this_cpu_ptr(&energy_cpus);
cpumask_clear(candidates); if (need_idle)
sync = ; if (sysctl_sched_sync_hint_enable && sync &&
bias_to_this_cpu(p, cpu, start_cpu)) { //满足3个调节：sync hint enable/flag：sync=1/bias to当前cpu
best_energy_cpu = cpu; //当前执行的cpu
fbt_env.fastpath = SYNC_WAKEUP;
goto done;
} if (is_many_wakeup(sibling_count_hint) && prev_cpu != cpu && //sibling_count_hint代表有多少个thread在当前event中唤醒
bias_to_this_cpu(p, prev_cpu, start_cpu)) {
best_energy_cpu = prev_cpu; //选择prev cpu
fbt_env.fastpath = MANY_WAKEUP;
goto done;
} rcu_read_lock();
pd = rcu_dereference(rd->pd);
if (!pd)
goto fail; /*
* Energy-aware wake-up happens on the lowest sched_domain starting
* from sd_asym_cpucapacity spanning over this_cpu and prev_cpu.
*/
sd = rcu_dereference(*this_cpu_ptr(&sd_asym_cpucapacity));
while (sd && !cpumask_test_cpu(prev_cpu, sched_domain_span(sd)))
sd = sd->parent;
if (!sd)
goto fail; sync_entity_load_avg(&p->se); //更新task所在sched_entity的PELT load
if (!task_util_est(p))
goto unlock; if (sched_feat(FIND_BEST_TARGET)) { //检查FIND_BEST_TARGET这个调度特性是否打开：目前是打开的
fbt_env.is_rtg = is_rtg;
fbt_env.placement_boost = placement_boost;
fbt_env.need_idle = need_idle;
fbt_env.start_cpu = start_cpu;
fbt_env.boosted = boosted;
fbt_env.strict_max = is_rtg &&
(task_boost == TASK_BOOST_STRICT_MAX);
fbt_env.skip_cpu = is_many_wakeup(sibling_count_hint) ?
cpu : -; find_best_target(NULL, candidates, p, &fbt_env); //（1）核心函数，最终是将找到的target_cpu和backup_cpu都存放进了candidates中
} else {
select_cpu_candidates(sd, candidates, pd, p, prev_cpu);
} /* Bail out if no candidate was found. */
weight = cpumask_weight(candidates); //判断如果没有找到target cpu和backup cpu时，直接goto unlock
if (!weight)
goto unlock; /* If there is only one sensible candidate, select it now. */
cpu = cpumask_first(candidates);
if (weight == && ((schedtune_prefer_idle(p) && idle_cpu(cpu)) || //如果只找到了1个cpu，task是prefer_idle并且这个cpu也是idle的；或者cpu就是prev_cpu
(cpu == prev_cpu))) {
best_energy_cpu = cpu; //那么就选这个cpu为【best_energy_cpu】
goto unlock;
} #ifdef CONFIG_SCHED_WALT
if (p->state == TASK_WAKING) //如果是新唤醒的task，获取task_util
delta = task_util(p);
#endif
if (task_placement_boost_enabled(p) || need_idle || boosted || //满足一下条件之一，那么第一个candidate cpu就作为【best_energy_cpu】不再考虑计算energy
is_rtg || __cpu_overutilized(prev_cpu, delta) || //打开了sched_boost、need_idle（PF_WAKE_UP_IDLE）、开了schedtune boost、related_thread_group限制使用小核、prev_cpu+delta没有overutil、
!task_fits_max(p, prev_cpu) || cpu_isolated(prev_cpu)) { //p放在prev_cpu上会misfit、prev_cpu处于isolated
best_energy_cpu = cpu;
goto unlock;
} if (cpumask_test_cpu(prev_cpu, &p->cpus_allowed)) //根据prev_cpu是否在task p的cpuset范围内
prev_energy = best_energy = compute_energy(p, prev_cpu, pd); //（2）在范围内，则计算p在prev_cpu上的energy
else
prev_energy = best_energy = ULONG_MAX; //不匹配，energy就设为最大，代表不合适 /* Select the best candidate energy-wise. */ //通过比较energy，挑选出best_energy_cpu、best_energy
for_each_cpu(cpu, candidates) {
if (cpu == prev_cpu) //过滤prev_cpu
continue;
cur_energy = compute_energy(p, cpu, pd); //计算p迁移到candidate cpu上的energy
trace_sched_compute_energy(p, cpu, cur_energy, prev_energy,
best_energy, best_energy_cpu);
if (cur_energy < best_energy) {
best_energy = cur_energy;
best_energy_cpu = cpu;
} else if (cur_energy == best_energy) {
if (select_cpu_same_energy(cpu, best_energy_cpu, //当candidate cpu的energy与best_cpu一样的话，怎么选
prev_cpu)) {
best_energy = cur_energy;
best_energy_cpu = cpu;
}
}
}
unlock:
rcu_read_unlock(); /*
* Pick the prev CPU, if best energy CPU can't saves at least 6% of
* the energy used by prev_cpu.
*/
if ((prev_energy != ULONG_MAX) && (best_energy_cpu != prev_cpu) && //找到了非prev_cpu的best_energy_cpu、且省电下来的energy要大于在prev_energy上的6%，那么best_energy_cpu则满足条件；否则仍然使用prev_cpu
((prev_energy - best_energy) <= prev_energy >> )) //这里巧妙地使用了位移：右移1位代表÷2，所以prev_energy/2/2/2/2 = prev_energy*6%
best_energy_cpu = prev_cpu; done: trace_sched_task_util(p, cpumask_bits(candidates)[], best_energy_cpu,
sync, need_idle, fbt_env.fastpath, placement_boost,
start_t, boosted, is_rtg, get_rtg_status(p), start_cpu); return best_energy_cpu; fail:
rcu_read_unlock();
eas_not_ready:
return -;
}
（1）find_best_target（）
static void find_best_target(struct sched_domain *sd, cpumask_t *cpus,
struct task_struct *p,
struct find_best_target_env *fbt_env)
{
unsigned long min_util = boosted_task_util(p); //获取p的boosted_task_util
unsigned long target_capacity = ULONG_MAX;
unsigned long min_wake_util = ULONG_MAX;
unsigned long target_max_spare_cap = ;
unsigned long best_active_util = ULONG_MAX;
unsigned long best_active_cuml_util = ULONG_MAX;
unsigned long best_idle_cuml_util = ULONG_MAX;
bool prefer_idle = schedtune_prefer_idle(p); //获取task prefer_idle配置
bool boosted = fbt_env->boosted;
/* Initialise with deepest possible cstate (INT_MAX) */
int shallowest_idle_cstate = INT_MAX;
struct sched_domain *start_sd;
struct sched_group *sg;
int best_active_cpu = -;
int best_idle_cpu = -;
int target_cpu = -;
int backup_cpu = -;
int i, start_cpu;
long spare_wake_cap, most_spare_wake_cap = ;
int most_spare_cap_cpu = -;
int prev_cpu = task_cpu(p);
bool next_group_higher_cap = false;
int isolated_candidate = -; /*
* In most cases, target_capacity tracks capacity_orig of the most
* energy efficient CPU candidate, thus requiring to minimise
* target_capacity. For these cases target_capacity is already
* initialized to ULONG_MAX.
* However, for prefer_idle and boosted tasks we look for a high
* performance CPU, thus requiring to maximise target_capacity. In this
* case we initialise target_capacity to 0.
*/
if (prefer_idle && boosted)
target_capacity = ; if (fbt_env->strict_max)
most_spare_wake_cap = LONG_MIN; /* Find start CPU based on boost value */
start_cpu = fbt_env->start_cpu;
/* Find SD for the start CPU */
start_sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, start_cpu)); //找到start cpu所在的sched domain，sd_asym_cpucapacity表示是非对称cpu capacity级别，应该就是DIE level，所以domain是cpu0-7
if (!start_sd)
goto out; /* fast path for prev_cpu */
if (((capacity_orig_of(prev_cpu) == capacity_orig_of(start_cpu)) || //prev cpu和start cpu的当前max_policy_freq下的capacity相等
asym_cap_siblings(prev_cpu, start_cpu)) &&
!cpu_isolated(prev_cpu) && cpu_online(prev_cpu) &&
idle_cpu(prev_cpu)) { if (idle_get_state_idx(cpu_rq(prev_cpu)) <= ) { //prev cpu idle state的index <1，说明休眠不深
target_cpu = prev_cpu; fbt_env->fastpath = PREV_CPU_FASTPATH;
goto target;
}
} /* Scan CPUs in all SDs */
sg = start_sd->groups;
do { //do-while循环，针对start cpu的调度域中的所有调度组进行遍历，由于domain是cpu0-7，那么调度组就是2个大小cluster：cpu0-3，cpu4-7
for_each_cpu_and(i, &p->cpus_allowed, sched_group_span(sg)) { //寻找task允许的cpuset和调度组可用cpu范围内
unsigned long capacity_curr = capacity_curr_of(i); //当前freq的cpu_capacity
unsigned long capacity_orig = capacity_orig_of(i); //当前max_policy_freq的cpu_capacity, >=capacity_curr
unsigned long wake_util, new_util, new_util_cuml;
long spare_cap;
int idle_idx = INT_MAX; trace_sched_cpu_util(i); if (!cpu_online(i) || cpu_isolated(i)) //cpu处于非online，或者isolate状态，则直接不考虑
continue; if (isolated_candidate == -)
isolated_candidate = i; /*
* This CPU is the target of an active migration that's
* yet to complete. Avoid placing another task on it.
* See check_for_migration()
*/
if (is_reserved(i)) //已经有task要迁移到上面，但是还没有迁移完成。所以这样的cpu不考虑
continue; if (sched_cpu_high_irqload(i)) //高irq load的cpu不考虑。irq load可以参考之前WALT文章：https://www.cnblogs.com/lingjiajun/p/12317090.html
continue; if (fbt_env->skip_cpu == i) //当前活动的cpu是否有很多event一起wakeup，如果有，那么也不考虑该cpu
continue; /*
* p's blocked utilization is still accounted for on prev_cpu
* so prev_cpu will receive a negative bias due to the double
* accounting. However, the blocked utilization may be zero.
*/
wake_util = cpu_util_without(i, p); //计算没有除了p以外的cpu_util（p不在该cpu rq的情况下，实际就是当前cpu_util）
new_util = wake_util + task_util_est(p); //计算cpu_util + p的task_util（p的task_util就是walt统计的demand_scaled）
spare_wake_cap = capacity_orig - wake_util; //剩余的capacity = capacity_orig - p以外的cpu_util if (spare_wake_cap > most_spare_wake_cap) {
most_spare_wake_cap = spare_wake_cap; //在循环中，找到有剩余capacity最多（最空闲）的cpu = i，并保存剩余的capacity
most_spare_cap_cpu = i;
} if (per_task_boost(cpu_rq(i)->curr) == //cpu【i】当前running_task的task_boost == TASK_BOOST_STRICT_MAX，那么不适合作为tager_cpu
TASK_BOOST_STRICT_MAX)
continue;
/*
* Cumulative demand may already be accounting for the
* task. If so, add just the boost-utilization to
* the cumulative demand of the cpu.
*/
if (task_in_cum_window_demand(cpu_rq(i), p)) //计算新的cpu【i】的cpu_util = cpu_util_cum + p的boosted_task_util
new_util_cuml = cpu_util_cum(i, ) + //特别地，如果p已经在cpu【i】的rq中，或者p的部分demand被统计在了walt中。那么防止统计2次，所以要减去p的task_util（denamd_scaled）
min_util - task_util(p);
else
new_util_cuml = cpu_util_cum(i, ) + min_util; /*
* Ensure minimum capacity to grant the required boost.
* The target CPU can be already at a capacity level higher
* than the one required to boost the task.
*/
new_util = max(min_util, new_util); //取 p的booted_task_util、加入p之后的cpu_util，之间的较大值
if (new_util > capacity_orig) //与capacity_orig比较，大于capacity_orig的情况下，不适合作为target_cpu
continue; /*
* Pre-compute the maximum possible capacity we expect
* to have available on this CPU once the task is
* enqueued here.
*/
spare_cap = capacity_orig - new_util; //预计算当p迁移到cpu【i】上后，剩余的可能最大capacity if (idle_cpu(i)) //判断当前cpu【i】是否处于idle，并获取idle index（idle的深度）
idle_idx = idle_get_state_idx(cpu_rq(i)); /*
* Case A) Latency sensitive tasks
*
* Unconditionally favoring tasks that prefer idle CPU to
* improve latency.
*
* Looking for:
* - an idle CPU, whatever its idle_state is, since
* the first CPUs we explore are more likely to be
* reserved for latency sensitive tasks.
* - a non idle CPU where the task fits in its current
* capacity and has the maximum spare capacity.
* - a non idle CPU with lower contention from other
* tasks and running at the lowest possible OPP.
*
* The last two goals tries to favor a non idle CPU
* where the task can run as if it is "almost alone".
* A maximum spare capacity CPU is favoured since
* the task already fits into that CPU's capacity
* without waiting for an OPP chance.
*
* The following code path is the only one in the CPUs
* exploration loop which is always used by
* prefer_idle tasks. It exits the loop with wither a
* best_active_cpu or a target_cpu which should
* represent an optimal choice for latency sensitive
* tasks.
*/
if (prefer_idle) { //对lantency有要求的task
/*
* Case A.1: IDLE CPU
* Return the best IDLE CPU we find:
* - for boosted tasks: the CPU with the highest
* performance (i.e. biggest capacity_orig)
* - for !boosted tasks: the most energy
* efficient CPU (i.e. smallest capacity_orig)
*/
if (idle_cpu(i)) { //如果cpu【i】是idle的
if (boosted &&
capacity_orig < target_capacity) //对于boosted task，cpu需要选择最大capacity_orig，不满足要continue
continue;
if (!boosted &&
capacity_orig > target_capacity) //对于非boosted task，cpu选择最小capacity_orig，不满足要continue
continue;
/*
* Minimise value of idle state: skip
* deeper idle states and pick the
* shallowest.
*/
if (capacity_orig == target_capacity &&
sysctl_sched_cstate_aware &&
idle_idx >= shallowest_idle_cstate) //包括下面的continue，都是为了挑选出处于idle最浅的cpu
continue; target_capacity = capacity_orig;
shallowest_idle_cstate = idle_idx;
best_idle_cpu = i; //选出【prefer_idle】best_idle_cpu
continue;
}
if (best_idle_cpu != -) //过滤上面已经找到best_idle_cpu的情况，不需要走下面流程了
continue; /*
* Case A.2: Target ACTIVE CPU
* Favor CPUs with max spare capacity.
*/
if (capacity_curr > new_util &&
spare_cap > target_max_spare_cap) { //找到capacity_curr满足包含进程p的cpu_util，并且找到空闲capacity最多的那个cpu
target_max_spare_cap = spare_cap;
target_cpu = i; //选出【prefer_idle】target_cpu
continue;
}
if (target_cpu != -) //如果cpu条件不满足，则continue，继续找target_cpu
continue; /*
* Case A.3: Backup ACTIVE CPU
* Favor CPUs with:
* - lower utilization due to other tasks
* - lower utilization with the task in
*/
if (wake_util > min_wake_util) //找出除了p以外的cpu_util最小的cpu
continue; /*
* If utilization is the same between CPUs,
* break the ties with WALT's cumulative
* demand
*/
if (new_util == best_active_util &&
new_util_cuml > best_active_cuml_util) //如果包含p的cpu_util相等，那么就挑选cpu_util + p的boosted_task_util最小的那个cpu
continue;
min_wake_util = wake_util;
best_active_util = new_util;
best_active_cuml_util = new_util_cuml;
best_active_cpu = i; //选出【prefer_idle】best_active_cpu
continue;
} /*
* Skip processing placement further if we are visiting
* cpus with lower capacity than start cpu
*/
if (capacity_orig < capacity_orig_of(start_cpu)) //cpu【i】capacity_orig < 【start_cpu】capacity_orig的不考虑
continue; /*
* Case B) Non latency sensitive tasks on IDLE CPUs.
*
* Find an optimal backup IDLE CPU for non latency
* sensitive tasks.
*
* Looking for:
* - minimizing the capacity_orig,
* i.e. preferring LITTLE CPUs
* - favoring shallowest idle states
* i.e. avoid to wakeup deep-idle CPUs
*
* The following code path is used by non latency
* sensitive tasks if IDLE CPUs are available. If at
* least one of such CPUs are available it sets the
* best_idle_cpu to the most suitable idle CPU to be
* selected.
*
* If idle CPUs are available, favour these CPUs to
* improve performances by spreading tasks.
* Indeed, the energy_diff() computed by the caller67jkkk
* will take care to ensure the minimization of energy
* consumptions without affecting performance.
*/ //对latency要求不高的task，并要求idle cpu作为target的情况
if (idle_cpu(i)) { //判断cpu【i】是否idle
/*
* Prefer shallowest over deeper idle state cpu,
* of same capacity cpus.
*/
if (capacity_orig == target_capacity && //选出capacity相同情况下，idle最浅的cpu
sysctl_sched_cstate_aware &&
idle_idx > shallowest_idle_cstate)
continue; if (shallowest_idle_cstate == idle_idx &&
target_capacity == capacity_orig &&
(best_idle_cpu == prev_cpu ||
(i != prev_cpu &&
new_util_cuml > best_idle_cuml_util))) //best_idle_cpu非prev_cpu，并且挑选cpu_util + p的boosted_task_util最小的
continue; target_capacity = capacity_orig;
shallowest_idle_cstate = idle_idx;
best_idle_cuml_util = new_util_cuml;
best_idle_cpu = i; //选出【normal-idle】best_idle_cpu
continue;
} /*
* Consider only idle CPUs for active migration.
*/
if (p->state == TASK_RUNNING) //task p正在运行，则不进行下面流程
continue; /*
* Case C) Non latency sensitive tasks on ACTIVE CPUs.
*
* Pack tasks in the most energy efficient capacities.
*
* This task packing strategy prefers more energy
* efficient CPUs (i.e. pack on smaller maximum
* capacity CPUs) while also trying to spread tasks to
* run them all at the lower OPP.
*
* This assumes for example that it's more energy
* efficient to run two tasks on two CPUs at a lower
* OPP than packing both on a single CPU but running
* that CPU at an higher OPP.
*
* Thus, this case keep track of the CPU with the
* smallest maximum capacity and highest spare maximum
* capacity.
*/ //对latency要求不高，并需要ACTIVE cpu作为target的情况 /* Favor CPUs with maximum spare capacity */
if (spare_cap < target_max_spare_cap) //找到迁移p之后，剩余capacity最多的cpu
continue; target_max_spare_cap = spare_cap;
target_capacity = capacity_orig;
target_cpu = i; //找出【normal-ACTIVe】的target_cpu
} //到此就是一个调度组（cluster）内cpu的循环查找 next_group_higher_cap = (capacity_orig_of(group_first_cpu(sg)) <
capacity_orig_of(group_first_cpu(sg->next))); //尝试查找下一个capacity更大的big cluster /*
* If we've found a cpu, but the boost is ON_ALL we continue
* visiting other clusters. If the boost is ON_BIG we visit
* next cluster if they are higher in capacity. If we are
* not in any kind of boost, we break.
*
* And always visit higher capacity group, if solo cpu group
* is not in idle.
*/
if (!prefer_idle && !boosted && //上面找到cpu但是boost=ON_ALL，那么还要查找其他cluster
((target_cpu != - && (sg->group_weight > || //上面找到cpu但是boost=ON_BIG，那么还要在capacity更大的cluster中查找
!next_group_higher_cap)) || //上面找到了cpu，并且不在任何boost。那么break
best_idle_cpu != -) && //如果上面group中，没有cpu是idle，那么always在capacity更大的cluster中查找
(fbt_env->placement_boost == SCHED_BOOST_NONE ||
!is_full_throttle_boost() ||
(fbt_env->placement_boost == SCHED_BOOST_ON_BIG &&
!next_group_higher_cap)))
break; /*
* if we are in prefer_idle and have found an idle cpu,
* break from searching more groups based on the stune.boost and
* group cpu capacity. For !prefer_idle && boosted case, don't
* iterate lower capacity CPUs unless the task can't be
* accommodated in the higher capacity CPUs.
*/
if ((prefer_idle && best_idle_cpu != -) || //如果设置了prefer_idle，并且找到了一个idle cpu；根据schedtune是否打开boost和是否有更大capacity的cluster进行判断是否break
(boosted && (best_idle_cpu != - || target_cpu != - || //没有prefer_idle，但是打开boost的情况，除非high capacity的cpu不能接受task，否则不用再遍历low capacity的cpu
(fbt_env->strict_max && most_spare_cap_cpu != -)))) {
if (boosted) {
if (!next_group_higher_cap)
break;
} else {
if (next_group_higher_cap)
break;
}
} } while (sg = sg->next, sg != start_sd->groups); adjust_cpus_for_packing(p, &target_cpu, &best_idle_cpu, //计算将task放在target_cpu时，在考虑20%的余量，和sched_load_boost之后，看capacity是否满足target_cpu当前freq的capacity
shallowest_idle_cstate, //另外检查rtg，看是否不考虑idle cpu
fbt_env, boosted); /*
* For non latency sensitive tasks, cases B and C in the previous loop,
* we pick the best IDLE CPU only if we was not able to find a target
* ACTIVE CPU. //latency要求不高的task选择cpu优先级：ACTIVE cpu > idle cpu；没有ACITVE，则选idle cpu
*
* Policies priorities:
*
* - prefer_idle tasks: //prefer_idle的task选择cpu优先级：idle cpu > ACTIVE cpu(包含task之后又更多spare capacity) > ACTIVE cpu(更小cpu_util+boosted_task_util)
*
* a) IDLE CPU available: best_idle_cpu
* b) ACTIVE CPU where task fits and has the bigger maximum spare
* capacity (i.e. target_cpu)
* c) ACTIVE CPU with less contention due to other tasks
* (i.e. best_active_cpu)
*
* - NON prefer_idle tasks: //非prefer_idle的task选择cpu优先级：ACTIVE cpu > idle cpu
*
* a) ACTIVE CPU: target_cpu
* b) IDLE CPU: best_idle_cpu
*/ if (prefer_idle && (best_idle_cpu != -)) { //prefer_idle的task，直接选择best_idle_cpu作为target
target_cpu = best_idle_cpu;
goto target;
} if (target_cpu == -) //假如target没有找到，那么重新找target：
target_cpu = prefer_idle
? best_active_cpu //1、prefer_idle的task选择best_active_cpu;
: best_idle_cpu; //2、而非prefer_idle的task选择best_idle_cpu
else
backup_cpu = prefer_idle //假如找到了target，那么再选backup_cpu：
? best_active_cpu //1、prefer_idle的task选择 best_active_cpu
: best_idle_cpu; //2、非prefer_idle的task选择 best_idle_cpu if (target_cpu == - && most_spare_cap_cpu != - &&
/* ensure we use active cpu for active migration */ //active migration（misfit task迁移）情况只选择active cpu
!(p->state == TASK_RUNNING && !idle_cpu(most_spare_cap_cpu)))
target_cpu = most_spare_cap_cpu; if (target_cpu == - && isolated_candidate != - && //假如没有找到target_cpu，prev_cpu又处于isolated，而task允许的所有cpu中有online并且unisolated的
cpu_isolated(prev_cpu))
target_cpu = isolated_candidate; //那么就选择最后一个online并unisolated的cpu作为target if (backup_cpu >= )
cpumask_set_cpu(backup_cpu, cpus); //将backup_cpu存放进cpus中
if (target_cpu >= ) {
target:
cpumask_set_cpu(target_cpu, cpus); //将找出的target cpu存放进cpus中
} out:
trace_sched_find_best_target(p, prefer_idle, min_util, start_cpu,
best_idle_cpu, best_active_cpu,
most_spare_cap_cpu,
target_cpu, backup_cpu);
}
（2）计算energy
/*
* compute_energy(): Estimates the energy that would be consumed if @p was
* migrated to @dst_cpu. compute_energy() predicts what will be the utilization
* landscape of the * CPUs after the task migration, and uses the Energy Model
* to compute what would be the energy if we decided to actually migrate that
* task.
*/
static long
compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
{
long util, max_util, sum_util, energy = ;
int cpu; for (; pd; pd = pd->next) {
max_util = sum_util = ;
/*
* The capacity state of CPUs of the current rd can be driven by
* CPUs of another rd if they belong to the same performance
* domain. So, account for the utilization of these CPUs too
* by masking pd with cpu_online_mask instead of the rd span.
*
* If an entire performance domain is outside of the current rd,
* it will not appear in its pd list and will not be accounted
* by compute_energy().
*/
for_each_cpu_and(cpu, perf_domain_span(pd), cpu_online_mask) { //在perf domain的cpu中找出online的
#ifdef CONFIG_SCHED_WALT
util = cpu_util_next_walt(cpu, p, dst_cpu); //计算迁移task p之后，每个cpu的util情况
#else
util = cpu_util_next(cpu, p, dst_cpu);
util += cpu_util_rt(cpu_rq(cpu));
util = schedutil_energy_util(cpu, util);
#endif
max_util = max(util, max_util); //找到perf domain中cpu util最大的值（同perf domain，即cluster，最大的util决定了freq的设定）
sum_util += util; //统计迁移之后，perf domain内的总util
} energy += em_pd_energy(pd->em_pd, max_util, sum_util); //计算perf domain的energy，并累计大小cluster的energy，就是整个系统energy
} return energy;
}
获取perf domain内的energy，在其中有2个重要的结构体：
/**
* em_cap_state - Capacity state of a performance domain
* @frequency: The CPU frequency in KHz, for consistency with CPUFreq
* @power: The power consumed by 1 CPU at this level, in milli-watts
* @cost: The cost coefficient associated with this level, used during
* energy calculation. Equal to: power * max_frequency / frequency
*/
struct em_cap_state {
unsigned long frequency;
unsigned long power;
unsigned long cost;
}; /**
* em_perf_domain - Performance domain
* @table: List of capacity states, in ascending order
* @nr_cap_states: Number of capacity states
* @cpus: Cpumask covering the CPUs of the domain
*
* A "performance domain" represents a group of CPUs whose performance is
* scaled together. All CPUs of a performance domain must have the same
* micro-architecture. Performance domains often have a 1-to-1 mapping with
* CPUFreq policies.
*/
struct em_perf_domain {
struct em_cap_state *table;
int nr_cap_states;
unsigned long cpus[];
};
em_pd_energy函数可以得到perf domain的energy。
/**
* em_pd_energy() - Estimates the energy consumed by the CPUs of a perf. domain
* @pd : performance domain for which energy has to be estimated
* @max_util : highest utilization among CPUs of the domain
* @sum_util : sum of the utilization of all CPUs in the domain
*
* Return: the sum of the energy consumed by the CPUs of the domain assuming
* a capacity state satisfying the max utilization of the domain.
*/
static inline unsigned long em_pd_energy(struct em_perf_domain *pd,
unsigned long max_util, unsigned long sum_util)
{
unsigned long freq, scale_cpu;
struct em_cap_state *cs;
int i, cpu; if (!sum_util)
return ; /*
* In order to predict the capacity state, map the utilization of the
* most utilized CPU of the performance domain to a requested frequency,
* like schedutil.
*/
cpu = cpumask_first(to_cpumask(pd->cpus));
scale_cpu = arch_scale_cpu_capacity(NULL, cpu); //获取cpu的max_capacity
cs = &pd->table[pd->nr_cap_states - ]; //获取capacity state，是为了获取最大频点（因为cs的table是升序排列的，所以最后一个配置就是最大的频点）
freq = map_util_freq(max_util, cs->frequency, scale_cpu); //利用上面获取的最大频点、max_capacity，根据当前的cpu util映射到当前的cpu freq /*
* Find the lowest capacity state of the Energy Model above the
* requested frequency.
*/
for (i = ; i < pd->nr_cap_states; i++) { //通过循环找到能满足当前cpu freq的最小的频点，及其对应的capacity state
cs = &pd->table[i]; //同样因为cs的table是升序排列的，所以递增找到第一个满足的，就是满足条件的最小频点
if (cs->frequency >= freq)
break;
} /*
* The capacity of a CPU in the domain at that capacity state (cs)
* can be computed as:
*
* cs->freq * scale_cpu
* cs->cap = -------------------- (1)
* cpu_max_freq
*
* So, ignoring the costs of idle states (which are not available in
* the EM), the energy consumed by this CPU at that capacity state is
* estimated as:
*
* cs->power * cpu_util
* cpu_nrg = -------------------- (2)
* cs->cap
*
* since 'cpu_util / cs->cap' represents its percentage of busy time.
*
* NOTE: Although the result of this computation actually is in
* units of power, it can be manipulated as an energy value
* over a scheduling period, since it is assumed to be
* constant during that interval.
*
* By injecting (1) in (2), 'cpu_nrg' can be re-expressed as a product
* of two terms:
*
* cs->power * cpu_max_freq cpu_util
* cpu_nrg = ------------------------ * --------- (3)
* cs->freq scale_cpu
*
* The first term is static, and is stored in the em_cap_state struct
* as 'cs->cost'.
*
* Since all CPUs of the domain have the same micro-architecture, they
* share the same 'cs->cost', and the same CPU capacity. Hence, the
* total energy of the domain (which is the simple sum of the energy of
* all of its CPUs) can be factorized as:
*
* cs->cost * \Sum cpu_util
* pd_nrg = ------------------------ (4)
* scale_cpu
*/
return cs->cost * sum_util / scale_cpu; //通过上面的注释以及公式，推导出energy计算公式，并计算出perf doamin的总energy
}
总结
1、find_best_target()函数主要是根据当前情况，找到task迁移的candidate cpu（target_cpu、backup cpu、prev_cpu）
2、在find_energy_efficient_cpu()后半段，计算task迁移到每个candidate cpu后的系统总energy。然后会通过总energy计算对比，找到省电又不影响性能的best_energy_cpu
补充：
在energy model与energy计算，目前还未弄清楚如何联系起来，后续需要找到如何联系。
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