smp_init过程解析
当你看到这样的log，会不会很慌张？
竟然由CPU没有启动成功，除了什么故障？
本文将结合我遇到的一个问题，将启动过程中bringup secondary cpu的过程分析一下。
smp_init代码如下：
602 void __init smp_init(void)
603 {
604 int num_nodes, num_cpus;
605 unsigned int cpu;
606
607 idle_threads_init();
608 cpuhp_threads_init();
609
610 pr_info("====cpuhp-walk:Bringing up secondary CPUs ...\n");
611
612 /* FIXME: This should be done in userspace --RR */
613 for_each_present_cpu(cpu) {
614 if (num_online_cpus() >= setup_max_cpus)
615 break;
616 pr_info("====cpuhp-walk:before bringing up cpu:%d\n", cpu);
617 if (!cpu_online(cpu))
618 cpu_up(cpu);
619 pr_info("====cpuhp-walk:after bringing up cpu:%d\n", cpu);
620 }
621
622 num_nodes = num_online_nodes();
623 num_cpus = num_online_cpus();
624 pr_info("====cpuhp-walk:Brought up %d node%s, %d CPU%s\n",
625 num_nodes, (num_nodes > 1 ? "s" : ""),
626 num_cpus, (num_cpus > 1 ? "s" : ""));
627
628 /* Any cleanup work */
629 smp_cpus_done(setup_max_cpus);
630 }
可以看得出在smp_init中，cpu0依次调用cpu_up启动其他的cpu，启动完成一个，再启动下一个，直到所有cpu都启动完毕，才继续内核剩余的启动流程，整个过程从smp_init开始，从smp_init结束。
下面分析具体的过程，我们将启动其他cpu成为BP(boot cpu)，将被启动的cpu成为AP(不知道A表示哪个单词)：
BP AP
smp_init
| do_cpu_up
| --> cpuhp_invoke_callback
| --> smpboot_create_threads
| perf_event_init_cpu
| workqueue_prepare_cpu
| hrtimers_prepare_cpu
| ...
| bringup_cpu
| -->__cpu_up
| -->boot_secondary (cpu_ops[cpu]->cpu_boot(cpu))
| --> cpu_psci_cpu_boot
| -->psci_cpu_on (invoke_psci_fn(0xc4000003, cpuid, secondary_entry, 0);)
| secondary_entry
next cpu --> secondary_startup
| --> __secondary_switched
| -->secondary_start_kernel
| --> notify_cpu_starting
| -->cpuhp_invoke_callback
| -->sched_cpu_starting
| gic_starting_cpu
| arch_timer_starting_cpu
| dummy_timer_starting_cpu
|
| bringup_wait_for_ap可以看出，BP通过bringup_cpu函数，调用psci的函数，请求firmware启动AP，然后原地等待AP启动的结果。
firmware给AP上电之后，AP进入head.S中的secondary_entry，然后进入secondary_start_kernel，完成一系列回调之后，通知BP自己启动成功。BP从bringup_wait_for_ap返回，继续启动下一个cpu。
在cpuhp_invoke_callback所调用的回调函数的定义在这：
kernel/cpu.c
1426 /* Boot processor state steps */
1427 static struct cpuhp_step cpuhp_hp_states[] = {
1428 [CPUHP_OFFLINE] = {
1429 .name = "offline",
1430 .startup.single = NULL,
1431 .teardown.single = NULL,
1432 },
1433 #ifdef CONFIG_SMP
1434 [CPUHP_CREATE_THREADS]= {
1435 .name = "threads:prepare",
1436 .startup.single = smpboot_create_threads,
1437 .teardown.single = NULL,
1438 .cant_stop = true,
1439 },
1440 [CPUHP_PERF_PREPARE] = {
1441 .name = "perf:prepare",
1442 .startup.single = perf_event_init_cpu,
1443 .teardown.single = perf_event_exit_cpu,
1444 },
1445 [CPUHP_WORKQUEUE_PREP] = {
1446 .name = "workqueue:prepare",
1447 .startup.single = workqueue_prepare_cpu,
1448 .teardown.single = NULL,
1449 },
1450 [CPUHP_HRTIMERS_PREPARE] = {
1451 .name = "hrtimers:prepare",
1452 .startup.single = hrtimers_prepare_cpu,
1453 .teardown.single = hrtimers_dead_cpu,
1454 },
1455 [CPUHP_SMPCFD_PREPARE] = {
1456 .name = "smpcfd:prepare",
1457 .startup.single = smpcfd_prepare_cpu,
1458 .teardown.single = smpcfd_dead_cpu,
1459 },
1460 [CPUHP_RELAY_PREPARE] = {
1461 .name = "relay:prepare",
1462 .startup.single = relay_prepare_cpu,
1463 .teardown.single = NULL,
1464 },
1465 [CPUHP_SLAB_PREPARE] = {
1466 .name = "slab:prepare",
1467 .startup.single = slab_prepare_cpu,
1468 .teardown.single = slab_dead_cpu,
1469 },
1470 [CPUHP_RCUTREE_PREP] = {
1471 .name = "RCU/tree:prepare",
...
1485 /* Kicks the plugged cpu into life */
1486 [CPUHP_BRINGUP_CPU] = {
1487 .name = "cpu:bringup",
1488 .startup.single = bringup_cpu,
1489 .teardown.single = NULL,
1490 .cant_stop = true,
1491 },
1492 /* Final state before CPU kills itself */
1493 [CPUHP_AP_IDLE_DEAD] = {
1494 .name = "idle:dead",
1495 },
1496 /*
1497 * Last state before CPU enters the idle loop to die. Transient state
1498 * for synchronization.
1499 */
1500 [CPUHP_AP_OFFLINE] = {
1501 .name = "ap:offline",
1502 .cant_stop = true,
1503 },
1504 /* First state is scheduler control. Interrupts are disabled */
1505 [CPUHP_AP_SCHED_STARTING] = {
1506 .name = "sched:starting",
1507 .startup.single = sched_cpu_starting,
1508 .teardown.single = sched_cpu_dying,
1509 },
1510 [CPUHP_AP_RCUTREE_DYING] = {
1511 .name = "RCU/tree:dying",
1512 .startup.single = NULL,
1513 .teardown.single = rcutree_dying_cpu,
1514 },
1515 [CPUHP_AP_SMPCFD_DYING] = {
1516 .name = "smpcfd:dying",
1517 .startup.single = NULL,
1518 .teardown.single = smpcfd_dying_cpu,
1519 },
1520 /* Entry state on starting. Interrupts enabled from here on. Transient
1521 * state for synchronsization */
1522 [CPUHP_AP_ONLINE] = {
1523 .name = "ap:online",
1524 },
1525 /*
1526 * Handled on controll processor until the plugged processor manages
1527 * this itself.
1528 */
1529 [CPUHP_TEARDOWN_CPU] = {
1530 .name = "cpu:teardown",
1531 .startup.single = NULL,
...
1535 /* Handle smpboot threads park/unpark */
1536 [CPUHP_AP_SMPBOOT_THREADS] = {
1537 .name = "smpboot/threads:online",
1538 .startup.single = smpboot_unpark_threads,
1539 .teardown.single = smpboot_park_threads,
1540 },
1541 [CPUHP_AP_IRQ_AFFINITY_ONLINE] = {
1542 .name = "irq/affinity:online",
1543 .startup.single = irq_affinity_online_cpu,
1544 .teardown.single = NULL,
1545 },
1546 [CPUHP_AP_PERF_ONLINE] = {
1547 .name = "perf:online",
1548 .startup.single = perf_event_init_cpu,
1549 .teardown.single = perf_event_exit_cpu,
1550 },
1551 [CPUHP_AP_WATCHDOG_ONLINE] = {
1552 .name = "lockup_detector:online",
1553 .startup.single = lockup_detector_online_cpu,
1554 .teardown.single = lockup_detector_offline_cpu,
1555 },
1556 [CPUHP_AP_WORKQUEUE_ONLINE] = {
1557 .name = "workqueue:online",
1558 .startup.single = workqueue_online_cpu,
1559 .teardown.single = workqueue_offline_cpu,
1560 },
1561 [CPUHP_AP_RCUTREE_ONLINE] = {
1562 .name = "RCU/tree:online",
1563 .startup.single = rcutree_online_cpu,
1564 .teardown.single = rcutree_offline_cpu,
1565 },
1566 #endif
1567 /*
1568 * The dynamically registered state space is here
1569 */
1570
1571 #ifdef CONFIG_SMP
1572 /* Last state is scheduler control setting the cpu active */
1573 [CPUHP_AP_ACTIVE] = {
1574 .name = "sched:active",
1575 .startup.single = sched_cpu_activate,
1576 .teardown.single = sched_cpu_deactivate,
1577 },
1578 #endif
1579
1580 /* CPU is fully up and running. */
1581 [CPUHP_ONLINE] = {
1582 .name = "online",
1583 .startup.single = NULL,
1584 .teardown.single = NULL,
1585 },
1586 };
1482,3-9 60所有的回调函数一次调用，最重要的是bringup_cpu，这个是通过firmware直接给cpu上电启动的过程。
psci通过设备树节点获取与固件通信的方法（method：smc）,smc具体的操作指令，可以在include/uapi/linux/psci.h中查找.
我在代码中增加一些打印语句，完整的smp_init的过程如下：
[ 0.064050] smp: ====cpuhp-walk:Bringing up secondary CPUs ...
[ 0.064051] smp: ====cpuhp-walk:before bringing up cpu:0
[ 0.064052] smp: ====cpuhp-walk:after bringing up cpu:0
[ 0.064053] smp: ====cpuhp-walk:before bringing up cpu:1
[ 0.064053] cpuhp_walk:cpu:1,do_cpu_up
[ 0.064060] cphp_walk:cpu:1,cpuhp_up_callbacks
[ 0.064061] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:smpboot_create_threads+0x0/0xe0
[ 0.104074] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:perf_event_init_cpu+0x0/0x140
[ 0.104080] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:workqueue_prepare_cpu+0x0/0x98
[ 0.112080] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:hrtimers_prepare_cpu+0x0/0xb0
[ 0.112084] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:smpcfd_prepare_cpu+0x0/0x70
[ 0.112089] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:relay_prepare_cpu+0x0/0xf8
[ 0.112092] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:rcutree_prepare_cpu+0x0/0x1c0
[ 0.112099] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:timers_prepare_cpu+0x0/0x80
[ 0.112101] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:bringup_cpu+0x0/0x140
[ 0.112104] cpuhp_walk:cpu:1,bringup_cpu
[ 0.112105] cpuhp_walk:cpu:1,__cpu_up
[ 0.112106] cpuhp_walk:cpu:1,boot_secondary, cpu_boot:cpu_psci_cpu_boot+0x0/0x94
[ 0.112109] psci: cpuhp_walk:cpu:1,cpu_psci_cpu_boot, cpu_on:psci_cpu_on+0x0/0x94
[ 0.112112] psci: cphp_walk:cpuid:1,psci_cpu_on, fn:0xc4000003
[ 0.145924] cphp_walk:cpu:1,secondary_start_kernel
[ 0.145934] Detected PIPT I-cache on CPU1
[ 0.145942] cpuhp_walk:cpu:1,notify_cpu_starting
[ 0.145943] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:sched_cpu_starting+0x0/0x120
[ 0.145946] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:gic_starting_cpu+0x0/0x40
[ 0.145950] GICv3: CPU1: found redistributor 1 region 1:0x00000000299a0000
[ 0.145958] GICv3: CPU1: using allocated LPI pending table @0x0000002176620000
[ 0.145964] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:arch_timer_starting_cpu+0x0/0x318
[ 0.145971] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:dummy_timer_starting_cpu+0x0/0x88
[ 0.145973] cpuhp_walk, CPU1: Booted secondary processor 0x0000000001 [0x701f6633]
[ 0.145978] cpuhp_walk:cpu:1,bringup_cpu, wait ap
[ 0.145980] cphp_walk:cpu:1,bringup_wait_for_ap, 1
[ 0.145984] cphp_walk:cpu:1,bringup_wait_for_ap, 2
[ 0.145987] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:smpboot_unpark_threads+0x0/0xb0
[ 0.145994] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:irq_affinity_online_cpu+0x0/0x110
[ 0.145998] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:perf_event_init_cpu+0x0/0x140
[ 0.146002] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:workqueue_online_cpu+0x0/0x228
[ 0.146019] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:rcutree_online_cpu+0x0/0x98
[ 0.146022] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:page_writeback_cpu_online+0x0/0x20
[ 0.146025] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:vmstat_cpu_online+0x0/0x70
[ 0.146029] cphp_walk:cpu:1,cpuhp_invoke_callback, 1.1, cb:sched_cpu_activate+0x0/0x168
[ 0.146035] smp: ====cpuhp-walk:after bringing up cpu:1
[ 0.146036] smp: ====cpuhp-walk:before bringing up cpu:2
[ 0.146037] cpuhp_walk:cpu:2,do_cpu_up
[ 0.146041] cphp_walk:cpu:2,cpuhp_up_callbacks
[ 0.146042] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:smpboot_create_threads+0x0/0xe0
[ 0.185939] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:perf_event_init_cpu+0x0/0x140
[ 0.185943] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:workqueue_prepare_cpu+0x0/0x98
[ 0.193946] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:hrtimers_prepare_cpu+0x0/0xb0
[ 0.193949] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:smpcfd_prepare_cpu+0x0/0x70
[ 0.193952] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:relay_prepare_cpu+0x0/0xf8
[ 0.193954] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:rcutree_prepare_cpu+0x0/0x1c0
[ 0.193959] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:timers_prepare_cpu+0x0/0x80
[ 0.193961] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:bringup_cpu+0x0/0x140
[ 0.193964] cpuhp_walk:cpu:2,bringup_cpu
[ 0.193964] cpuhp_walk:cpu:2,__cpu_up
[ 0.193965] cpuhp_walk:cpu:2,boot_secondary, cpu_boot:cpu_psci_cpu_boot+0x0/0x94
[ 0.193967] psci: cpuhp_walk:cpu:2,cpu_psci_cpu_boot, cpu_on:psci_cpu_on+0x0/0x94
[ 0.193969] psci: cphp_walk:cpuid:256,psci_cpu_on, fn:0xc4000003
[ 0.227952] cphp_walk:cpu:2,secondary_start_kernel
[ 0.227961] Detected PIPT I-cache on CPU2
[ 0.227967] cpuhp_walk:cpu:2,notify_cpu_starting
[ 0.227968] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:sched_cpu_starting+0x0/0x120
[ 0.227972] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:gic_starting_cpu+0x0/0x40
[ 0.227976] GICv3: CPU2: found redistributor 100 region 2:0x00000000299c0000
[ 0.227994] GICv3: CPU2: using allocated LPI pending table @0x0000002176630000
[ 0.227999] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:arch_timer_starting_cpu+0x0/0x318
[ 0.228007] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:dummy_timer_starting_cpu+0x0/0x88
[ 0.228010] cpuhp_walk, CPU2: Booted secondary processor 0x0000000100 [0x701f6633]
[ 0.228017] cpuhp_walk:cpu:2,bringup_cpu, wait ap
[ 0.228019] cphp_walk:cpu:2,bringup_wait_for_ap, 1
[ 0.228022] cphp_walk:cpu:2,bringup_wait_for_ap, 2
[ 0.228029] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:smpboot_unpark_threads+0x0/0xb0
[ 0.228039] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:irq_affinity_online_cpu+0x0/0x110
[ 0.228042] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:perf_event_init_cpu+0x0/0x140
[ 0.228047] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:workqueue_online_cpu+0x0/0x228
[ 0.228070] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:rcutree_online_cpu+0x0/0x98
[ 0.228073] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:page_writeback_cpu_online+0x0/0x20
[ 0.228076] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:vmstat_cpu_online+0x0/0x70
[ 0.228080] cphp_walk:cpu:2,cpuhp_invoke_callback, 1.1, cb:sched_cpu_activate+0x0/0x168
[ 0.228086] smp: ====cpuhp-walk:after bringing up cpu:2
[ 0.228087] smp: ====cpuhp-walk:before bringing up cpu:3
[ 0.228088] cpuhp_walk:cpu:3,do_cpu_up
[ 0.228092] cphp_walk:cpu:3,cpuhp_up_callbacks
[ 0.228093] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:smpboot_create_threads+0x0/0xe0
[ 0.267968] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:perf_event_init_cpu+0x0/0x140
[ 0.267972] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:workqueue_prepare_cpu+0x0/0x98
[ 0.275974] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:hrtimers_prepare_cpu+0x0/0xb0
[ 0.275977] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:smpcfd_prepare_cpu+0x0/0x70
[ 0.275980] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:relay_prepare_cpu+0x0/0xf8
[ 0.275983] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:rcutree_prepare_cpu+0x0/0x1c0
[ 0.275988] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:timers_prepare_cpu+0x0/0x80
[ 0.275990] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:bringup_cpu+0x0/0x140
[ 0.275993] cpuhp_walk:cpu:3,bringup_cpu
[ 0.275994] cpuhp_walk:cpu:3,__cpu_up
[ 0.275995] cpuhp_walk:cpu:3,boot_secondary, cpu_boot:cpu_psci_cpu_boot+0x0/0x94
[ 0.275996] psci: cpuhp_walk:cpu:3,cpu_psci_cpu_boot, cpu_on:psci_cpu_on+0x0/0x94
[ 0.275998] psci: cphp_walk:cpuid:257,psci_cpu_on, fn:0xc4000003
[ 0.309980] cphp_walk:cpu:3,secondary_start_kernel
[ 0.309985] Detected PIPT I-cache on CPU3
[ 0.309989] cpuhp_walk:cpu:3,notify_cpu_starting
[ 0.309990] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:sched_cpu_starting+0x0/0x120
[ 0.309993] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:gic_starting_cpu+0x0/0x40
[ 0.309996] GICv3: CPU3: found redistributor 101 region 3:0x00000000299e0000
[ 0.310002] GICv3: CPU3: using allocated LPI pending table @0x0000002176640000
[ 0.310007] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:arch_timer_starting_cpu+0x0/0x318
[ 0.310012] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:dummy_timer_starting_cpu+0x0/0x88
[ 0.310014] cpuhp_walk, CPU3: Booted secondary processor 0x0000000101 [0x701f6633]
[ 0.310019] cpuhp_walk:cpu:3,bringup_cpu, wait ap
[ 0.310021] cphp_walk:cpu:3,bringup_wait_for_ap, 1
[ 0.310024] cphp_walk:cpu:3,bringup_wait_for_ap, 2
[ 0.310028] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:smpboot_unpark_threads+0x0/0xb0
[ 0.310036] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:irq_affinity_online_cpu+0x0/0x110
[ 0.310039] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:perf_event_init_cpu+0x0/0x140
[ 0.310043] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:workqueue_online_cpu+0x0/0x228
[ 0.310050] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:rcutree_online_cpu+0x0/0x98
[ 0.310052] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:page_writeback_cpu_online+0x0/0x20
[ 0.310054] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:vmstat_cpu_online+0x0/0x70
[ 0.310057] cphp_walk:cpu:3,cpuhp_invoke_callback, 1.1, cb:sched_cpu_activate+0x0/0x168
[ 0.310063] smp: ====cpuhp-walk:after bringing up cpu:3
[ 0.310064] smp: ====cpuhp-walk:Brought up 1 node, 4 CPUs
[ 0.310066] SMP: Total of 4 processors activated.
[ 0.310067] CPU features: detected: 32-bit EL0 Support
[ 0.310069] CPU features: detected: CRC32 instructions
[ 0.310303] CPU: All CPU(s) started at EL2经查，是CPU厂商firmware的问题， 也就是psci_cpu_on, fn:0xc4000003的问题。
所以以后遇到这样的问题，熟悉了过程，就不慌了。
本文的psci部分由我的同事提供，同时，在阅读和分析代码之前，我还参照了如下链接的信息：
https://www.cnblogs.com/pengdonglin137/p/11925299.html
https://zhuanlan.zhihu.com/p/538782115
https://blog.csdn.net/weixin_45264425/article/details/127895018
https://zhuanlan.zhihu.com/p/545550388
此外，由于功耗控制的原因，系统运行期间，还会通过cpu idle来间断的关闭和打开cpu，也就是cpu hotplug，打开cpu的操作和上述过程相同，关闭cpu的过程与cpu up相反，读者自行阅读代码，也是通过psci来关闭cpu。