sched/cpuacct: Fix cpuacct charge

get_irq_regs only work for current running cpu, but the task, whose
cpuacct will be charged, maybe run different cpu, like Cpu 2 wake
up a kernel thread to CPU 3, cause CPU 3 task are charged with the
following stack

  cpuacct_charge+0xd8/0x100
  update_curr+0xe1/0x1e0
  enqueue_entity+0x144/0x6e0
  enqueue_task_fair+0x93/0x900
  ttwu_do_activate+0x4b/0x90
  try_to_wake_up+0x20b/0x530
  ? update_dl_rq_load_avg+0x10f/0x210
  swake_up_locked.part.1+0x13/0x40
  swake_up_one+0x27/0x40
  rcu_process_callbacks+0x484/0x4f0
  ? run_rebalance_domains_bt+0x5a/0x180
  __do_softirq+0xe3/0x308
  irq_exit+0xf0/0x100
  smp_apic_timer_interrupt+0x74/0x160
  apic_timer_interrupt+0xf/0x20
  </IRQ>
 RIP: 0033:0x456947

so define a get_irq_regs_cpu which returns the required cpu irq registers

BUT it should be not safe, and do not know what it should be like in MIPS?

Fixes: dbe9337 "(sched/cpuacct: Fix charge cpuacct.usage_sys)"
Co-developed-by: Zhao Jie <zhaojie17@baidu.com>
Signed-off-by: Zhao Jie <zhaojie17@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>

include/asm-generic/irq_regs.h

@@ -21,6 +21,11 @@ static inline struct pt_regs *get_irq_regs(void)
 	return __this_cpu_read(__irq_regs);
 }
 
+static inline struct pt_regs *get_irq_regs_cpu(int cpu)
+{
+	return per_cpu(__irq_regs, cpu);
+}
+
 static inline struct pt_regs *set_irq_regs(struct pt_regs *new_regs)
 {
 	struct pt_regs *old_regs;

kernel/sched/cpuacct.c

@@ -340,7 +340,8 @@ void cpuacct_charge(struct task_struct *tsk, u64 cputime)
 {
 	struct cpuacct *ca;
 	int index = CPUACCT_STAT_SYSTEM;
-	struct pt_regs *regs = get_irq_regs() ? : task_pt_regs(tsk);
+	int cpu = task_cpu(tsk);
+	struct pt_regs *regs = get_irq_regs_cpu(cpu) ? : task_pt_regs(tsk);
 
 	if (regs && user_mode(regs))
 		index = CPUACCT_STAT_USER;