Re: [PATCH][RFC] sched/cpuacct: Fix cpuacct charge

[Peter Zijlstra]

On Tue, Jul 20, 2021 at 02:04:41PM +0800, Li RongQing wrote:
> 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: dbe9337109c2 "(sched/cpuacct: Fix charge cpuacct.usage_sys)"
> Co-developed-by: Zhao Jie <zhaojie17@xxxxxxxxx>
> Signed-off-by: Zhao Jie <zhaojie17@xxxxxxxxx>
> Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
> ---
>  include/asm-generic/irq_regs.h | 5 +++++
>  kernel/sched/cpuacct.c         | 3 ++-
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/include/asm-generic/irq_regs.h b/include/asm-generic/irq_regs.h
> index 2e7c6e8..93e2579 100644
> --- a/include/asm-generic/irq_regs.h
> +++ b/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);
> +}

This primitive just cannot be right... it'll get you some random data.