Re: [PATCH v2] sched/cputime: Fix using smp_processor_id() in preemptible

[Frederic Weisbecker <fweisbec@gmail.com>]

On Fri, Jul 07, 2017 at 02:08:25AM -0700, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@hotmail.com>
> 
>  BUG: using smp_processor_id() in preemptible [00000000] code: 99-trinity/181
>  caller is debug_smp_processor_id+0x17/0x19
>  CPU: 0 PID: 181 Comm: 99-trinity Not tainted 4.12.0-01059-g2a42eb9 #1
>  Call Trace:
>   dump_stack+0x82/0xb8
>   check_preemption_disabled+0xd1/0xe3
>   debug_smp_processor_id+0x17/0x19
>   vtime_delta+0xd/0x2c
>   task_cputime+0x89/0xdb
>   thread_group_cputime+0x11b/0x1ed
>   thread_group_cputime_adjusted+0x1f/0x47
>   wait_consider_task+0x2a9/0xaf9
>   ? lock_acquire+0x97/0xa4
>   do_wait+0xdf/0x1f4
>   SYSC_wait4+0x8e/0xb5
>   ? list_add+0x34/0x34
>   SyS_wait4+0x9/0xb
>   do_syscall_64+0x70/0x82
>   entry_SYSCALL64_slow_path+0x25/0x25
> 
> This patch fixes it by replacing sched_clock_cpu() in vtime_delta() by 
> local_clock() for effectively raw_smp_processor_id().

That's also broken because task_cputime() can be called from a different CPU than
where the target task is running on, even though there shouldn't be practical effect
as the clock must be stable but still the code would be confusing.

No I think you can still use sched_clock(), just make sure you also use it on
arch_vtime_task_switch() and vtime_init_idle().

Thanks.