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

[Peter Zijlstra]

On Thu, Jul 06, 2017 at 08:18:41PM -0700, Wanpeng Li wrote:
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -683,7 +683,7 @@ static u64 vtime_delta(struct vtime *vtime)
>  {
>  	unsigned long long clock;
>  
> -	clock = sched_clock_cpu(smp_processor_id());
> +	clock = sched_clock();
>  	if (clock < vtime->starttime)
>  		return 0;
>  

This is broken. sched_clock_cpu() and sched_clock() do not have the
exact same value :-/

So this ends up doing:

	vtime->starttime = sched_clock_cpu(cpu);

	/* ... */

	clock = sched_clock();

	delta = clock - vtime->starttime;


And you compute a delta using two different clocks.