Re: Utime and stime are less when getrusage (RUSAGE_THREAD) is executed on a tickless CPU.

[hasegawa-hitomi@xxxxxxxxxxx]

Hi Frederic, Peter, and Mel

> If necessary I guess we can do something like the below, which
> would only add the overhead where it's required:
> 
> diff --git a/include/linux/sched/cputime.h b/include/linux/sched/cputime.h
> index 6c9f19a33865..ce3c58286062 100644
> --- a/include/linux/sched/cputime.h
> +++ b/include/linux/sched/cputime.h
> @@ -18,15 +18,16 @@
>  #endif /* CONFIG_VIRT_CPU_ACCOUNTING_NATIVE */
>  
>  #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
> -extern void task_cputime(struct task_struct *t,
> +extern bool task_cputime(struct task_struct *t,
>  			 u64 *utime, u64 *stime);
>  extern u64 task_gtime(struct task_struct *t);
>  #else
> -static inline void task_cputime(struct task_struct *t,
> +static inline bool task_cputime(struct task_struct *t,
>  				u64 *utime, u64 *stime)
>  {
>  	*utime = t->utime;
>  	*stime = t->stime;
> +	return false;
>  }
>  
>  static inline u64 task_gtime(struct task_struct *t)
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 872e481d5098..9392aea1804e 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -615,7 +615,8 @@ void task_cputime_adjusted(struct task_struct *p, u64 *ut, u64 *st)
>  		.sum_exec_runtime = p->se.sum_exec_runtime,
>  	};
>  
> -	task_cputime(p, &cputime.utime, &cputime.stime);
> +	if (task_cputime(p, &cputime.utime, &cputime.stime))
> +		cputime.sum_exec_runtime = task_sched_runtime(p);
>  	cputime_adjust(&cputime, &p->prev_cputime, ut, st);
>  }
>  EXPORT_SYMBOL_GPL(task_cputime_adjusted);
> @@ -828,19 +829,21 @@ u64 task_gtime(struct task_struct *t)
>   * add up the pending nohz execution time since the last
>   * cputime snapshot.
>   */
> -void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
> +bool task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
>  {
>  	struct vtime *vtime = &t->vtime;
>  	unsigned int seq;
>  	u64 delta;
> +	int ret;
>  
>  	if (!vtime_accounting_enabled()) {
>  		*utime = t->utime;
>  		*stime = t->stime;
> -		return;
> +		return false;
>  	}
>  
>  	do {
> +		ret = false;
>  		seq = read_seqcount_begin(&vtime->seqcount);
>  
>  		*utime = t->utime;
> @@ -850,6 +853,7 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
>  		if (vtime->state < VTIME_SYS)
>  			continue;
>  
> +		ret = true;
>  		delta = vtime_delta(vtime);
>  
>  		/*
> @@ -861,6 +865,8 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
>  		else
>  			*utime += vtime->utime + delta;
>  	} while (read_seqcount_retry(&vtime->seqcount, seq));
> +
> +	return ret;
>  }
>  
>  static int vtime_state_fetch(struct vtime *vtime, int cpu)

I tested this and confirmed that it gives accurate values.
Also, task_sched_runtime () is done only on nohz_full CPU, so I think
it's a better idea than before.
I hope you will incorporate this fix into your mainline.

Thanks.
Hitomi Hasegawa