Re: [PATCH] posix-cpu-timers:clock_gettime(CLOCK_THREAD_CPUTIME_ID) goes backward

[Ingo Molnar]

* Hidetoshi Seto <seto.hidetoshi@xxxxxxxxxxxxxx> wrote:

> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -4064,6 +4064,33 @@ DEFINE_PER_CPU(struct kernel_stat, kstat);
>  EXPORT_PER_CPU_SYMBOL(kstat);
>  
>  /*
> + * Return total of runtime which already banked and which not yet
> + * @p in case that task is currently running.
> + */
> +unsigned long long task_total_exec(struct task_struct *p)
> +{
> +	unsigned long flags;
> +	struct rq *rq;
> +	u64 ns = 0;
> +
> +	rq = task_rq_lock(p, &flags);
> +
> +	if (task_current(rq, p)) {
> +		u64 delta_exec;
> +
> +		update_rq_clock(rq);
> +		delta_exec = rq->clock - p->se.exec_start;
> +		if ((s64)delta_exec > 0)
> +			ns = delta_exec;
> +	}
> +	ns += p->se.sum_exec_runtime;
> +
> +	task_rq_unlock(rq, &flags);
> +
> +	return ns;
> +}
> +

ok - the issue is real, but there's a few implementational details. Did 
you know about task_delta_exec()? This new function duplicates that 
function's behavior - and task_delta_exec() is already used by posix 
task/cpu timers.

I find your new task_total_exec() interface better (it's easier to use), 
could you have a look at eliminating task_delta_exec() users and migrating 
them over to task_total_exec()?

Maybe even change the name of sum_exec_runtime over to __sum_exec_runtime, 
to make sure nobody uses the 'raw' value - which is always stale up to one 
tick's worth of execution. Accessing that raw value would only make sense 
after the task has finished executing. (i.e. at cutime/cstime collection 
time)

	Ingo
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