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

["hasegawa-hitomi@fujitsu.com" <hasegawa-hitomi@fujitsu.com>]

Hello.

I found that when I run getrusage(RUSAGE_THREAD) on a tickless CPU, the utime and stime I get are less than the actual time, unlike when I run getrusage(RUSAGE_SELF) on a single thread.
This problem seems to be caused by the fact that se.sum_exec_runtime is not updated just before getting the information from 'current'.
In the current implementation, task_cputime_adjusted() calls task_cputime() to get the 'current' utime and stime, then calls cputime_adjust() to adjust the sum of utime and stime to be equal to cputime.sum_exec_runtime. On a tickless CPU, sum_exec_runtime is not updated periodically, so there seems to be a discrepancy with the actual time.
Therefore, I think I should include a process to update se.sum_exec_runtime just before getting the information from 'current' (as in other processes except RUSAGE_THREAD). I'm thinking of the following improvement.

@@ void getrusage(struct task_struct *p, int who, struct rusage *r)
        if (who == RUSAGE_THREAD) {
+               task_sched_runtime(current);
                task_cputime_adjusted(current, &utime, &stime);

Is there any possible problem with this?

Thanks.
Hitomi Hasegawa