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From:   "hasegawa-hitomi@fujitsu.com" <hasegawa-hitomi@fujitsu.com>
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Subject: Utime and stime are less when getrusage (RUSAGE_THREAD) is executed
 on a tickless CPU.
Thread-Topic: Utime and stime are less when getrusage (RUSAGE_THREAD) is
 executed on a tickless CPU.
Thread-Index: AddG3PD5Dm9CD2RNQ7+TrQrb7bYc8A==
Date:   Wed, 12 May 2021 03:28:02 +0000
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Hello.

I found that when I run getrusage(RUSAGE_THREAD) on a tickless CPU, the uti=
me and stime I get are less than the actual time, unlike when I run getrusa=
ge(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 adjus=
t 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 =3D=3D RUSAGE_THREAD) {
+               task_sched_runtime(current);
                task_cputime_adjusted(current, &utime, &stime);

Is there any possible problem with this?

Thanks.
Hitomi Hasegawa