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From:   "hasegawa-hitomi@fujitsu.com" <hasegawa-hitomi@fujitsu.com>
To:     Peter Zijlstra <peterz@infradead.org>,
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Subject: Re: 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+TrQrb7bYc8AE2IspgAAJjxoAAIq+F+A==
Date:   Wed, 19 May 2021 06:30:36 +0000
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Hi Ingo, Peter, Juri, and Vincent.=0A=
=0A=
=0A=
> Your email is malformed.=0A=
=0A=
I'm sorry. I was sent in the wrong format. I correct it and resend.=0A=
Thank you, Peter, for pointing this out.=0A=
=0A=
=0A=
I found that when I run getrusage(RUSAGE_THREAD) on a tickless CPU,=0A=
the utime and stime I get are less than the actual time, unlike when I run=
=0A=
getrusage(RUSAGE_SELF) on a single thread.=0A=
This problem seems to be caused by the fact that se.sum_exec_runtime is not=
=0A=
updated just before getting the information from 'current'.=0A=
In the current implementation, task_cputime_adjusted() calls task_cputime()=
 to=0A=
get the 'current' utime and stime, then calls cputime_adjust() to adjust th=
e=0A=
sum of utime and stime to be equal to cputime.sum_exec_runtime. On a tickle=
ss=0A=
CPU, sum_exec_runtime is not updated periodically, so there seems to be a=
=0A=
discrepancy with the actual time.=0A=
Therefore, I think I should include a process to update se.sum_exec_runtime=
=0A=
just before getting the information from 'current' (as in other processes=
=0A=
except RUSAGE_THREAD). I'm thinking of the following improvement.=0A=
=0A=
@@ void getrusage(struct task_struct *p, int who, struct rusage *r)=0A=
        if (who =3D=3D RUSAGE_THREAD) {=0A=
+               task_sched_runtime(current);=0A=
                task_cputime_adjusted(current, &utime, &stime);=0A=
=0A=
Is there any possible problem with this?=0A=
=0A=
=0A=
Thanks.=0A=
Hitomi Hasegawa=