linux-kernel - Utime and stime are less when getrusage (RUSAGE

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Message-ID: <OSBPR01MB21837C8931D90AE55AF4A955EB529@OSBPR01MB2183.jpnprd01.prod.outlook.com>
Date:   Wed, 12 May 2021 03:28:02 +0000
From:   "hasegawa-hitomi@...itsu.com" <hasegawa-hitomi@...itsu.com>
To:     "'fweisbec@...il.com'" <fweisbec@...il.com>,
        "'tglx@...utronix.de'" <tglx@...utronix.de>,
        "'mingo@...nel.org'" <mingo@...nel.org>,
        "'peterz@...radead.org'" <peterz@...radead.org>,
        "'juri.lelli@...hat.com'" <juri.lelli@...hat.com>,
        "'vincent.guittot@...aro.org'" <vincent.guittot@...aro.org>
CC:     "'dietmar.eggemann@....com'" <dietmar.eggemann@....com>,
        "'rostedt@...dmis.org'" <rostedt@...dmis.org>,
        "'bsegall@...gle.com'" <bsegall@...gle.com>,
        "'mgorman@...e.de'" <mgorman@...e.de>,
        "'bristot@...hat.com'" <bristot@...hat.com>,
        "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>
Subject: Utime and stime are less when getrusage (RUSAGE_THREAD) is executed
 on a tickless CPU.

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