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SubjectRe: [PATCH] sched/cputime: Ensure correct utime and stime proportion
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Hi Peter,

On 7/5/18 9:21 PM, Xunlei Pang wrote:
> On 7/5/18 6:46 PM, Peter Zijlstra wrote:
>> On Wed, Jun 27, 2018 at 08:22:42PM +0800, Xunlei Pang wrote:
>>> tick-based whole utime is utime_0, tick-based whole stime
>>> is stime_0, scheduler time is rtime_0.
>>
>>> For a long time, the process runs mainly in userspace with
>>> run-sleep patterns, and because two different clocks, it
>>> is possible to have the following condition:
>>> rtime_0 < utime_0 (as with little stime_0)
>>
>> I don't follow... what?
>>
>> Why are you, and why do you think it makes sense to, compare rtime_0
>> against utime_0 ?
>>
>> The [us]time_0 are, per your earlier definition, ticks. They're not an
>> actual measure of time. Do not compare the two, that makes no bloody
>> sense.
>>
>
> [us]time_0 is task_struct:utime{stime}, I cited directly from
> cputime_adjust(), both in nanoseconds. I assumed "rtime_0 < utime_0"
> here to simple the following proof to help explain the problem we met.
>

Please see the enclosure for the reproducer cputime_adjust.tgz
(process_top.sh, usr_sys.c):
gcc usr_sys.c -o usr_sys
Firstly, the function consume_sys() in usr_sys.c yields 100% sys
which can be verified as follows:
$ taskset -c 0 ./usr_sys 1
$ ./process_top.sh $(pidof usr_sys)
0.0 usr, 100.0 sys
0.0 usr, 100.0 sys


Tested on my local box on 4.17.0 by executing "taskset -c 0 ./usr_sys",
then executing "./process_top.sh $(pidof usr_sys)" to watch.
1) Before this patch
50.0 usr, 0.0 sys
50.0 usr, 1.0 sys
50.0 usr, 0.0 sys
50.0 usr, 0.0 sys
49.0 usr, 4.0 sys //switch to consume 100% sys, ignore this line
12.0 usr, 88.0 sys
11.0 usr, 89.0 sys
10.0 usr, 90.0 sys
10.0 usr, 90.0 sys
9.0 usr, 91.0 sys
8.0 usr, 91.0 sys

Obviously there were around 10% sys wrongly goes to usr

2) After this patch
50.0 usr, 0.0 sys
50.0 usr, 0.0 sys
50.0 usr, 0.0 sys
50.0 usr, 0.0 sys
11.0 usr, 76.0 sys //switch to consume 100% sys, ignore this line
1.0 usr, 100.0 sys
0.0 usr, 100.0 sys
1.0 usr, 100.0 sys
0.0 usr, 100.0 sys
0.0 usr, 100.0 sys

So it displayed the correct result as we expected after this patch.

Thanks,
Xunlei
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