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From: Sergei Shtylyov <sshtylyov@ru.mvista.com>
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Subject: Re: New time code miscalculates cpu usage
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Cc: Olof Johansson <olof@lixom.net>, linuxppc-dev@ozlabs.org, paulus@samba.org
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Hello.

Benjamin Herrenschmidt wrote:

>>>Not sure when this started happening, but I wanted to report it. I'll
>>>start bisecting in a day or two if noone else has gotten around to
>>>looking at it:

>>>$ echo "int main(void) { while(1); }" > test.c ; gcc test.c
>>>$ time ./a.out & sleep 2 ; killall a.out
>>>real    0m2.008s
>>>user    0m4.014s
>>>sys     0m0.002s

>>>Seen on POWER5 and PA6T, haven't tried anything else yet.

>>    I'm not surprised -- the kernel accounts twice for each tick.

> Your input would be much more valuable if you actually pointed out where
> that happens and why since you seem to know it.

   I've already pointed out the reason, yet it won't hurt to repeat indeed.
timer_interrupt() calls both account_process_time() and the hrtimers event 
handler which leads to:

- if determenistic accounting is off, account_process_time() is the same as
   update_process_times() which gets also calleed by hrtimers via the event
   handler, so we get each tick accounted for twice;

- if determenistic accounting is on, then timer_interrupt() calls
   account_process_time() and update_process_times() is still called by
   hrtimers, so each tick's gets accounted for userspace twice...

> Ben.

WBR, Sergei