From: Sergei Shtylyov <sshtylyov@ru•mvista.com>
To: benh@kernel•crashing.org
Cc: Olof Johansson <olof@lixom•net>,
linuxppc-dev@ozlabs•org, paulus@samba•org
Subject: Re: New time code miscalculates cpu usage
Date: Thu, 25 Oct 2007 16:46:25 +0400 [thread overview]
Message-ID: <47209021.4080501@ru.mvista.com> (raw)
In-Reply-To: <1193262969.6653.32.camel@pasglop>
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
prev parent reply other threads:[~2007-10-25 12:46 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-10-16 20:25 New time code miscalculates cpu usage Olof Johansson
2007-10-17 6:07 ` Tony Breeds
2007-10-24 20:19 ` Olof Johansson
2007-10-24 20:17 ` Sergei Shtylyov
2007-10-24 21:56 ` Benjamin Herrenschmidt
2007-10-25 0:19 ` Paul Mackerras
2007-10-25 0:53 ` Benjamin Herrenschmidt
2007-10-25 12:46 ` Sergei Shtylyov [this message]
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