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Subject: Re: [RFC][PATCH 09/10] taskstats: Fix exit CPU time accounting
From: Michael Holzheu <holzheu@linux.vnet.ibm.com>
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To: Oleg Nesterov <oleg@redhat.com>
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Date: Mon, 27 Sep 2010 15:23:04 +0200
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Hello Oleg,

On Sun, 2010-09-26 at 20:11 +0200, Oleg Nesterov wrote:
> > I think one place was "khelper" (kmod.c). It is created with
> > kernel_thread() and it exits without having accounted the times with
> > sys_wait() to the parent's ctimes
> 
> No. Well yes, it is not accounted, but this is not because it is
> kthread.
> 
> To simplify the discussion, lets talk about utime/cutime only,
> and lets forget about the multithreading.
> 
> It is very simple, currently linux accounts the exiting task's
> utime and adds its to ->cutime _only_ if parent does do_wait().
> If parent ignores SIGCHLD, the child reaps itself and it is not
> accounted.
> 
> I do not know why it was done this way, but I'm afraid we can't
> change this historical behaviour.

Ok thanks, I didn't know this. So time can disappear, if the parent
ignores SIGCHLD.

I am a bit disappointed, because I thought by looking at all tasks of a
system it should be possible to evaluate all consumed CPU time from now
to the time where the system has been booted. That would have been a
nice thing.

> > Ok, the problem is that I did not consider exiting threads that are no
> > thread group leaders. When they exit the ctime of the parent is not
> > updated. Instead the time is accumulated in the signal struct.
> 
> I think I am a bit confused, but see above. With or without threads
> the whole process can exit without accounting.

Sorry that I couldn't explain my thoughts clear enough, believe me, I
tried my best :-)

Michael