linux-kernel - Re: [RFC][PATCH 09/10] taskstats: Fix exit CPU time accounting

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Date:	Sun, 26 Sep 2010 20:11:27 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
Cc:	Shailabh Nagar <nagar1234@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	John stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 09/10] taskstats: Fix exit CPU time accounting

Hi,

On 09/24, Michael Holzheu wrote:
>
> On Thu, 2010-09-23 at 19:10 +0200, Oleg Nesterov wrote:
> >
> > On 09/23, Michael Holzheu wrote:
> > >
> > > Currently there are code pathes (e.g. for kthreads) where the consumed
> > > CPU time is not accounted to the parents cumulative counters.
> >
> > Could you explain more?
>
> 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, 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.

Oleg.

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