linux-kernel - Re: [patch v2 4/4] taskstats: Export "cdata

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Date:	Mon, 6 Dec 2010 14:36:54 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Shailabh Nagar <nagar1234@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	John stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Roland McGrath <roland@...hat.com>, Valdis.Kletnieks@...edu,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [patch v2 4/4] taskstats: Export "cdata_wait" CPU times with
 taskstats

* Michael Holzheu <holzheu@...ux.vnet.ibm.com> [2010-12-02 17:34:01]:

> On Wed, 2010-12-01 at 19:51 +0100, Oleg Nesterov wrote:
> > But in fact I don't really understand this anyway. This is called
> > before we reparent our children. This means that ac_cutime/ac_cstime
> > can be changed after that (multithreading, or full_cdata_enabled).
> > 
> > Say, taskstats_exit()->fill_stats()->bacct_add_tsk(). Every thread
> > does this, including the group_leader. But, it is possible that
> > group_leader exits first, before other threads. IOW, what
> > stats->ac_cXtime actually mean?
> 
> Because I worked mostly with the ptop tool, I was not so much focused on
> the taskstats exit events, but instead more on the taskstats commands to
> query data for running tasks.
> 
> For the query scenario stats->ac_cXtime means:
> 
> 1) full_cdata=0: "Sum of CPU time of exited child processes where
>    sys_wait() have been done (up to this time)"
> 2) full_cdata=1: "Sum of CPU time of exited child processes where
>    sys_wait() have been done plus exited child processes where
>    the parents ignored SIGCHLD or have set SA_NOCLDWAIT (up to
>    this time)"
> 
> Regarding taskstats_exit(): Do you have something like the following
> scenario in mind?
> 
> 1) You have a thread group with several threads
> 2) Thread group leader dies and reports cdata_wait in taskstats_exit()
> 3) Thread group leader stays around as zombie until the thread
>    group dies
> 4) Other forked processes of this thread group die
> 5) cdata_wait of thread group is increased
> 6) The new cdata is not reported by any exit event of the thread group
> 
> So maybe we should remove the thread_group_leader() check and report
> cdata_wait for all threads and not only for the thread group leader? We
> also should add ac_tgid to taskstats so that userspace can find the
> corresponding thread group for each thread.
> 
> When the last thread exits and the process/thread group dies,
> taskstats_exit() sends an additional taskstats struct to userspace that
> aggregates the thread accounting data. Currently only the delay
> accounting data is aggregated (see
> taskstats_exit->fill_tgid_exit->delayacct_add_tsk). Not sure, why the
> other information is not aggregated. We perhaps also should include
> ac_cXtime in the aggregated taskstats.
>

The delay accounting data is aggregated, the other part tsacct do not
care much about aggregation (IIRC). tsacct was added to export pid
data and never tgid data again IIRC. This subsystem is the one that
owns ac_*, perhaps if tsacct supported tgid's then what you say makes
sense.

-- 
	Three Cheers,
	Balbir
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