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

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Date:	Thu, 02 Dec 2010 17:34:01 +0100
From:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	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>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	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

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.

Michael

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