linux-kernel - Re: [patch 0/4] taskstats: Improve cumulative time accounting

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Message-ID: <20101122072129.GK12043@balbir.in.ibm.com>
Date:	Mon, 22 Nov 2010 12:51:29 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
	Shailabh Nagar <nagar1234@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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>,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [patch 0/4] taskstats: Improve cumulative time accounting

* Oleg Nesterov <oleg@...hat.com> [2010-11-20 16:17:11]:

> On 11/19, Peter Zijlstra wrote:
> >
> > On Fri, 2010-11-19 at 21:11 +0100, Michael Holzheu wrote:
> > > Due to POSIX POSIX.1-2001, the CPU time of processes is not accounted
> > > to the cumulative time of the parents, if the parents ignore SIGCHLD
> > > or have set SA_NOCLDWAIT. This behaviour has the major drawback that
> > > it is not possible to calculate all consumed CPU time of a system by
> > > looking at the current tasks. CPU time can be lost.
> > >
> > > To solve this problem, this patch set duplicates the cumulative accounting
> > > data in the signal_struct. In the second set (cdata_acct) the complete
> > > cumulative resource counters are stored. The new cumulative CPU time (utime
> > > and stime) is then exported via the taskstats interface.
> >
> > Maybe this has been treated earlier in the threads and I missed it, but
> > the obvious solution doesn't get mentioned:
> 
> IIRC, the first version did this.
> 
> And it was me who spoiled this approach. But! only because I wasn't sure
> this user-visible change is acceptable, and because there was some
> misunderstanding. See http://marc.info/?l=linux-kernel&m=128552495203050&w=2
> 
> But,
> 
> > What would break if we
> 
> say, any test-case which does getrusage() after fork() with ignored
> SIGCHLD/SA_NOCLDWAIT?.
> 
> > violate this silly POSIX rule and account time of
> > childs regardless of SIGCHLD/SA_NOCLDWAIT?
> 
> +1.
> 
> Personally, I'd certainly prefer this way, because I don't care about
> POSIX at all ;)
> 
> 
> Still. Once again, this breaks the current rules, and we never do
> this without strong reason.
> 
> I think we should ask Roland. If he thinks this is OK, I'd certainly
> agree.
>

The Linux man page states

In Linux kernel versions before 2.6.9, if the disposition of SIGCHLD
is set to SIG_IGN then the resource usages of child processes are
automatically included in the value returned by RUSAGE_CHILDREN,
although POSIX.1-2001 explicitly prohibits this. This nonconformance
is rectified in Linux 2.6.9 and later. 



-- 
	Three Cheers,
	Balbir
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