linux-kernel - Re: [patch 3/4] taskstats: Introduce cdata_acct for complete cumulative accounting

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Message-ID: <20101125132155.GA25836@redhat.com>
Date:	Thu, 25 Nov 2010 14:21:55 +0100
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>,
	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>,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [patch 3/4] taskstats: Introduce cdata_acct for complete
	cumulative accounting

On 11/25, Michael Holzheu wrote:
>
> Hello Oleg,
>
> On Tue, 2010-11-23 at 17:59 +0100, Oleg Nesterov wrote:
> > On 11/19, Michael Holzheu wrote:
> > > TODO:
> > > -----
> > > With this patch we take the siglock twice. First for the dead task
> > > and second for the parent of the dead task. This give the following
> > > lockdep warning (probably a lockdep annotation is needed here):
> >
> > And we already discussed this ;) We do not need 2 siglock's, only
> > parent's. Just move the callsite in __exit_signal() down, under
> > another (lockless) group_dead check.
> >
> > Or I missed something?
>
> The problem with moving this down to the second group_dead check is that
> after __unhash_process() is called, pid_alive(tsk) which is checked in
> thread_group_cputime() returns false. Therefore we always get zero CPU
> times.

I see, thanks.

> So I probably have to introduce a second group_dead check at the
> beginning of __exit_signal():

Probably...

But in fact this reminds we should cleanup this code somehow.
By the time we call thread_group_times() there are no other
threads.

> My personal feeling is that probably the only acceptable thing would be
> to make the new behavior configurable with a sysctl and define the
> default as it currently is (POSIX compliant).
>
> This would only introduce two additional checks in __exit_signal() and
> wait_task_zombie() and would not add any new fields to the
> signal_struct.

Yeah, it would be nice to avoid new fields.


Hmm. Somehow I forgot about 4/4, please see another email...

Oleg.

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