Re: [patch v2 4/4] taskstats: Export "cdata_wait" CPU times withtaskstats

[Michael Holzheu]

Hello Oleg,

On Sat, 2010-12-11 at 18:39 +0100, Oleg Nesterov wrote:
> On 12/10, Michael Holzheu wrote:
> >
> > > > 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.
> > >
> > > Not sure I understand... Do you mean
> > >
> > > 	if (is_thread_group)
> > > 		fill_tgid_exit(tsk);
> > >
> > > ? Afaics, this is not "When the last thread exits", this is
> > > "this program is multithreaded, it has (or had) other threads".
> >
> > fill_tgid_exit() adds the data of the dying thread to the thread group
> > data (tsk->signal->stats). Currently only for delay accounting.
> >
> > But the accumulated data is sent to userspace only after the last thread
> > has died:
> >
> >         if (!is_thread_group || >>>!group_dead<<<)
> >                 goto send;
> 
> Ah, right you are.
> 
> And this looks racy, or I missed something again. group_dead can be
> true, but this doesn't mean all other threads have already passed
> taskstats_exit()->fill_tgid_exit()->delayacct_add_tsk().

I think you are right.

One way to fix that could be to separate the aggregation from the
sending. We could call fill_tgid_exit()->delayacct_add_tsk() before
atomic_dec_and_test(&tsk->signal->live) in do_exit() and
taskstats_exit() with the sender part afterwards.

> And. Why "if (is_thread_group)" does "size *= 2" unconditionally?
> IOW, perhaps the patch below makes sense?

Yes, I think the patch makes sense. Balbir?

> 
> Oleg.
> 
> --- x/kernel/taskstats.c
> +++ x/kernel/taskstats.c
> @@ -576,7 +576,8 @@ void taskstats_exit(struct task_struct *
>  	is_thread_group = !!taskstats_tgid_alloc(tsk);
>  	if (is_thread_group) {
>  		/* PID + STATS + TGID + STATS */
> -		size = 2 * size;
> +		if (group_dead)
> +			size *= 2;
>  		/* fill the tsk->signal->stats structure */
>  		fill_tgid_exit(tsk);
>  	}
> 


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