Re: [RFC][PATCH 09/10] taskstats: Fix exit CPU time accounting

[Oleg Nesterov <oleg@redhat.com>]

Hi,

On 09/24, Michael Holzheu wrote:
>
> On Thu, 2010-09-23 at 19:10 +0200, Oleg Nesterov wrote:
> >
> > On 09/23, Michael Holzheu wrote:
> > >
> > > Currently there are code pathes (e.g. for kthreads) where the consumed
> > > CPU time is not accounted to the parents cumulative counters.
> >
> > Could you explain more?
>
> I think one place was "khelper" (kmod.c). It is created with
> kernel_thread() and it exits without having accounted the times with
> sys_wait() to the parent's ctimes

No. Well yes, it is not accounted, but this is not because it is
kthread.

To simplify the discussion, lets talk about utime/cutime only,
and lets forget about the multithreading.

It is very simple, currently linux accounts the exiting task's
utime and adds its to ->cutime _only_ if parent does do_wait().
If parent ignores SIGCHLD, the child reaps itself and it is not
accounted.

I do not know why it was done this way, but I'm afraid we can't
change this historical behaviour.

> Ok, the problem is that I did not consider exiting threads that are no
> thread group leaders. When they exit the ctime of the parent is not
> updated. Instead the time is accumulated in the signal struct.

I think I am a bit confused, but see above. With or without threads
the whole process can exit without accounting.

Oleg.