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SubjectRe: [PATCH] Read THREAD_CPUTIME clock from other processes.
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On Thu, 2010-12-23 at 17:44 +0100, Oleg Nesterov wrote: 
> > Therefore, this patch removes such limitation and enables the
> > following behaviour, for the threaded and process-based case,
> > respectively:
>
> Can't comment, I never understood this.
>
If I can ask... What's that you never understood? Why the limitation is
there? Or something else?

> > rcu_read_lock();
> > p = find_task_by_vpid(pid);
> > - if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
> > - same_thread_group(p, current) : has_group_leader_pid(p))) {
> > + if (!p)
> > error = -EINVAL;
> > - }
>
> This changes the behaviour of sys_clock_settime(). Probably doesn't
> matter since it does nothing, but perhaps !CPUCLOCK_PERTHREAD &&
> !group_leader should result in -EINAVL as before.
>
Oops, sure, you're right, I can fix this. :-)

> > @@ -349,18 +347,21 @@ int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
> > rcu_read_lock();
> > p = find_task_by_vpid(pid);
> > if (p) {
> > - if (CPUCLOCK_PERTHREAD(which_clock)) {
> > - if (same_thread_group(p, current)) {
> > - error = cpu_clock_sample(which_clock,
> > - p, &rtn);
> > - }
> > +
> > + if (CPUCLOCK_PERTHREAD(which_clock) &&
> > + same_thread_group(p, current)) {
> > + error = cpu_clock_sample(which_clock,
> > + p, &rtn);
> > } else {
> > read_lock(&tasklist_lock);
> > - if (thread_group_leader(p) && p->sighand) {
> > + if (!CPUCLOCK_PERTHREAD(which_clock) &&
> > + thread_group_leader(p) && p->sighand)
> > error =
> > cpu_clock_sample_group(which_clock,
> > - p, &rtn);
> > - }
> > + p, &rtn);
> > + else
> > + error = cpu_clock_sample(which_clock,
> > + p, &rtn);
> > read_unlock(&tasklist_lock);
>
> Can't understand... why did you duplicate cpu_clock_sample() ?
>
> IOW, it seems to me you could simply kill the
> "if (same_thread_group(p, current)) {" line with the same efect, no?
>
Well, yes, but looking at the original code I thought that in the !
same_thread_group() case I might need the tasklist_lock...

Am I wrong? Is it there just because of cpu_clock_sample_group()?

Thanks and Regards,
Dario

--
<<This happens because I choose it to happen!>> (Raistlin Majere)
----------------------------------------------------------------------
Dario Faggioli, ReTiS Lab, Scuola Superiore Sant'Anna, Pisa (Italy)
http://retis.sssup.it/people/faggioli -- dario.faggioli@jabber.org
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