Re: [PATCH] Read THREAD_CPUTIME clock from other processes.

[Oleg Nesterov <oleg@redhat.com>]

On 12/23, Dario Faggioli wrote:
>
> 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?

Yes. IOW, I agree it looks strange, clock_gettime() can sample the
whole group but not a single thread.

> > > @@ -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()?

Yes, it is because of _group (we are going to sample all sub-threads),
not because of !same_thread_group().

Oh. In fact we should remove this tasklist, but this is another story.

Oleg.