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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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