Re: posix-cpu-timers revamp

Frank Mayhar
2008-04-02T20:43:55Z

On Wed, 2008-04-02 at 12:48 -0700, Roland McGrath wrote:
> > One quick note: this inline isn't needed for the 2b solution (allocate
> > percpu storage in copy_signal CLONE_THREAD case), since if there's more
> > than one thread there'll always be a percpu area and if there's only one
> > thread the summation code won't be entered.
>
> That's true. I still think it's a good idea to have it, even if it winds
> up being empty in the variants we really use. The principle is that the
> new set of types/functions could be used to implement exactly what we have
> now. In fact, it's usually best to do a series of small patches that start
> with introducing the abstraction while not changing anything.

Ah, okay. Well, except that the whole point of this exercise is to fix
that hang. :-) But yeah, I understand.

> > And another quick note: It appears that with the "allocate percpu
> > storage in copy_signal CLONE_THREAD case" mechanism, I don't need to
> > worry about allocating it anywhere else. If I need it (which is only in
> > the case of multiple threads and an interval timer) then I'll have it
> > because it was allocated with the second thread.
>
> That's correct.
>
> > So I just eliminate the allocation in do_setitimer() entirely.
>
> Again, I'd leave the call to the inline that would do it.
> For this implementation plan, its body is:
> BUG_ON(!task->signal->cputime.totals && !thread_group_empty(task));

BTW I did look at allocating it in posix_cpu_timer_set() and
set_process_cpu_timer() but the first at least is doing stuff with locks
held. I'll keep looking at it, though.

One little gotcha we just ran into, though: When checking
tsk->signal->(anything) in run_posix_cpu_timers(), we have to hold
tasklist_lock to avoid a race with release_task(). This is going to
make even the null case always cost more than before.