On Tue, Oct 08, 2013 at 10:34:28PM +0200, Frederic Weisbecker wrote:
> On Mon, Oct 07, 2013 at 08:39:55AM -0700, Paul E. McKenney wrote:
> > Hello, Frederic!
> >
> > The following patch seems to me to be a good idea to better handle
> > task nesting. Any reason why it would be a bad thing?
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > rcu: Allow task-level idle entry/exit nesting
> >
> > The current task-level idle entry/exit code forces an entry/exit on
> > each call, regardless of the nesting level. This commit therefore
> > properly accounts for nesting.
> >
> > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
>
> Looks good. In fact, the current code is even buggy because two nesting
> rcu_user_eqs()
> as in:
>
> rcu_eqs_enter()
> rcu_eqs_enter()
> rcu_eqs_exit()
> rcu_eqs_exit()
>
> would result in rdtp->dynticks wrong increment, right?
That was my thought, but I figured I should run it past you in case
there was some subtle tie-in to NO_HZ_FULL.
> So that's even a bug fix. I wonder if it's a regression. That said
> rcu_eqs_enter_common()
> should warn on such miscount, so may be these functions actually don't nest
> in practice
> or you would have received such warnings.
And the lack of such warnings was another reason I felt the need to check
with you.
> So I wonder, do we want to continue to allow this nesting? I remember that
> DYNTICK_TASK_NEST_*
> stuff is there to protects against non finishing interrupts on some archs (I
> also remember that
> this, or at least a practical scenario for this, was hard to really define
> though :o)
> But then wouldn't it involve other kind of scenario like this?
>
> rcu_irq_enter()
> rcu_eqs_enter()
> rcu_eqs_exit()
> ...
>
> Anyway, that's just random thougths on further simplifications, in any case,
> this
> patch looks good.
Yep, if no task-level nesting is ever required, things could be a bit
simpler. I would be a bit slow about making such a change, though.
After all, the need to deal with Hotel California interrupts means that
handling nesting isn't that big of a deal comparatively. ;-)
May I add your Reviewed-by?
Thanx, Paul
> Thanks.
>
> >
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 106f7f5cdd1d..f0be20886617 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -411,11 +411,12 @@ static void rcu_eqs_enter(bool user)
> > rdtp = this_cpu_ptr(&rcu_dynticks);
> > oldval = rdtp->dynticks_nesting;
> > WARN_ON_ONCE((oldval & DYNTICK_TASK_NEST_MASK) == 0);
> > - if ((oldval & DYNTICK_TASK_NEST_MASK) == DYNTICK_TASK_NEST_VALUE)
> > + if ((oldval & DYNTICK_TASK_NEST_MASK) == DYNTICK_TASK_NEST_VALUE) {
> > rdtp->dynticks_nesting = 0;
> > - else
> > + rcu_eqs_enter_common(rdtp, oldval, user);
> > + } else {
> > rdtp->dynticks_nesting -= DYNTICK_TASK_NEST_VALUE;
> > - rcu_eqs_enter_common(rdtp, oldval, user);
> > + }
> > }
> >
> > /**
> > @@ -533,11 +534,12 @@ static void rcu_eqs_exit(bool user)
> > rdtp = this_cpu_ptr(&rcu_dynticks);
> > oldval = rdtp->dynticks_nesting;
> > WARN_ON_ONCE(oldval < 0);
> > - if (oldval & DYNTICK_TASK_NEST_MASK)
> > + if (oldval & DYNTICK_TASK_NEST_MASK) {
> > rdtp->dynticks_nesting += DYNTICK_TASK_NEST_VALUE;
> > - else
> > + } else {
> > rdtp->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
> > - rcu_eqs_exit_common(rdtp, oldval, user);
> > + rcu_eqs_exit_common(rdtp, oldval, user);
> > + }
> > }
> >
> > /**
> >
>
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