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?
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.
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.
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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