Re: [RFC PATCH v2 16/20] rcu: Make RCU dynticks counter size configurable

Paul E. McKenney
2023-07-21T16:09:20Z

On Fri, Jul 21, 2023 at 04:08:10PM +0100, Valentin Schneider wrote:
> On 21/07/23 07:10, Paul E. McKenney wrote:
> > On Fri, Jul 21, 2023 at 09:17:53AM +0100, Valentin Schneider wrote:
> >> On 20/07/23 17:30, Valentin Schneider wrote:
> >> > index bdd7eadb33d8f..1ff2aab24e964 100644
> >> > --- a/kernel/rcu/Kconfig
> >> > +++ b/kernel/rcu/Kconfig
> >> > @@ -332,4 +332,37 @@ config RCU_DOUBLE_CHECK_CB_TIME
> >> > Say Y here if you need tighter callback-limit enforcement.
> >> > Say N here if you are unsure.
> >> >
> >> > +config RCU_DYNTICKS_RANGE_BEGIN
> >> > + int
> >> > + depends on !RCU_EXPERT
> >> > + default 31 if !CONTEXT_TRACKING_WORK
> >>
> >> You'll note that this should be 30 really, because the lower *2* bits are
> >> taken by the context state (CONTEXT_GUEST has a value of 3).
> >>
> >> This highlights the fragile part of this: the Kconfig values are hardcoded,
> >> but they depend on CT_STATE_SIZE, CONTEXT_MASK and CONTEXT_WORK_MAX. The
> >> static_assert() will at least capture any misconfiguration, but having that
> >> enforced by the actual Kconfig ranges would be less awkward.
> >>
> >> Do we currently have a way of e.g. making a Kconfig file depend on and use
> >> values generated by a C header?
> >
> > Why not just have something like a boolean RCU_DYNTICKS_TORTURE Kconfig
> > option and let the C code work out what the number of bits should be?
> >
> > I suppose that there might be a failure whose frequency depended on
> > the number of bits, which might be an argument for keeping something
> > like RCU_DYNTICKS_RANGE_BEGIN for fault isolation. But still using
> > RCU_DYNTICKS_TORTURE for normal testing.
> >
> > Thoughts?
> >
>
> AFAICT if we run tests with the minimum possible width, then intermediate
> values shouldn't have much value.
>
> Your RCU_DYNTICKS_TORTURE suggestion sounds like a saner option than what I
> came up with, as we can let the context tracking code figure out the widths
> itself and not expose any of that to Kconfig.

Agreed. If a need for variable numbers of bits ever does arise, we can
worry about it at that time. And then we would have more information
on what a variable-bit facility should look like.

Thanx, Paul