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

On Fri, Jul 21, 2023 at 04:08:10PM +0100, Valentin Schneider wrote:

On 21/07/23 07:10, Paul E. McKenney wrote:

quoted

On Fri, Jul 21, 2023 at 09:17:53AM +0100, Valentin Schneider wrote:

quoted

On 20/07/23 17:30, Valentin Schneider wrote:

quoted

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