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

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Message-ID: <28d4abb7-8496-45ec-b270-ea2b6164537b@paulmck-laptop>
Date:   Fri, 21 Jul 2023 07:10:03 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Valentin Schneider <vschneid@...hat.com>
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Subject: Re: [RFC PATCH v2 16/20] rcu: Make RCU dynticks counter size
 configurable

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?

							Thanx, Paul