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

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Message-ID: <xhsmhjzutu18u.mognet@vschneid.remote.csb>
Date:   Fri, 21 Jul 2023 09:17:53 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, kvm@...r.kernel.org, linux-mm@...ck.org,
        bpf@...r.kernel.org, x86@...nel.org, rcu@...r.kernel.org,
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Subject: Re: [RFC PATCH v2 16/20] rcu: Make RCU dynticks counter size
 configurable

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?