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

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Message-ID: <xhsmhbkg5ti91.mognet@vschneid.remote.csb>
Date:   Fri, 21 Jul 2023 16:08:10 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     paulmck@...nel.org
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Subject: Re: [RFC PATCH v2 16/20] rcu: Make RCU dynticks counter size
 configurable

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.

>                                                       Thanx, Paul