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From:   Ankur Arora <ankur.a.arora@oracle.com>
To:     paulmck@kernel.org
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ankur Arora <ankur.a.arora@oracle.com>,
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Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED
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Paul E. McKenney <paulmck@kernel.org> writes:

> Thomas!
>
> On Thu, Oct 19, 2023 at 02:21:35AM +0200, Thomas Gleixner wrote:
>> Paul!
>>
>> On Wed, Oct 18 2023 at 10:19, Paul E. McKenney wrote:
>> > On Wed, Oct 18, 2023 at 03:16:12PM +0200, Thomas Gleixner wrote:
>> >> On Tue, Oct 17 2023 at 18:03, Paul E. McKenney wrote:
>> >> In the end there is no CONFIG_PREEMPT_XXX anymore. The only knob
>> >> remaining would be CONFIG_PREEMPT_RT, which should be renamed to
>> >> CONFIG_RT or such as it does not really change the preemption
>> >> model itself. RT just reduces the preemption disabled sections with the
>> >> lock conversions, forced interrupt threading and some more.
>> >
>> > Again, please, no.
>> >
>> > There are situations where we still need rcu_read_lock() and
>> > rcu_read_unlock() to be preempt_disable() and preempt_enable(),
>> > repectively.  Those can be cases selected only by Kconfig option, not
>> > available in kernels compiled with CONFIG_PREEMPT_DYNAMIC=y.
>>
>> Why are you so fixated on making everything hardcoded instead of making
>> it a proper policy decision problem. See above.
>
> Because I am one of the people who will bear the consequences.
>
> In that same vein, why are you so opposed to continuing to provide
> the ability to build a kernel with CONFIG_PREEMPT_RCU=n?  This code
> is already in place, is extremely well tested, and you need to handle
> preempt_disable()/preeempt_enable() regions of code in any case.  What is
> the real problem here?

I have a somewhat related question. What ties PREEMPTION=y to PREEMPT_RCU=y?

I see e72aeafc66 ("rcu: Remove prompt for RCU implementation") from
2015, stating that the only possible choice for PREEMPTION=y kernels
is PREEMPT_RCU=y:

    The RCU implementation is chosen based on PREEMPT and SMP config options
    and is not really a user-selectable choice.  This commit removes the
    menu entry, given that there is not much point in calling something a
    choice when there is in fact no choice..  The TINY_RCU, TREE_RCU, and
    PREEMPT_RCU Kconfig options continue to be selected based solely on the
    values of the PREEMPT and SMP options.

As far as I can tell (which isn't all that far), TREE_RCU=y makes strictly
stronger forward progress guarantees with respect to rcu readers (in
that they can't be preempted.)

So, can PREEMPTION=y run with, say TREE_RCU=y? Or maybe I'm missing something
obvious there.


Thanks

--
ankur