linux-kernel - Re: [PATCH 15/30] rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT

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Date: Mon, 11 Mar 2024 20:16:12 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Thomas Gleixner <tglx@...utronix.de>,
        Ankur Arora
 <ankur.a.arora@...cle.com>, paulmck@...nel.org,
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Subject: Re: [PATCH 15/30] rcu: handle quiescent states for PREEMPT_RCU=n,
 PREEMPT_COUNT=y


Joel Fernandes <joel@...lfernandes.org> writes:

> Hi, Thomas,
> Thanks for your reply! I replied below.
>
> On 3/11/2024 3:12 PM, Thomas Gleixner wrote:
>> On Mon, Mar 11 2024 at 11:25, Joel Fernandes wrote:

   [ ... ]

>> What's wrong with the combination of PREEMPT_AUTO=y and PREEMPT_RCU=n?
>> Paul and me agreed long ago that this needs to be supported.
>
> There's nothing wrong with it. Its just a bit quirky (again just a point of
> view), that for a configuration that causes preemption (similar to
> CONFIG_PREEMPT=y), that PREEMPT_RCU can be disabled. After all, again with
> CONFIG_PREEMPT=y, PREEMPT_RCU cannot be currently disabled.

I think the argument was that PREEMPT_RCU=y is suboptimal for certain
workloads, and those configurations might prefer the stronger
forward-progress guarantees that PREEMPT_RCU=n provides.

See this:
https://lore.kernel.org/lkml/73ecce1c-d321-4579-b892-13b1e0a0620a@paulmck-laptop/T/#m6aab5a6fd5f1fd4c3dc9282ce564e64f2fa6cdc3

and the surrounding thread.

Thanks

--
ankur