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

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Message-ID: <8734swf2gt.fsf@oracle.com>
Date: Mon, 11 Mar 2024 22:23:14 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Ankur Arora <ankur.a.arora@...cle.com>,
        Thomas Gleixner
 <tglx@...utronix.de>, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, peterz@...radead.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:

> On 3/11/2024 11:16 PM, Ankur Arora wrote:
>>
>> 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 for the link. Sorry for any noise due to being late to the party. Based
> on the discussions, I concur with everyone on the goal of getting rid of

No worries. Given the unending context, easy enough to miss.

> CONFIG_PREEMPT_DYNAMIC and the various cond_resched()/might_sleep() things. I'll
> also go look harder at what else we need to get CONFIG_PREEMPT_RCU=y/n working
> with CONFIG_PREEMPT_AUTO=y.

Sounds great. Thanks.

And, please keep the review comments coming.

--
ankur