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

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Message-ID: <7a26275f-7ed9-49a8-90ec-798f2f2de195@joelfernandes.org>
Date: Mon, 11 Mar 2024 23:24:15 -0400
From: Joel Fernandes <joel@...lfernandes.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, paulmck@...nel.org,
 linux-kernel@...r.kernel.org, peterz@...radead.org,
 torvalds@...ux-foundation.org, akpm@...ux-foundation.org, luto@...nel.org,
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 boris.ostrovsky@...cle.com, konrad.wilk@...cle.com, rcu@...r.kernel.org
Subject: Re: [PATCH 15/30] rcu: handle quiescent states for PREEMPT_RCU=n,
 PREEMPT_COUNT=y



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
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.

thanks,

- Joel