linux-kernel - Re: [PATCH] tick/nohz: Fix cpu_is_hotpluggable() by checking with nohz subsystem

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Date:   Fri, 20 Jan 2023 08:47:41 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Zhouyi Zhou <zhouzhouyi@...il.com>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        rcu <rcu@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] tick/nohz: Fix cpu_is_hotpluggable() by checking with nohz subsystem


> On Jan 20, 2023, at 8:44 AM, Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:
> 
> On Fri, Jan 20, 2023 at 08:32:30AM -0500, Joel Fernandes wrote:
>> 
>> 
>>>> On Jan 20, 2023, at 2:05 AM, Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:
>>> 
>>> On Thu, Jan 19, 2023 at 08:44:35PM +0000, Joel Fernandes (Google) wrote:
>>>> For CONFIG_NO_HZ_FULL systems, the tick_do_timer_cpu cannot be offlined.
>>>> However, cpu_is_hotpluggable() still returns true for those CPUs. This causes
>>>> torture tests that do offlining to end up trying to offline this CPU causing
>>>> test failures. Such failure happens on all architectures.
>>>> 
>>>> Fix it by asking the opinion of the nohz subsystem on whether the CPU can
>>>> be hotplugged.
>>>> 
>>>> [ Apply Frederic Weisbecker feedback on refactoring tick_nohz_cpu_down(). ]
>>>> 
>>>> Cc: Frederic Weisbecker <frederic@...nel.org>
>>>> Cc: "Paul E. McKenney" <paulmck@...nel.org>
>>>> Cc: Zhouyi Zhou <zhouzhouyi@...il.com>
>>>> Cc: Will Deacon <will@...nel.org>
>>>> Cc: Marc Zyngier <maz@...nel.org>
>>>> Cc: rcu <rcu@...r.kernel.org>
>>>> Fixes: 2987557f52b9 ("driver-core/cpu: Expose hotpluggability to the rest of the kernel")
>>>> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>>> 
>>> Also want to cc: stable on the patch?
>> 
>> Oh sure, sorry. For some reason I thought Sasha and your AI scripts
>> were looking at the Linux-kernel list as well. Or are they, and a Cc
>> to stable is just to be doubly sure?
> 
> As per the rules we have had for the last 15+ years, always add a cc:
> stable to be sure that the patch will be considered for stable releases.
> If not, you are on you own and sometimes we might notice it, others not.
> 
> See:
>    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
> for the details.

Ah my bad, I did read that already but somehow assumed anything merged with a Fixes tag was already considered for stable. I will always Cc stable henceforth if I want something in stable.

Thank you!

 - Joel


> 
> thanks,
> 
> greg k-h