linux-kernel - Re: [PATCH v4 22/30] context_tracking: Exit CT_STATE

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Message-ID: <xhsmh8qqdk8h7.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Mon, 10 Feb 2025 18:36:20 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
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Subject: Re: [PATCH v4 22/30] context_tracking: Exit CT_STATE_IDLE upon
 irq/nmi entry

On 07/02/25 19:37, Frederic Weisbecker wrote:
> Le Fri, Feb 07, 2025 at 06:06:45PM +0100, Valentin Schneider a écrit :
>>
>> Soooo I've been thinking...
>>
>> Isn't
>>
>>   (context_tracking.state & CT_RCU_WATCHING)
>>
>> pretty much a proxy for knowing whether a CPU is executing in kernelspace,
>> including NMIs?
>
> You got it!
>

Yay!

>>
>> NMI interrupts userspace/VM/idle -> ct_nmi_enter()   -> it becomes true
>> IRQ interrupts idle              -> ct_irq_enter()   -> it becomes true
>> IRQ interrupts userspace         -> __ct_user_exit() -> it becomes true
>> IRQ interrupts VM                -> __ct_user_exit() -> it becomes true
>>
>> IOW, if I gate setting deferred work by checking for this instead of
>> explicitely CT_STATE_KERNEL, "it should work" and prevent the
>> aforementioned issue? Or should I be out drinking instead? :-)
>
> Exactly it should work! Now that doesn't mean you can't go out
> for a drink :-)
>

Well, drinks were had very shortly after sending this email :D

> Thanks.