Re: [PATCH v4 22/30] context_tracking: Exit CT_STATE_IDLE upon irq/nmi entry

Valentin Schneider
2025-02-07T17:06:54Z

On 27/01/25 12:17, Valentin Schneider wrote:
> On 22/01/25 01:22, Frederic Weisbecker wrote:
>> And NMIs interrupting userspace don't call
>> enter_from_user_mode(). In fact they don't call irqentry_enter_from_user_mode()
>> like regular IRQs but irqentry_nmi_enter() instead. Well that's for archs
>> implementing common entry code, I can't speak for the others.
>>
>
> That I didn't realize, so thank you for pointing it out. Having another
> look now, I mistook DEFINE_IDTENTRY_RAW(exc_int3) for the general case
> when it really isn't :(
>
>> Unifying the behaviour between user and idle such that the IRQs/NMIs exit the
>> CT_STATE can be interesting but I fear this may not come for free. You would
>> need to save the old state on IRQ/NMI entry and restore it on exit.
>>
>
> That's what I tried to avoid, but it sounds like there's no nice way around it.
>
>> Do we really need it?
>>
>
> Well, my problem with not doing IDLE->KERNEL transitions on IRQ/NMI is that
> this leads the IPI deferral logic to observe a technically-out-of-sync sate
> for remote CPUs. Consider:
>
> CPUx CPUy
> state := CT_STATE_IDLE
> ...
> ~>IRQ
> ...
> ct_nmi_enter()
> [in the kernel proper by now]
>
> text_poke_bp_batch()
> ct_set_cpu_work(CPUy, CT_WORK_SYNC)
> READ CPUy ct->state
> `-> CT_IDLE_STATE
> `-> defer IPI
>
>
> I thought this meant I would need to throw out the "defer IPIs if CPU is
> idle" part, but AIUI this also affects CT_STATE_USER and CT_STATE_GUEST,
> which is a bummer :(

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?

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? :-)