linux-kernel - Re: [PATCH v16 3/8] cpu/hotplug: Add dynamic parallel bringup states before CPUHP_BRINGUP

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Message-ID: <f363eb51084bcaa2adb383568965b23d573e91b1.camel@infradead.org>
Date:   Fri, 24 Mar 2023 15:33:17 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Usama Arif <usama.arif@...edance.com>, kim.phillips@....com,
        brgerst@...il.com
Cc:     piotrgorski@...hyos.org, oleksandr@...alenko.name,
        arjan@...ux.intel.com, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, x86@...nel.org,
        pbonzini@...hat.com, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rcu@...r.kernel.org, mimoja@...oja.de, hewenliang4@...wei.com,
        thomas.lendacky@....com, seanjc@...gle.com, pmenzel@...gen.mpg.de,
        fam.zheng@...edance.com, punit.agrawal@...edance.com,
        simon.evans@...edance.com, liangma@...ngbit.com,
        gpiccoli@...lia.com
Subject: Re: [PATCH v16 3/8] cpu/hotplug: Add dynamic parallel bringup
 states before CPUHP_BRINGUP_CPU

On Fri, 2023-03-24 at 14:57 +0100, Thomas Gleixner wrote:
> 
> Why? Simply because of this:
> 
>   BP                    AP              state
>   kick()                                BRINGUP_CPU
>                         startup()                      
>   sync()                sync() 
>                         starting()      advances to AP_ONLINE
>   sync()                sync()
>   TSC_sync()            TSC_sync()
>   wait_for_online()     set_online()
>                         cpu_startup_entry() AP_ONLINE_IDLE
>   wait_for_completion() complete()
> 
> This works correctly today because bringup_cpu() does not modify state
> and excpects the state to be advanced by the AP once the completion is
> done.
> 
> So you _cannot_ just throw some magic dynamic states before BRINGUP_CPU
> and then expect that the state machine is consistent when the AP is
> allowed to run the starting callbacks in parallel.

Aha! I see.

Yes, when the AP calls notify_cpu_starting(), which x86 does from
smp_callin(), the AP takes *itself* forward through the states from
there.

That happens when the BP gets to do_wait_cpu_initialized(). So yes, the
actual code in the existing series of patches is entirely safe, but
you're right that we do only want that *one* additional state for
parallelising the "kick AP"  before CPUHP_BRINGUP_CPU. The rest need to
come afterwards and be handled differently.


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