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

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Message-ID: <8dff6ae5ffaebfbcc55a01c04420fd478070b830.camel@infradead.org>
Date:   Thu, 23 Mar 2023 23:12:21 +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 00:05 +0100, Thomas Gleixner wrote:
> Still the rest can be simplified as below.

...

> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1504,13 +1504,45 @@ int bringup_hibernate_cpu(unsigned int s
>  
>  void bringup_nonboot_cpus(unsigned int setup_max_cpus)
>  {
> -       unsigned int cpu;
> +       unsigned int cpu, n = 1;
>  
> +       /*
> +        * On architectures which have setup the CPUHP_BP_PARALLEL_STARTUP
> +        * state, this invokes all BP prepare states and the parallel
> +        * startup state sends the startup IPI to each of the to be onlined
> +        * APs. This avoids waiting for each AP to respond to the startup
> +        * IPI in CPUHP_BRINGUP_CPU. The APs proceed through the low level
> +        * bringup code and then wait for the control CPU to release them
> +        * one by one for the final onlining procedure in the loop below.
> +        *
> +        * For architectures which do not support parallel bringup all
> +        * states are fully serialized in the loop below.
> +        */
> +       if (!cpuhp_step_empty(true, CPUHP_BP_PARALLEL_STARTUP) {

I'll take using cpuhp_step_empty().

> +                       for_each_present_cpu(cpu) {
> +                               if (n++ >= setup_max_cpus)
> +                                       break;
> +                               cpu_up(cpu, CPUHP_BP_PARALLEL_STARTUP);
> +                       }
> +       }
> +
> +       /* Do the per CPU serialized bringup to ONLINE state */
>         for_each_present_cpu(cpu) {
>                 if (num_online_cpus() >= setup_max_cpus)
>                         break;
> -               if (!cpu_online(cpu))
> -                       cpu_up(cpu, CPUHP_ONLINE);
> +
> +               if (!cpu_online(cpu)) {
> +                       struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
> +                       int ret = cpu_up(cpu, CPUHP_ONLINE);
> +
> +                       /*
> +                        * Due to the above preparation loop a failed online attempt
> +                        * might have only rolled back to CPUHP_BP_PARALLEL_STARTUP. Do the
> +                        * remaining cleanups. NOOP for the non parallel case.
> +                        */
> +                       if (ret && can_rollback_cpu(st))
> +                               WARN_ON(cpuhp_invoke_callback_range(false, cpu, st, CPUHP_OFFLINE));
> +               }

And I'll take doing this bit unconditionally (it's basically a no-op if
they already got rolled all the way back to CPUHP_OFFLINE, right?).

But the additional complexity of having multiple steps is fairly
minimal, and I'm already planning to *use* another one even in x86, as
discussed.




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