linux-kernel - Re: [External] Re: [PATCH v9 0/8] Parallel CPU bringup for x86

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Message-ID: <d91f96eb-86a1-c794-761d-a96e39076b13@bytedance.com>
Date:   Tue, 21 Feb 2023 10:47:31 +0000
From:   Usama Arif <usama.arif@...edance.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>
Cc:     Kim Phillips <kim.phillips@....com>, tglx@...utronix.de,
        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,
        "Limonciello, Mario" <Mario.Limonciello@....com>,
        Piotr Gorski <piotrgorski@...hyos.org>
Subject: Re: [External] Re: [PATCH v9 0/8] Parallel CPU bringup for x86_64



On 21/02/2023 10:27, David Woodhouse wrote:
> 
> 
> On 21 February 2023 09:49:51 GMT, Oleksandr Natalenko <oleksandr@...alenko.name> wrote:
>> On 21.02.2023 10:06, David Woodhouse wrote:
>>> Why does arch/x86/kernel/acpi/sleep.c::x86_acpi_suspend_lowlevel() set
>>>
>>>      initial_gs = per_cpu_offset(smp_processor_id()) ?
>>>
>>> Would it not be CPU#0 that comes back up, and should it not get
>>> per_cpu_offset(0) ?
>>
>> Wanna me try `initial_gs = per_cpu_offset(0);` too?
> 

I think it might be smp_processor_id() and not 0 incase CPU0 was offline 
at the point the system was suspended?

> Hm, yes please. There's another one to make zero on the next line up, I think?
> 
>>> Or maybe we should just set up smpboot_control for the CPU to find its
>>> own stuff, *even* on waking. Since the structures are already set up,
>>> it isn't like a clean boot.
>>>
>>> If you let it boot in parallel mode, what if you just *remove* the line
>>> that sets smpboot_control=0 ?
>>
>> If the `smpboot_control = 0;` line in arch/x86/kernel/acpi/sleep.c::x86_acpi_suspend_lowlevel() is commented out, and the system is booted in parallel mode, then suspend/resume works.
> 
> Well that's entertaining. Now, can we come up with any theory which doesn't leave us wondering why it ever worked in the first place...?