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

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Message-ID: <056c22523573943347906d0235708d2a@natalenko.name>
Date:   Tue, 21 Feb 2023 22:04:29 +0100
From:   Oleksandr Natalenko <oleksandr@...alenko.name>
To:     Usama Arif <usama.arif@...edance.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        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 21:04, Usama Arif wrote:
> On 21/02/2023 19:10, David Woodhouse wrote:
>> On Tue, 2023-02-21 at 13:14 +0100, Oleksandr Natalenko wrote:
>>> 
>>> With this in place:
>>> 
>>> ```
>>>          early_gdt_descr.address = (unsigned long)get_cpu_gdt_rw(0);
>>>          initial_gs = per_cpu_offset(0);
>>>          smpboot_control = 0;
>>> ```
>>> 
>>> the resume does not work.
>> 
>> Yeah, I think it's always running on CPU0 after the other CPUs are
>> taken down anyway.
>> 
>> We definitely *do* need to clear smpboot_control because we absolutely
>> want it using the temp_stack we explicitly put into initial_stack, not
>> finding its own idle thread.
>> 
>> The problem was that it was never being restored to STARTUP_SECONDARY
>> in the parallel modes, because that's a one-time setup in
>> native_smp_prepare_cpus(). So we can just restore it in
>> arch_thaw_secondary_cpus_begin() too, by working this into patch 6 of
>> the series.
>> 
>> (Usama, I think my tree is fairly out of date now so I'll let you do
>> that? Thanks!).
> 
> Sounds good! Will send out the next revision with below diff, 
> checkpatch
> fixes and rebased to 6.2 (testing it now). The below fix looks good! 
> Oleksandr, would you mind testing out suspend/resume with the below 
> diff on your AMD machine just to make sure it fixes the issue before I 
> send out the next revision with it. Thanks!

Right, so I applied the whole series + this fix, and the suspend/resume 
works. Thanks!

Reported-by: Oleksandr Natalenko <oleksandr@...alenko.name>
Tested-by: Oleksandr Natalenko <oleksandr@...alenko.name>

There's another open question pending though: why would this series 
cause booting up one CPU only on an older AMD CPU. But I'd expect 
Piotr's fellow to chime in occasionally.

>> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
>> index 50621793671d..3db77a257ae2 100644
>> --- a/arch/x86/kernel/smpboot.c
>> +++ b/arch/x86/kernel/smpboot.c
>> @@ -1571,6 +1571,17 @@ void __init native_smp_prepare_cpus(unsigned 
>> int max_cpus)
>>     void arch_thaw_secondary_cpus_begin(void)
>>   {
>> +	/*
>> +	 * On suspend, smpboot_control will have been zeroed to allow the
>> +	 * boot CPU to use explicitly passed values including a temporary
>> +	 * stack. Since native_smp_prepare_cpus() won't be called again,
>> +	 * restore the appropriate value for the parallel startup modes.
>> +	 */
>> +	if (do_parallel_bringup) {
>> +		smpboot_control = STARTUP_SECONDARY |
>> +			(x2apic_mode ? STARTUP_APICID_CPUID_0B : STARTUP_APICID_CPUID_01);
>> +	}
>> +
>>   	set_cache_aps_delayed_init(true);
>>   }
>> 

-- 
   Oleksandr Natalenko (post-factum)