linux-kernel - Re: [PATCH v7 1/4] cpu/SMT: Provide a default topology_is_primary

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Message-ID: <a8a41066-e569-44c4-aa96-176d4788c7b6@arm.com>
Date: Thu, 7 Nov 2024 18:20:29 +0100
From: Pierre Gondois <pierre.gondois@....com>
To: Yicong Yang <yangyicong@...wei.com>, Thomas Gleixner
 <tglx@...utronix.de>, catalin.marinas@....com, will@...nel.org,
 sudeep.holla@....com, peterz@...radead.org, mpe@...erman.id.au,
 linux-arm-kernel@...ts.infradead.org, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, dietmar.eggemann@....com
Cc: yangyicong@...ilicon.com, linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
 linux-kernel@...r.kernel.org, morten.rasmussen@....com, msuchanek@...e.de,
 gregkh@...uxfoundation.org, rafael@...nel.org, jonathan.cameron@...wei.com,
 prime.zeng@...ilicon.com, linuxarm@...wei.com, xuwei5@...wei.com,
 guohanjun@...wei.com
Subject: Re: [PATCH v7 1/4] cpu/SMT: Provide a default
 topology_is_primary_thread()



On 10/31/24 13:17, Yicong Yang wrote:
> On 2024/10/30 22:55, Thomas Gleixner wrote:
>> On Wed, Oct 30 2024 at 20:54, Yicong Yang wrote:
>>>   
>>> +#ifndef topology_is_primary_thread
>>> +#define topology_is_primary_thread topology_is_primary_thread
>>
>> Please do not glue defines and functions together w/o a newline in between.
>>
> 
> sure, will add a newline here.
> 
>>> +static inline bool topology_is_primary_thread(unsigned int cpu)
>>> +{
>>> +	/*
>>> +	 * On SMT hotplug the primary thread of the SMT won't be disabled.
>>> +	 * Architectures do have a special primary thread (e.g. x86) need
>>> +	 * to override this function. Otherwise just make the first thread
>>> +	 * in the SMT as the primary thread.
>>> +	 */
>>> +	return cpu == cpumask_first(topology_sibling_cpumask(cpu));
>>
>> How is that supposed to work? Assume both siblings are offline, then the
>> sibling mask is empty and you can't boot the CPU anymore.
>>
> 
> For architectures' using arch_topology, topology_sibling_cpumask() will at least
> contain the tested CPU itself. This is initialized in
> drivers/base/arch_topology.c:reset_cpu_topology(). So it won't be empty here.
> 
> Besides we don't need to check topology_is_primary_thread() at boot time:
> -> cpu_up(cpu)
>       cpu_bootable()
>         if (cpu_smt_control == CPU_SMT_ENABLED &&
>             cpu_smt_thread_allowed(cpu)) // will always return true if !CONFIG_SMT_NUM_THREADS_DYNAMIC
>           return true; // we'll always return here and @cpu is always bootable
> 
> Also tested fine in practice.
> 
> Thanks.
> 
> 

FWIW, I also tested the case where:
- setting maxcpus=1 in the kernel cmdline to have CPUs that never booted
- setting smt to off:
   'echo off > /sys/devices/system/cpu/smt/control'
and effectively the primary CPUs can boot and secondary CPUs can't,
so it works as expected.