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

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Message-ID: <488d8289-b538-01cb-5de7-d691f49c0d7b@huawei.com>
Date: Thu, 31 Oct 2024 20:17:57 +0800
From: Yicong Yang <yangyicong@...wei.com>
To: 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>,
	<pierre.gondois@....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 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.