linux-kernel - Re: [PATCH v10 2/4] arch_topology: Support SMT control for OF based system

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Message-ID: <935a29ae-7bf4-fcaa-061c-8ff77e53385d@huawei.com>
Date: Tue, 24 Dec 2024 20:23:23 +0800
From: Yicong Yang <yangyicong@...wei.com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
CC: <yangyicong@...ilicon.com>, <catalin.marinas@....com>, <will@...nel.org>,
	<sudeep.holla@....com>, <tglx@...utronix.de>, <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>,
	<linuxppc-dev@...ts.ozlabs.org>, <x86@...nel.org>,
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	<msuchanek@...e.de>, <gregkh@...uxfoundation.org>, <rafael@...nel.org>,
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	<guohanjun@...wei.com>
Subject: Re: [PATCH v10 2/4] arch_topology: Support SMT control for OF based
 system

On 2024/12/24 0:33, Jonathan Cameron wrote:
> On Fri, 20 Dec 2024 15:53:11 +0800
> Yicong Yang <yangyicong@...wei.com> wrote:
> 
>> From: Yicong Yang <yangyicong@...ilicon.com>
>>
>> On building the topology from the devicetree, we've already
>> gotten the SMT thread number of each core. Update the largest
>> SMT thread number and enable the SMT control by the end of
>> topology parsing.
>>
>> The core's SMT control provides two interface to the users [1]:
>> 1) enable/disable SMT by writing on/off
>> 2) enable/disable SMT by writing thread number 1/max_thread_number
>>
>> If a system have more than one SMT thread number the 2) may
>> not handle it well, since there're multiple thread numbers in the
>> system and 2) only accept 1/max_thread_number. So issue a warning
>> to notify the users if such system detected.
>>
>> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/ABI/testing/sysfs-devices-system-cpu#n542
>> Signed-off-by: Yicong Yang <yangyicong@...ilicon.com>
> Hi Yicong,
> 
> Apologies that I'm late to the game on this one.
> 
> A few comments inline.  Only important one is whether to bail out early
> on error from parse_cluster()
> 

thanks for the comments.

> Thanks,
> 
> Jonathan
> 
> 
>> ---
>>  drivers/base/arch_topology.c | 26 ++++++++++++++++++++++++++
>>  1 file changed, 26 insertions(+)
>>
>> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
>> index 3ebe77566788..9e81060144c7 100644
>> --- a/drivers/base/arch_topology.c
>> +++ b/drivers/base/arch_topology.c
>> @@ -11,6 +11,7 @@
>>  #include <linux/cleanup.h>
>>  #include <linux/cpu.h>
>>  #include <linux/cpufreq.h>
>> +#include <linux/cpu_smt.h>
>>  #include <linux/device.h>
>>  #include <linux/of.h>
>>  #include <linux/slab.h>
>> @@ -506,6 +507,10 @@ core_initcall(free_raw_capacity);
>>  #endif
>>  
>>  #if defined(CONFIG_ARM64) || defined(CONFIG_RISCV)
>> +
>> +/* Maximum SMT thread number detected used to enable the SMT control */
>> +static unsigned int max_smt_thread_num;
>> +
>>  /*
>>   * This function returns the logic cpu number of the node.
>>   * There are basically three kinds of return values:
>> @@ -565,6 +570,17 @@ static int __init parse_core(struct device_node *core, int package_id,
>>  		i++;
>>  	} while (1);
>>  
>> +	/*
>> +	 * If max_smt_thread_num has been initialized and doesn't match
>> +	 * the thread number of this entry, then the system has
>> +	 * heterogeneous SMT topology.
>> +	 */
>> +	if (max_smt_thread_num && max_smt_thread_num != i)
>> +		pr_warn_once("Heterogeneous SMT topology is partly supported by SMT control\n");
>> +
>> +	if (max_smt_thread_num < i)
>> +		max_smt_thread_num = i;
> 
> Maybe more self documenting if you use min()? I'm not sure...
> 	max_smt_thread_num = min(max_smt_thread_num, i);
> 

sure, will use max_t here (since we'd like to know the maximum thread number).

> 
>> +
>>  	cpu = get_cpu_for_node(core);
>>  	if (cpu >= 0) {
>>  		if (!leaf) {
>> @@ -677,6 +693,16 @@ static int __init parse_socket(struct device_node *socket)
>>  	if (!has_socket)
>>  		ret = parse_cluster(socket, 0, -1, 0);
> 
> Is it appropriate to check ret before setting num threads?
> 	if (!has_socket) {
> 		ret = parse_cluster(socket, 0, -1, 0);
> 		if (ret)
> 			return ret;
> 	}
> ...
> 
> 	return 0;
> 

thanks for catching this. since we always need to notify the framework of
the SMT thread number if HOTPLUG_SMT selected, we should make it to 1 if
topology parsing failed. Since on failure the topology information will
be reset, a thread_number of 1 will be handled as SMT not supported.

>>  
>> +	/*
>> +	 * Notify the CPU framework of the SMT support. Initialize the
>> +	 * max_smt_thread_num to 1 if no SMT support detected. A thread
>> +	 * number of 1 can be handled by the framework so we don't need
>> +	 * to check max_smt_thread_num to see we support SMT or not.
>> +	 */
>> +	if (!max_smt_thread_num)
>> +		max_smt_thread_num = 1;
>> +
>> +	cpu_smt_set_num_threads(max_smt_thread_num, max_smt_thread_num);
> 
> Trivial but I'd put a blank line here.
> 

ok.

Thanks.