Re: [PATCH v2 1/4] powerpc/topology: Introduce llc_override kernel parameter

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Chen Yu, Thanks for taking a look.

On 10/23/26 9:12 PM, Chen Yu wrote:
> Hi Shrikanth,
> 
> On Fri, Oct 23, 2026 at 06:46:43PM +0530, Shrikanth Hegde wrote:
> 
> [ ... ]
> 
>> ---
>>   arch/powerpc/kernel/smp.c | 23 +++++++++++++++++++++++
>>   1 file changed, 23 insertions(+)
>>
>> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
>> index 6a5a5469aaae..6b1201ea3572 100644
>> --- a/arch/powerpc/kernel/smp.c
>> +++ b/arch/powerpc/kernel/smp.c
>> @@ -1059,6 +1059,29 @@ static bool has_coregroup_support(void)
>>   	return coregroup_enabled;
>>   }
>>   
>> +enum llc_override_values {
>> +	LLC_OVERRIDE_MC		= 1,
>> +	LLC_OVERRIDE_PKG	= 2,
>> +};
>> +
>> +static int llc_override;
>> +
>> +static int __init set_llc_override(char *str)
>> +{
>> +	if (!strcasecmp(str, "mc") && has_coregroup_support())
>> +		WRITE_ONCE(llc_override, LLC_OVERRIDE_MC);
> 
> There is only 1 booting CPU at the moment, maybe WRITE_ONCE() is not needed.

Yes.

> 
>> +	else if (!strcasecmp(str, "pkg"))
>> +		WRITE_ONCE(llc_override, LLC_OVERRIDE_PKG);
>> +	else {
>> +		pr_warn("Unsupported llc_override=%s\n", str);
>> +		return 1;
>> +	}
>> +
>> +	pr_info("llc_override set at domain: %s\n", str);
>> +	return 1;
>> +}
>> +__setup("llc_override=", set_llc_override);
>> +
>>   static int __init init_big_cores(void)
>>   {
>>   	int cpu;
> 
> On PowerPC, it seems that get_effective_llc_bytes() always returns 0,
> which makes exceed_llc_capacity() return true, so cache-aware scheduling
> is disabled on PowerPC. We might need to provide an arch_llc_bytes() that
> is adjusted based on llc_override - sometimes calculate the sum of L2 across
> several cores.
> 
> thanks,
> Chenyu

Ah!. I hadn't checked that. Thanks for pointing out.

I am thinking right path would be to move powerpc cacheinfo into a generic one.
I see there was an effort a while ago. Need to look into how to pull it off
if it is possible.

We could think of arch_llc_bytes if above is not possible.