Re: [PATCH 06/33] ACPI / PPTT: Add a helper to fill a cpumask from a cache_id

[James Morse <james.morse@arm.com>]

Hi Dave,

On 27/08/2025 11:53, Dave Martin wrote:
> On Fri, Aug 22, 2025 at 03:29:47PM +0000, James Morse wrote:
>> MPAM identifies CPUs by the cache_id in the PPTT cache structure.
>>
>> The driver needs to know which CPUs are associated with the cache,
>> the CPUs may not all be online, so cacheinfo does not have the
>> information.
> 
> Nit: cacheinfo lacking the information is not a consequence of the
> driver needing it.
> 
> Maybe split the sentence:
> 
> -> "[...] associated with the cache. The CPUs may not [...]"

Sure,


>> Add a helper to pull this information out of the PPTT.

>> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
>> index 660457644a5b..cb93a9a7f9b6 100644
>> --- a/drivers/acpi/pptt.c
>> +++ b/drivers/acpi/pptt.c
>> @@ -971,3 +971,65 @@ int find_acpi_cache_level_from_id(u32 cache_id)
>>  
>>  	return -ENOENT;
>>  }
>> +
>> +/**
>> + * acpi_pptt_get_cpumask_from_cache_id() - Get the cpus associated with the
>> + *					   specified cache
>> + * @cache_id: The id field of the unified cache
>> + * @cpus: Where to build the cpumask
>> + *
>> + * Determine which CPUs are below this cache in the PPTT. This allows the property
>> + * to be found even if the CPUs are offline.
>> + *
>> + * The PPTT table must be rev 3 or later,
>> + *
>> + * Return: -ENOENT if the PPTT doesn't exist, or the cache cannot be found.
>> + * Otherwise returns 0 and sets the cpus in the provided cpumask.
>> + */
>> +int acpi_pptt_get_cpumask_from_cache_id(u32 cache_id, cpumask_t *cpus)
>> +{
>> +	u32 acpi_cpu_id;
>> +	int level, cpu, num_levels;
>> +	struct acpi_pptt_cache *cache;
>> +	struct acpi_pptt_cache_v1 *cache_v1;
>> +	struct acpi_pptt_processor *cpu_node;
>> +	struct acpi_table_header *table __free(acpi_table) = acpi_get_table_ret(ACPI_SIG_PPTT, 0);
>> +
>> +	cpumask_clear(cpus);
>> +
>> +	if (IS_ERR(table))
>> +		return -ENOENT;
>> +
>> +	if (table->revision < 3)
>> +		return -ENOENT;
>> +
>> +	/*
>> +	 * If we found the cache first, we'd still need to walk from each cpu.
>> +	 */
>> +	for_each_possible_cpu(cpu) {
>> +		acpi_cpu_id = get_acpi_id_for_cpu(cpu);
>> +		cpu_node = acpi_find_processor_node(table, acpi_cpu_id);
>> +		if (!cpu_node)
>> +			return 0;
>> +		num_levels = acpi_count_levels(table, cpu_node, NULL);
>> +
>> +		/* Start at 1 for L1 */
>> +		for (level = 1; level <= num_levels; level++) {
>> +			cache = acpi_find_cache_node(table, acpi_cpu_id,
>> +						     ACPI_PPTT_CACHE_TYPE_UNIFIED,
>> +						     level, &cpu_node);
>> +			if (!cache)
>> +				continue;
>> +
>> +			cache_v1 = ACPI_ADD_PTR(struct acpi_pptt_cache_v1,
>> +						cache,
>> +						sizeof(struct acpi_pptt_cache));
>> +
>> +			if (cache->flags & ACPI_PPTT_CACHE_ID_VALID &&
>> +			    cache_v1->cache_id == cache_id)
>> +				cpumask_set_cpu(cpu, cpus);

> Again, it feels like we are repeating the same walk multiple times to
> determine how deep the table is (on which point the table is self-
> describing anyway), and then again to derive some static property, and
> then we are then doing all of that work multiple times to derive
> different static properties, etc.
> 
> Can we not just walk over the tables once and stash the derived
> properties somewhere?

That is possible - but its a more invasive change to the PPTT parsing code.
Before the introduction of the leaf flag, the search for a processor also included a
search to check if the discovered node was a leaf.

I think this is trading time - walking over the table multiple times, against the memory
you'd need to de-serialise the tree to find the necessary properties quickly. I think the
reason Jeremy L went this way was because there may never be another request into this
code, so being ready with a quick answer was a waste of memory.

MPAM doesn't change this - all these things are done up front during driver probing, and
the values are cached by the driver.


> I'm still getting my head around this parsing code, so I'm not saying
> that the approach is incorrect here -- just wondering whether there is
> a way to make it simpler.

It's walked at boot, and on cpu-hotplug. Neither are particularly performance critical.

I agree that as platforms get bigger, there will be a tipping point ... I don't think
anyone has complained yet!


Thanks,

James