Re: [PATCH v11 3/4] arm64: topology: Support SMT control on ACPI based system

[Sudeep Holla <sudeep.holla@arm.com>]

On Fri, Feb 28, 2025 at 06:51:16PM +0100, Pierre Gondois wrote:
> 
> 
> On 2/28/25 14:56, Sudeep Holla wrote:
> > On Tue, Feb 18, 2025 at 10:10:17PM +0800, Yicong Yang wrote:
> > > From: Yicong Yang <yangyicong@hisilicon.com>
> > > 
> > > For ACPI we'll build the topology from PPTT and we cannot directly
> > > get the SMT number of each core. Instead using a temporary xarray
> > > to record the heterogeneous information (from ACPI_PPTT_ACPI_IDENTICAL)
> > > and SMT information of the first core in its heterogeneous CPU cluster
> > > when building the topology. Then we can know the largest SMT number
> > > in the system. If a homogeneous system's using ACPI 6.2 or later,
> > > all the CPUs should be under the root node of PPTT. There'll be
> > > only one entry in the xarray and all the CPUs in the system will
> > > be assumed identical.
> > > 
> > > 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
> > > 
> > > Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > > Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> > > ---
> > >   arch/arm64/kernel/topology.c | 66 ++++++++++++++++++++++++++++++++++++
> > >   1 file changed, 66 insertions(+)
> > > 
> > > diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> > > index 1a2c72f3e7f8..6eba1ac091ee 100644
> > > --- a/arch/arm64/kernel/topology.c
> > > +++ b/arch/arm64/kernel/topology.c
> > > @@ -15,8 +15,10 @@
> > >   #include <linux/arch_topology.h>
> > >   #include <linux/cacheinfo.h>
> > >   #include <linux/cpufreq.h>
> > > +#include <linux/cpu_smt.h>
> > >   #include <linux/init.h>
> > >   #include <linux/percpu.h>
> > > +#include <linux/xarray.h>
> > >   #include <asm/cpu.h>
> > >   #include <asm/cputype.h>
> > > @@ -37,17 +39,28 @@ static bool __init acpi_cpu_is_threaded(int cpu)
> > >   	return !!is_threaded;
> > >   }
> > > +struct cpu_smt_info {
> > > +	unsigned int thread_num;
> > > +	int core_id;
> > > +};
> > > +
> > >   /*
> > >    * Propagate the topology information of the processor_topology_node tree to the
> > >    * cpu_topology array.
> > >    */
> > >   int __init parse_acpi_topology(void)
> > >   {
> > > +	unsigned int max_smt_thread_num = 0;
> > > +	struct cpu_smt_info *entry;
> > > +	struct xarray hetero_cpu;
> > > +	unsigned long hetero_id;
> > >   	int cpu, topology_id;
> > >   	if (acpi_disabled)
> > >   		return 0;
> > > +	xa_init(&hetero_cpu);
> > > +
> > >   	for_each_possible_cpu(cpu) {
> > >   		topology_id = find_acpi_cpu_topology(cpu, 0);
> > >   		if (topology_id < 0)
> > > @@ -57,6 +70,34 @@ int __init parse_acpi_topology(void)
> > >   			cpu_topology[cpu].thread_id = topology_id;
> > >   			topology_id = find_acpi_cpu_topology(cpu, 1);
> > >   			cpu_topology[cpu].core_id   = topology_id;
> > > +
> > > +			/*
> > > +			 * In the PPTT, CPUs below a node with the 'identical
> > > +			 * implementation' flag have the same number of threads.
> > > +			 * Count the number of threads for only one CPU (i.e.
> > > +			 * one core_id) among those with the same hetero_id.
> > > +			 * See the comment of find_acpi_cpu_topology_hetero_id()
> > > +			 * for more details.
> > > +			 *
> > > +			 * One entry is created for each node having:
> > > +			 * - the 'identical implementation' flag
> > > +			 * - its parent not having the flag
> > > +			 */
> > > +			hetero_id = find_acpi_cpu_topology_hetero_id(cpu);
> > > +			entry = xa_load(&hetero_cpu, hetero_id);
> > > +			if (!entry) {
> > > +				entry = kzalloc(sizeof(*entry), GFP_KERNEL);
> > > +				WARN_ON_ONCE(!entry);
> > > +
> > > +				if (entry) {
> > > +					entry->core_id = topology_id;
> > > +					entry->thread_num = 1;
> > > +					xa_store(&hetero_cpu, hetero_id,
> > > +						 entry, GFP_KERNEL);
> > > +				}
> > > +			} else if (entry->core_id == topology_id) {
> > > +				entry->thread_num++;
> > > +			}
> > >   		} else {
> > >   			cpu_topology[cpu].thread_id  = -1;
> > >   			cpu_topology[cpu].core_id    = topology_id;
> > > @@ -67,6 +108,31 @@ int __init parse_acpi_topology(void)
> > >   		cpu_topology[cpu].package_id = topology_id;
> > >   	}
> > > +	/*
> > > +	 * This should be a short loop depending on the number of heterogeneous
> > > +	 * CPU clusters. Typically on a homogeneous system there's only one
> > > +	 * entry in the XArray.
> > > +	 */
> > > +	xa_for_each(&hetero_cpu, hetero_id, entry) {
> > > +		if (entry->thread_num != max_smt_thread_num && max_smt_thread_num)
> > > +			pr_warn_once("Heterogeneous SMT topology is partly supported by SMT control\n");
> > 
> > Ditto as previous patch about handling no threaded cores with threaded cores
> > in the system. I am not sure if that is required but just raising it here.
> > 
> > > +
> > > +		max_smt_thread_num = max(max_smt_thread_num, entry->thread_num);
> > > +		xa_erase(&hetero_cpu, hetero_id);
> > > +		kfree(entry);
> > > +	}
> > > +
> > > +	/*
> > > +	 * 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;
> > > +
> > 
> > Ditto as previous patch, can get rid if it is default 1.
> > 
> 
> On non-SMT platforms, not calling cpu_smt_set_num_threads() leaves
> cpu_smt_num_threads uninitialized to UINT_MAX:
> 
> smt/active:0
> smt/control:-1
> 
> If cpu_smt_set_num_threads() is called:
> active:0
> control:notsupported
> 
> So it might be slightly better to still initialize max_smt_thread_num.
>

Sure, what I meant is to have max_smt_thread_num set to 1 by default is
that is what needed anyways and the above code does that now.

Why not start with initialised to 1 instead ?
Of course some current logic needs to change around testing it for zero.

-- 
Regards,
Sudeep