Re: git: 7b26353a59d6 - main - hwpstate_intel: Disable package control on hybrid CPU

During CPU frequency initialization, a sysctl callback is used to 
retrieve all available frequency profiles. A user-space daemon (powerd) 
obtains this list and uses both the available profiles and the current 
workload to determine the appropriate operating frequency. It then calls 
cpufreq_set() to switch the CPU to the selected profile.

In my presentation at AsiaBSDCon, I proposed moving this functionality 
from user space into the kernel. One limitation of the current design is 
that when a task migrates from one CPU core to another, powerd must 
effectively relearn or re-evaluate the workload characteristics before 
making frequency decisions. The kernel, however, already has direct 
visibility into workload scheduling and task migration events. By 
keeping the policy logic in the kernel, workload information can be 
carried across core migrations, enabling more responsive and efficient 
CPU frequency management.

For the interface,  cpufreq driver exposes min and max performance value 
(can be frequency or an abstracted value like cppc provided) it can 
provides. The kernel detects the work load and set the desired 
performance between min and max performance.

On 6/3/26 02:25, Adrian Chadd wrote:
> .. it sounds like we want to split this up into package versus cluster
> frequency or something?
>
> What's the API look like right now?
>
>
>
> -adrian
>
> On Mon, 1 Jun 2026 at 23:42, ShengYi Hung <aokblast@freebsd.org> wrote:
>> The branch main has been updated by aokblast:
>>
>> URL: https://cgit.FreeBSD.org/src/commit/?id=7b26353a59d66dc1bc611fd042a49b9e3bd13699
>>
>> commit 7b26353a59d66dc1bc611fd042a49b9e3bd13699
>> Author:     ShengYi Hung <aokblast@FreeBSD.org>
>> AuthorDate: 2026-06-01 09:46:37 +0000
>> Commit:     ShengYi Hung <aokblast@FreeBSD.org>
>> CommitDate: 2026-06-02 06:41:41 +0000
>>
>>      hwpstate_intel: Disable package control on hybrid CPU
>>
>>      In package control mode, the performance of all cores depends on the
>>      most recent value written to the request field. If the last write comes
>>      from an E-core, all cores are forced to align with the E-core
>>      performance level, resulting in significant performance degradation.
>>      Therefore, package control is disabled on hybrid-core systems.
>>
>>      Reviewed by:    olce
>>      MFC after:      2 weeks
>>      Sponsored by:   The FreeBSD Foundation
>>      Sponsored by:   Framework Computer Inc
>>      Differential Revision: https://reviews.freebsd.org/D57377
>> ---
>>   sys/x86/cpufreq/hwpstate_intel.c | 21 +++++++++++++++++++++
>>   1 file changed, 21 insertions(+)
>>
>> diff --git a/sys/x86/cpufreq/hwpstate_intel.c b/sys/x86/cpufreq/hwpstate_intel.c
>> index 3d2cc0a5966b..db8600d7b89a 100644
>> --- a/sys/x86/cpufreq/hwpstate_intel.c
>> +++ b/sys/x86/cpufreq/hwpstate_intel.c
>> @@ -321,9 +321,19 @@ out:
>>          return (ret);
>>   }
>>
>> +static void
>> +intel_hwpstate_hybrid_cb(void *ctx)
>> +{
>> +       uint32_t *small_cores = ctx;
>> +
>> +       atomic_add_32(small_cores, PCPU_GET(small_core));
>> +}
>> +
>>   void
>>   intel_hwpstate_identify(driver_t *driver, device_t parent)
>>   {
>> +       uint32_t small_cores = 0;
>> +
>>          if (device_find_child(parent, "hwpstate_intel", DEVICE_UNIT_ANY) != NULL)
>>                  return;
>>
>> @@ -343,6 +353,17 @@ intel_hwpstate_identify(driver_t *driver, device_t parent)
>>          if ((cpu_power_eax & CPUTPM1_HWP) == 0)
>>                  return;
>>
>> +       /*
>> +        * On hybrid-core systems, package-level control cannot be used.
>> +        * It may cause all cores to run at the E-core frequency because
>> +        * the resulting package frequency depends on the last core that
>> +        * sets the frequency.
>> +        */
>> +       smp_rendezvous_cpus(all_cpus, smp_no_rendezvous_barrier,
>> +           intel_hwpstate_hybrid_cb, smp_no_rendezvous_barrier, &small_cores);
>> +       if (small_cores > 0 && small_cores < mp_ncores)
>> +               hwpstate_pkg_ctrl_enable = false;
>> +
>>          if (BUS_ADD_CHILD(parent, 10, "hwpstate_intel", device_get_unit(parent))
>>              == NULL)
>>                  device_printf(parent, "hwpstate_intel: add child failed\n");
>>