linux-kernel - Re: [PATCH] cpufreq: intel_pstate: Enable asym capacity only when cpu smt is not possible

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Message-ID: <2fe6e22c-aa8d-4514-9f10-91265facedd4@kylinos.cn>
Date: Thu, 29 Jan 2026 09:21:31 +0800
From: Yaxiong Tian <tianyaxiong@...inos.cn>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: srinivas.pandruvada@...ux.intel.com, lenb@...nel.org,
 viresh.kumar@...aro.org, ricardo.neri-calderon@...ux.intel.com,
 linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: intel_pstate: Enable asym capacity only when cpu
 smt is not possible


在 2026/1/29 05:21, Rafael J. Wysocki 写道:
> On Wed, Jan 28, 2026 at 4:15 AM Yaxiong Tian <tianyaxiong@...inos.cn> wrote:
>> According to the description in the intel_pstate.rst documentation,
>> Capacity-Aware Scheduling and Energy-Aware Scheduling are only
>> supported on a hybrid processor without SMT. Previously, the system
>> used sched_smt_active() for judgment, which is not a strict condition
>> because users can switch it on or off via /sys at any time.
>>
>> This could lead to incorrect driver settings in certain scenarios.
>>   For example, on a CPU that supports SMT, a user can disable SMT
>> via the nosmt parameter to enable asym capacity, and then re-enable
>> SMT via /sys. In such cases, some settings in the driver would no
>> longer be correct.
>>
>> To address this issue, replace sched_smt_active() with cpu_smt_possible(),
>> and only enable asym capacity when cpu smt is not possible.
>>
>> Fixes: 929ebc93ccaa ("cpufreq: intel_pstate: Set asymmetric CPU capacity on hybrid systems")
>> Signed-off-by: Yaxiong Tian <tianyaxiong@...inos.cn>
>> ---
>>   drivers/cpufreq/intel_pstate.c | 15 +++++++--------
>>   1 file changed, 7 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
>> index ec4abe374573..8105c41861a3 100644
>> --- a/drivers/cpufreq/intel_pstate.c
>> +++ b/drivers/cpufreq/intel_pstate.c
>> @@ -1142,8 +1142,11 @@ static void hybrid_refresh_cpu_capacity_scaling(void)
>>
>>   static void hybrid_init_cpu_capacity_scaling(bool refresh)
>>   {
>> -       /* Bail out if enabling capacity-aware scheduling is prohibited. */
>> -       if (no_cas)
>> +       /*
>> +        * Bail out if capacity-aware scheduling is prohibited, or if SMT is
>> +        * possible, as the capacity of SMT threads cannot be determined reliably.
>> +        */
>> +       if (no_cas || cpu_smt_possible())
>>                  return;
>>
>>          /*
>> @@ -1156,12 +1159,8 @@ static void hybrid_init_cpu_capacity_scaling(bool refresh)
>>                  return;
>>          }
>>
>> -       /*
>> -        * On hybrid systems, use asym capacity instead of ITMT, but because
>> -        * the capacity of SMT threads is not deterministic even approximately,
>> -        * do not do that when SMT is in use.
>> -        */
>> -       if (hwp_is_hybrid && !sched_smt_active() && arch_enable_hybrid_capacity_scale()) {
> Why don't you replace sched_smt_active() here with cpu_smt_possible()?
>
> There's no point calling arch_enable_hybrid_capacity_scale() if the
> latter is true.
Because I think cpu_smt_possible() becomes fixed after kernel startup. 
Placing this check earlier can also prevent calling 
arch_enable_hybrid_capacity_scale(). Additionally, when users switch the 
cpufreq driver via /sys/devices/system/cpu/intel_pstate/status, it's 
unnecessary to proceed with further checks.
>> +       /* On hybrid systems, use asym capacity instead of ITMT */
>> +       if (hwp_is_hybrid && arch_enable_hybrid_capacity_scale()) {
>>                  hybrid_refresh_cpu_capacity_scaling();
>>                  /*
>>                   * Disabling ITMT causes sched domains to be rebuilt to disable asym
>> --