Re: [PATCH 04/14] cpufreq/amd-pstate: Overhaul locking

[Dhananjay Ugwekar <Dhananjay.Ugwekar@amd.com>]

On 2/12/2025 3:24 AM, Mario Limonciello wrote:
> On 2/10/2025 23:02, Dhananjay Ugwekar wrote:
>> On 2/7/2025 3:26 AM, Mario Limonciello wrote:
>>> From: Mario Limonciello <mario.limonciello@amd.com>
>>>
>>> amd_pstate_cpu_boost_update() and refresh_frequency_limits() both
>>> update the policy state and have nothing to do with the amd-pstate
>>> driver itself.
>>>
>>> A global "limits" lock doesn't make sense because each CPU can have
>>> policies changed independently.  Instead introduce locks into to the
>>> cpudata structure and lock each CPU independently.
>>>
>>> The remaining "global" driver lock is used to ensure that only one
>>> entity can change driver modes at a given time.
>>>
>>> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
>>> ---
>>>   drivers/cpufreq/amd-pstate.c | 27 +++++++++++++++++----------
>>>   drivers/cpufreq/amd-pstate.h |  2 ++
>>>   2 files changed, 19 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
>>> index 77bc6418731ee..dd230ed3b9579 100644
>>> --- a/drivers/cpufreq/amd-pstate.c
>>> +++ b/drivers/cpufreq/amd-pstate.c
>>> @@ -196,7 +196,6 @@ static inline int get_mode_idx_from_str(const char *str, size_t size)
>>>       return -EINVAL;
>>>   }
>>>   -static DEFINE_MUTEX(amd_pstate_limits_lock);
>>>   static DEFINE_MUTEX(amd_pstate_driver_lock);
>>>     static u8 msr_get_epp(struct amd_cpudata *cpudata)
>>> @@ -283,6 +282,8 @@ static int msr_set_epp(struct amd_cpudata *cpudata, u8 epp)
>>>       u64 value, prev;
>>>       int ret;
>>>   +    lockdep_assert_held(&cpudata->lock);
>>
>> After making the perf_cached variable writes atomic, do we still need a cpudata->lock ?
> 
> My concern was specifically that userspace could interact with multiple sysfs files that influence the atomic perf variable (and the HW) at the same time.  So you would not have a deterministic behavior if they raced.  But if you take the mutex on all the paths that this could happen it will be a FIFO.

I guess, the lock still wont guarantee the ordering right? It will just ensure that one thread executes 
that code path for a specific CPU at a time. And do we even care about the ordering ? I'm having a hard 
time thinking of a scenario where we'll need the lock. Can you or Gautham think of any such scenario? 

> 
>>
>> Regards,
>> Dhananjay
>>
>>> +
>>>       value = prev = READ_ONCE(cpudata->cppc_req_cached);
>>>       value &= ~AMD_CPPC_EPP_PERF_MASK;
>>>       value |= FIELD_PREP(AMD_CPPC_EPP_PERF_MASK, epp);
>>> @@ -315,6 +316,8 @@ static int shmem_set_epp(struct amd_cpudata *cpudata, u8 epp)
>>>       int ret;
>>>       struct cppc_perf_ctrls perf_ctrls;
>>>   +    lockdep_assert_held(&cpudata->lock);
>>> +
>>>       if (epp == cpudata->epp_cached)
>>>           return 0;
>>>   @@ -335,6 +338,8 @@ static int amd_pstate_set_energy_pref_index(struct cpufreq_policy *policy,
>>>       struct amd_cpudata *cpudata = policy->driver_data;
>>>       u8 epp;
>>>   +    guard(mutex)(&cpudata->lock);
>>> +
>>>       if (!pref_index)
>>>           epp = cpudata->epp_default;
>>>       else
>>> @@ -750,7 +755,6 @@ static int amd_pstate_set_boost(struct cpufreq_policy *policy, int state)
>>>           pr_err("Boost mode is not supported by this processor or SBIOS\n");
>>>           return -EOPNOTSUPP;
>>>       }
>>> -    guard(mutex)(&amd_pstate_driver_lock);
>>>         ret = amd_pstate_cpu_boost_update(policy, state);
>>>       refresh_frequency_limits(policy);
>>> @@ -973,6 +977,9 @@ static int amd_pstate_cpu_init(struct cpufreq_policy *policy)
>>>         cpudata->cpu = policy->cpu;
>>>   +    mutex_init(&cpudata->lock);
>>> +    guard(mutex)(&cpudata->lock);
>>> +
>>>       ret = amd_pstate_init_perf(cpudata);
>>>       if (ret)
>>>           goto free_cpudata1;
>>> @@ -1179,8 +1186,6 @@ static ssize_t store_energy_performance_preference(
>>>       if (ret < 0)
>>>           return -EINVAL;
>>>   -    guard(mutex)(&amd_pstate_limits_lock);
>>> -
>>>       ret = amd_pstate_set_energy_pref_index(policy, ret);
>>>         return ret ? ret : count;
>>> @@ -1353,8 +1358,10 @@ int amd_pstate_update_status(const char *buf, size_t size)
>>>       if (mode_idx < 0 || mode_idx >= AMD_PSTATE_MAX)
>>>           return -EINVAL;
>>>   -    if (mode_state_machine[cppc_state][mode_idx])
>>> +    if (mode_state_machine[cppc_state][mode_idx]) {
>>> +        guard(mutex)(&amd_pstate_driver_lock);
>>>           return mode_state_machine[cppc_state][mode_idx](mode_idx);
>>> +    }
>>>         return 0;
>>>   }
>>> @@ -1375,7 +1382,6 @@ static ssize_t status_store(struct device *a, struct device_attribute *b,
>>>       char *p = memchr(buf, '\n', count);
>>>       int ret;
>>>   -    guard(mutex)(&amd_pstate_driver_lock);
>>>       ret = amd_pstate_update_status(buf, p ? p - buf : count);
>>>         return ret < 0 ? ret : count;
>>> @@ -1472,6 +1478,9 @@ static int amd_pstate_epp_cpu_init(struct cpufreq_policy *policy)
>>>         cpudata->cpu = policy->cpu;
>>>   +    mutex_init(&cpudata->lock);
>>> +    guard(mutex)(&cpudata->lock);
>>> +
>>>       ret = amd_pstate_init_perf(cpudata);
>>>       if (ret)
>>>           goto free_cpudata1;
>>> @@ -1558,6 +1567,8 @@ static int amd_pstate_epp_update_limit(struct cpufreq_policy *policy)
>>>       union perf_cached perf;
>>>       u8 epp;
>>>   +    guard(mutex)(&cpudata->lock);
>>> +
>>>       amd_pstate_update_min_max_limit(policy);
>>>         if (cpudata->policy == CPUFREQ_POLICY_PERFORMANCE)
>>> @@ -1646,8 +1657,6 @@ static int amd_pstate_epp_cpu_offline(struct cpufreq_policy *policy)
>>>       if (cpudata->suspended)
>>>           return 0;
>>>   -    guard(mutex)(&amd_pstate_limits_lock);
>>> -
>>>       if (trace_amd_pstate_epp_perf_enabled()) {
>>>           trace_amd_pstate_epp_perf(cpudata->cpu, perf.highest_perf,
>>>                         AMD_CPPC_EPP_BALANCE_POWERSAVE,
>>> @@ -1684,8 +1693,6 @@ static int amd_pstate_epp_resume(struct cpufreq_policy *policy)
>>>       struct amd_cpudata *cpudata = policy->driver_data;
>>>         if (cpudata->suspended) {
>>> -        guard(mutex)(&amd_pstate_limits_lock);
>>> -
>>>           /* enable amd pstate from suspend state*/
>>>           amd_pstate_epp_reenable(policy);
>>>   diff --git a/drivers/cpufreq/amd-pstate.h b/drivers/cpufreq/amd-pstate.h
>>> index a140704b97430..6d776c3e5712a 100644
>>> --- a/drivers/cpufreq/amd-pstate.h
>>> +++ b/drivers/cpufreq/amd-pstate.h
>>> @@ -96,6 +96,8 @@ struct amd_cpudata {
>>>       bool    boost_supported;
>>>       bool    hw_prefcore;
>>>   +    struct mutex    lock;
>>> +
>>>       /* EPP feature related attributes*/
>>>       u8    epp_cached;
>>>       u32    policy;
>>
>