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

[Mario Limonciello <superm1@kernel.org>]

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.

> 
> 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;
>