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Message-ID: <bfcafbaf-c407-412b-a5e4-d152e2a565d7@kernel.org>
Date: Tue, 11 Feb 2025 15:54:53 -0600
From: Mario Limonciello <superm1@...nel.org>
To: Dhananjay Ugwekar <Dhananjay.Ugwekar@....com>,
"Gautham R . Shenoy" <gautham.shenoy@....com>,
Perry Yuan <perry.yuan@....com>
Cc: "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<linux-kernel@...r.kernel.org>,
"open list:CPU FREQUENCY SCALING FRAMEWORK" <linux-pm@...r.kernel.org>,
Mario Limonciello <mario.limonciello@....com>
Subject: Re: [PATCH 04/14] cpufreq/amd-pstate: Overhaul locking
On 2/10/2025 23:02, Dhananjay Ugwekar wrote:
> On 2/7/2025 3:26 AM, Mario Limonciello wrote:
>> From: Mario Limonciello <mario.limonciello@....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@....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;
>
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