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Message-ID: <04f036bf-e863-45c1-87de-7b61f8bc510d@kernel.org>
Date: Wed, 12 Feb 2025 16:05:22 -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 2/11/2025 23:15, Dhananjay Ugwekar wrote:
> 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@....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.
>
> 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?
>
You're right; I can't really think of one either. Let me take out the
private lock for the per-cpu device and I'll just overhaul the global
lock locations.
>>
>>>
>>> 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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