Date: Sat, 27 Apr 2024 15:24:40 -0500
From: Mario Limonciello <mario.limonciello@....com>
To: Perry Yuan <perry.yuan@....com>, rafael.j.wysocki@...el.com,
 viresh.kumar@...aro.org, Ray.Huang@....com, gautham.shenoy@....com,
 Borislav.Petkov@....com
Cc: Alexander.Deucher@....com, Xinmei.Huang@....com, Xiaojian.Du@....com,
 Li.Meng@....com, linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 3/6] cpufreq: amd-pstate: implement cpb_boost sysfs
 entry for boost control

On 4/26/2024 09:07, Mario Limonciello wrote:
> On 4/26/2024 01:34, Perry Yuan wrote:
>> From: Perry Yuan <Perry.Yuan@....com>
>>
>> With this new sysfs entry `cpb_boost`created, user can change CPU boost
>> state dynamically under `active`, `guided` and `passive` modes.
>> And the highest perf and frequency will also be updated as the boost
>> state changing.
>>
>> 0): check current boost state
>> cat /sys/devices/system/cpu/amd_pstate/cpb_boost
>>
>> 1): disable CPU boost
>> sudo bash -c "echo 0 > /sys/devices/system/cpu/amd_pstate/cpb_boost"
>>
>> 2): enable CPU boost
>> sudo bash -c "echo 1 > /sys/devices/system/cpu/amd_pstate/cpb_boost"
>>
>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217931
>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217618
>> Signed-off-by: Perry Yuan <Perry.Yuan@....com>
>> ---
>>   drivers/cpufreq/amd-pstate.c | 99 ++++++++++++++++++++++++++++++++++++
>>   1 file changed, 99 insertions(+)
>>
>> diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
>> index aa17a3134497..53ef39c6dbfa 100644
>> --- a/drivers/cpufreq/amd-pstate.c
>> +++ b/drivers/cpufreq/amd-pstate.c
>> @@ -1295,6 +1295,103 @@ static ssize_t prefcore_show(struct device *dev,
>>       return sysfs_emit(buf, "%s\n", 
>> str_enabled_disabled(amd_pstate_prefcore));
>>   }
>> +static int amd_cpu_boost_update(struct amd_cpudata *cpudata, u32 on)
>> +{
>> +    struct cpufreq_policy *policy = cpufreq_cpu_acquire(cpudata->cpu);
>> +    struct cppc_perf_ctrls perf_ctrls;
>> +    u32 highest_perf, nominal_perf, nominal_freq, max_freq;
>> +    int ret;
>> +
>> +    if (!policy)
>> +        return -ENODATA;
>> +
>> +    highest_perf = READ_ONCE(cpudata->highest_perf);
>> +    nominal_perf = READ_ONCE(cpudata->nominal_perf);
>> +    nominal_freq = READ_ONCE(cpudata->nominal_freq);
>> +    max_freq = READ_ONCE(cpudata->max_freq);
>> +
>> +    if (boot_cpu_has(X86_FEATURE_CPPC)) {
>> +        u64 value = READ_ONCE(cpudata->cppc_req_cached);
>> +
>> +        value &= ~GENMASK_ULL(7, 0);
>> +        value |= on ? highest_perf : nominal_perf;
>> +        WRITE_ONCE(cpudata->cppc_req_cached, value);
>> +
>> +        wrmsrl_on_cpu(cpudata->cpu, MSR_AMD_CPPC_REQ, value);
>> +    } else {
>> +        perf_ctrls.max_perf = on ? highest_perf : nominal_perf;
>> +        ret = cppc_set_epp_perf(cpudata->cpu, &perf_ctrls, 1);
>> +        if (ret) {
>> +            cpufreq_cpu_release(policy);
>> +            pr_debug("failed to set energy perf value (%d)\n", ret);
>> +            return ret;
>> +        }
>> +    }
>> +
>> +    if (on)
>> +        policy->cpuinfo.max_freq = max_freq;
>> +    else
>> +        policy->cpuinfo.max_freq = nominal_freq * 1000;
>> +
>> +    policy->max = policy->cpuinfo.max_freq;
>> +
>> +    if (cppc_state == AMD_PSTATE_PASSIVE) {
>> +        ret = freq_qos_update_request(&cpudata->req[1],
>> +                      policy->cpuinfo.max_freq);
>> +    }
>> +
>> +    cpufreq_cpu_release(policy);
>> +
>> +    return ret;
>> +}
>> +
>> +static ssize_t cpb_boost_show(struct device *dev,
>> +               struct device_attribute *attr, char *buf)
>> +{
>> +    return sysfs_emit(buf, "%u\n", amd_pstate_global_params.cpb_boost);
>> +}
>> +
>> +static ssize_t cpb_boost_store(struct device *dev, struct 
>> device_attribute *b,
>> +                const char *buf, size_t count)
>> +{
>> +    bool new_state;
>> +    ssize_t ret;
>> +    int cpu;
>> +
>> +    if (!amd_pstate_global_params.cpb_supported) {
>> +        pr_err("Boost mode is not supported by this processor or 
>> SBIOS\n");
>> +        return -EINVAL;
>> +    }
> 
> As this is information that will be known when you first probe, I feel 
> it would be better to affect the visibility of the file than let a user 
> discover it doesn't work when they try to write it.
> 
> Thinking down the road software like power-profiles-daemon and tuned 
> will probably adapt to this new file and if their contract is that the 
> file exists means they should write it that's going to turn into 
> needless errors in any such system's log on every boot.
> 
>> +
>> +    ret = kstrtobool(buf, &new_state);
>> +    if (ret)
>> +        return ret;
>> +
>> +    mutex_lock(&amd_pstate_driver_lock);
>> +    amd_pstate_global_params.cpb_boost = !!new_state;
>> +
>> +    for_each_present_cpu(cpu) {
> 
> It seems to me that by making an attribute for every single CPU this is 
> wrong.  It means that writing boost for one CPU's files applies to all 
> other CPUs too.
> 
> If it's going to be a global attribute that is shared by all CPUs it 
> should be a single file.
> 
> Otherwise this is going to be a problem if (for example) software tries 
> to turn on boost for only 1 CPU.  Think of this sequence:
> 
> 1) 16 CPUs, cpb_boost is currently turned off.
> 2) Software tries to turn CPB boost on for the "first" CPU only.
> 3) It reads the value of the first CPU and sees it's 0.  So It changes 
> the value for the first CPU (which causes a global change).
> 4) It reads the value for the second CPU and sees it's 1.  It tries to 
> change this back to zero, which again causes a global change.
> 5) It checks all the others and they're all 0 and it does nothing.
> 
> So you see by having a global attribute which is shared with every 
> single CPU you now have a flow problem that userspace can introduce.
> 

Hi,

Sorry, I can see now that it current IS a global attribute in this 
implementation (IE it's DEVICE_ATTR_RW not cpufreq_freq_attr_rw).

However as you're already actually applying it to each CPU individually, 
I do think it is better to make it a per CPU attribute.

There could be cases that software only wants some of the cores to be 
boosted, so could you please instead consider to convert to use 
cpufreq_freq_attr_rw for the attribute and only apply one CPU at a time?

Thanks!

>> +
>> +        struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
>> +        struct amd_cpudata *cpudata = policy->driver_data;
>> +
>> +        if (!cpudata) {
>> +            pr_err("cpudata is NULL\n");
>> +            ret = -ENODATA;
>> +            cpufreq_cpu_put(policy);
>> +            goto err_exit;
>> +        }
>> +
>> +        amd_cpu_boost_update(cpudata, 
>> amd_pstate_global_params.cpb_boost);
>> +        refresh_frequency_limits(policy);
>> +        cpufreq_cpu_put(policy);
>> +    }
>> +
>> +err_exit:
>> +    mutex_unlock(&amd_pstate_driver_lock);
>> +    return ret < 0 ? ret : count;
>> +}
>> +
>>   cpufreq_freq_attr_ro(amd_pstate_max_freq);
>>   cpufreq_freq_attr_ro(amd_pstate_lowest_nonlinear_freq);
>> @@ -1305,6 +1402,7 @@ 
>> cpufreq_freq_attr_rw(energy_performance_preference);
>>   cpufreq_freq_attr_ro(energy_performance_available_preferences);
>>   static DEVICE_ATTR_RW(status);
>>   static DEVICE_ATTR_RO(prefcore);
>> +static DEVICE_ATTR_RW(cpb_boost);
>>   static struct freq_attr *amd_pstate_attr[] = {
>>       &amd_pstate_max_freq,
>> @@ -1329,6 +1427,7 @@ static struct freq_attr *amd_pstate_epp_attr[] = {
>>   static struct attribute *pstate_global_attributes[] = {
>>       &dev_attr_status.attr,
>>       &dev_attr_prefcore.attr,
>> +    &dev_attr_cpb_boost.attr,
>>       NULL
>>   };
> 

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