linux-kernel - Re: [PATCH 2/3] cpufreq: CPPC: Keep the target core awake when reading its cpufreq rate

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Message-ID: <28a6e60c-4492-105b-5fcf-3129ca868349@nvidia.com>
Date:   Wed, 25 Oct 2023 20:27:23 +0530
From:   Sumit Gupta <sumitg@...dia.com>
To:     Zeng Heng <zengheng4@...wei.com>,
        Beata Michalska <beata.michalska@....com>
CC:     <broonie@...nel.org>, <joey.gouly@....com>, <will@...nel.org>,
        <amit.kachhap@....com>, <rafael@...nel.org>,
        <catalin.marinas@....com>, <james.morse@....com>, <maz@...nel.org>,
        <viresh.kumar@...aro.org>, <yang@...amperecomputing.com>,
        <linux-kernel@...r.kernel.org>, <linux-pm@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <wangxiongfeng2@...wei.com>, <xiexiuqi@...wei.com>,
        Ionela Voinescu <ionela.voinescu@....com>,
        linux-tegra <linux-tegra@...r.kernel.org>,
        "Mark Rutland" <mark.rutland@....com>,
        Sumit Gupta <sumitg@...dia.com>
Subject: Re: [PATCH 2/3] cpufreq: CPPC: Keep the target core awake when
 reading its cpufreq rate




> [adding Ionela]
> 
> On Wed, Oct 25, 2023 at 05:38:46PM +0800, Zeng Heng wrote:
>> As ARM AMU's document says, all counters are subject to any changes
>> in clock frequency, including clock stopping caused by the WFI and WFE
>> instructions.
>>
>> Therefore, using smp_call_on_cpu() to trigger target CPU to
>> read self's AMU counters, which ensures the counters are working
>> properly while cstate feature is enabled.
> 
> IIUC there's a pretty deliberate split with all the actual reading of the AMU
> living in arch/arm64/kernel/topolgy.c, and the driver code being (relatively)
> generic.
> 
> We already have code in arch/arm64/kernel/topolgy.c to read counters on a
> specific CPU; why can't e reuse that (and avoid exporting cpu_has_amu_feat())?


This patch seems mostly based on my previous patch [1] and discussed 
here [2] already. Beata [CCed] shared an alternate approach [3] 
leveraging existing code from 'topology.c' to get the average freq for 
last tick period.


Beata,

Could you share v2 of [3] with the request to merge. We can try to solve 
the issue with CPU IDLE case later on top?

Additionally, also please include the fix in [4] if it looks fine.

Best Regards,
Sumit Gupta

[1] https://lore.kernel.org/all/20230418113459.12860-7-sumitg@nvidia.com/
[2] 
https://lore.kernel.org/lkml/cde1d8a9-3a21-e82b-7895-40603a14d898@nvidia.com/T/#m2174305de4706006e0bd9c103a0e5ff61cea7a12
[3] 
https://lore.kernel.org/lkml/20230606155754.245998-1-beata.michalska@arm.com/
[4] 
https://lore.kernel.org/lkml/6a5710f6-bfbb-5dfd-11cd-0cd02220cee7@nvidia.com/


>>
>> Reported-by: Sumit Gupta <sumitg@...dia.com>
>> Link: https://lore.kernel.org/all/20230418113459.12860-7-sumitg@nvidia.com/
>> Signed-off-by: Zeng Heng <zengheng4@...wei.com>
>> ---
>>   drivers/cpufreq/cppc_cpufreq.c | 39 ++++++++++++++++++++++++++--------
>>   1 file changed, 30 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
>> index fe08ca419b3d..321a9dc9484d 100644
>> --- a/drivers/cpufreq/cppc_cpufreq.c
>> +++ b/drivers/cpufreq/cppc_cpufreq.c
>> @@ -90,6 +90,12 @@ static int cppc_perf_from_fbctrs(struct cppc_cpudata *cpu_data,
>>                                 struct cppc_perf_fb_ctrs *fb_ctrs_t0,
>>                                 struct cppc_perf_fb_ctrs *fb_ctrs_t1);
>>
>> +struct fb_ctr_pair {
>> +     u32 cpu;
>> +     struct cppc_perf_fb_ctrs fb_ctrs_t0;
>> +     struct cppc_perf_fb_ctrs fb_ctrs_t1;
>> +};
>> +
>>   /**
>>    * cppc_scale_freq_workfn - CPPC arch_freq_scale updater for frequency invariance
>>    * @work: The work item.
>> @@ -840,9 +846,24 @@ static int cppc_perf_from_fbctrs(struct cppc_cpudata *cpu_data,
>>        return (reference_perf * delta_delivered) / delta_reference;
>>   }
>>
>> +static int cppc_get_perf_ctrs_pair(void *val)
>> +{
>> +     struct fb_ctr_pair *fb_ctrs = val;
>> +     int cpu = fb_ctrs->cpu;
>> +     int ret;
>> +
>> +     ret = cppc_get_perf_ctrs(cpu, &fb_ctrs->fb_ctrs_t0);
>> +     if (ret)
>> +             return ret;
>> +
>> +     udelay(2); /* 2usec delay between sampling */
>> +
>> +     return cppc_get_perf_ctrs(cpu, &fb_ctrs->fb_ctrs_t1);
>> +}
>> +
>>   static unsigned int cppc_cpufreq_get_rate(unsigned int cpu)
>>   {
>> -     struct cppc_perf_fb_ctrs fb_ctrs_t0 = {0}, fb_ctrs_t1 = {0};
>> +     struct fb_ctr_pair fb_ctrs = { .cpu = cpu, };
>>        struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
>>        struct cppc_cpudata *cpu_data = policy->driver_data;
>>        u64 delivered_perf;
>> @@ -850,18 +871,18 @@ static unsigned int cppc_cpufreq_get_rate(unsigned int cpu)
>>
>>        cpufreq_cpu_put(policy);
>>
>> -     ret = cppc_get_perf_ctrs(cpu, &fb_ctrs_t0);
>> -     if (ret)
>> -             return 0;
>> -
>> -     udelay(2); /* 2usec delay between sampling */
>> +     if (cpu_has_amu_feat(cpu))
>> +             ret = smp_call_on_cpu(cpu, cppc_get_perf_ctrs_pair,
>> +                                   &fb_ctrs, false);
>> +     else
>> +             ret = cppc_get_perf_ctrs_pair(&fb_ctrs);
>>
>> -     ret = cppc_get_perf_ctrs(cpu, &fb_ctrs_t1);
>>        if (ret)
>>                return 0;
>>
>> -     delivered_perf = cppc_perf_from_fbctrs(cpu_data, &fb_ctrs_t0,
>> -                                            &fb_ctrs_t1);
>> +     delivered_perf = cppc_perf_from_fbctrs(cpu_data,
>> +                                           &fb_ctrs.fb_ctrs_t0,
>> +                                           &fb_ctrs.fb_ctrs_t1);
>>
>>        return cppc_cpufreq_perf_to_khz(cpu_data, delivered_perf);
>>   }
>> --
>> 2.25.1
>>