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

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Message-ID: <438e230d-5169-32b3-3992-9fef5ba71f05@huawei.com>
Date:   Thu, 26 Oct 2023 17:05:31 +0800
From:   Zeng Heng <zengheng4@...wei.com>
To:     Sudeep Holla <sudeep.holla@....com>
CC:     <broonie@...nel.org>, <joey.gouly@....com>, <will@...nel.org>,
        <amit.kachhap@....com>, <rafael@...nel.org>,
        <catalin.marinas@....com>, <james.morse@....com>,
        <mark.rutland@....com>, <maz@...nel.org>,
        <viresh.kumar@...aro.org>, <sumitg@...dia.com>,
        <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>
Subject: Re: [PATCH 2/3] cpufreq: CPPC: Keep the target core awake when
 reading its cpufreq rate


在 2023/10/26 16:53, Sudeep Holla 写道:
> On Thu, Oct 26, 2023 at 10:24:54AM +0800, Zeng Heng wrote:
>> 在 2023/10/25 19:13, Sudeep Holla 写道:
>>> 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.
>>>>
>>>> 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
>>> [...]
>>>
>>>> @@ -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))
>>> Have you compiled this on x86 ? Even if you have somehow managed to,
>>> this is not the right place to check the presence of AMU feature on
>>> the CPU.
>>> If AMU registers are used in CPPC, they must be using FFH GAS, in which
>>> case the interpretation of FFH is architecture dependent code.
>> According to drivers/cpufreq/Makefile, cppc_cpufreq.c is only compiled with
>> ARM architecture.
>>
> Well that's true but this change doesn't belong to cppc_cpufreq.c, it must
> be part of drivers/acpi/cppc_acpi.c IMO and sorry I assumed that without
> explicitly mentioning that here.
>
>> But here, I would change cpu_has_amu_feat() with cpc_ffh_supported(), which
>> belongs to FFH APIs.
>>
> It is not like that. cppc_acpi.c will know the GAS is FFH based so no need to
> check anything there. I see counters_read_on_cpu() called from cpc_ffh_read()
> already takes care of reading the AMUs on the right CPU. What exactly is
> the issue you are seeing ? I don't if this change is needed at all.
>
> --
> Regards,
> Sudeep


In this scenario, both topology.c and cppc_acpi.c do not provide an API 
to keep the AMU online

during the whole sampling period. Just using cpc_read_ffh() at the start 
and end of the sampling

period is not enough.


Zeng Heng