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

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Date:   Thu, 26 Oct 2023 09:53:21 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Zeng Heng <zengheng4@...wei.com>
Cc:     <broonie@...nel.org>, <joey.gouly@....com>, <will@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>, <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

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