linux-kernel - Re: [PATCH v7 3/4] arm64: Provide an AMU-based version of arch_freq_avg_get_on

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Message-ID: <5y3yz2ct2o42c53dc6rwpse3andstjx74lowt2b3hohj4ogbct@nu2szdnxvxid>
Date: Thu, 3 Oct 2024 14:54:22 -0700
From: Vanshidhar Konda <vanshikonda@...amperecomputing.com>
To: Beata Michalska <beata.michalska@....com>
Cc: Sumit Gupta <sumitg@...dia.com>, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-pm@...r.kernel.org, ionela.voinescu@....com, 
	sudeep.holla@....com, will@...nel.org, catalin.marinas@....com, rafael@...nel.org, 
	viresh.kumar@...aro.org, yang@...amperecomputing.com, lihuisong@...wei.com, 
	zhanjie9@...ilicon.com, linux-tegra <linux-tegra@...r.kernel.org>, 
	Bibek Basu <bbasu@...dia.com>
Subject: Re: [PATCH v7 3/4] arm64: Provide an AMU-based version of
 arch_freq_avg_get_on_cpu

On Thu, Oct 03, 2024 at 11:39:54PM GMT, Beata Michalska wrote:
>On Thu, Sep 26, 2024 at 04:21:14PM -0700, Vanshidhar Konda wrote:
>> On Thu, Sep 26, 2024 at 12:34:01PM GMT, Beata Michalska wrote:
>> > On Tue, Sep 17, 2024 at 05:41:09PM +0530, Sumit Gupta wrote:
>> > > Hi Beata,
>> > Hi Sumit,
>> > >
>> > > Thank you for the patches.
>> > Thank you for having a look at those.
>> > >
>> > > On 13/09/24 18:59, Beata Michalska wrote:
>> > > > External email: Use caution opening links or attachments
>> > > >
>> > > >
>> > > > With the Frequency Invariance Engine (FIE) being already wired up with
>> > > > sched tick and making use of relevant (core counter and constant
>> > > > counter) AMU counters, getting the average frequency for a given CPU,
>> > > > can be achieved by utilizing the frequency scale factor which reflects
>> > > > an average CPU frequency for the last tick period length.
>> > > >
>> > > > The solution is partially based on APERF/MPERF implementation of
>> > > > arch_freq_get_on_cpu.
>> > > >
>> > > > Suggested-by: Ionela Voinescu <ionela.voinescu@....com>
>> > > > Signed-off-by: Beata Michalska <beata.michalska@....com>
>> > > > ---
>> > > >   arch/arm64/kernel/topology.c | 109 +++++++++++++++++++++++++++++++----
>> > > >   1 file changed, 99 insertions(+), 10 deletions(-)
>> > > >

--- snip ----

>> > >
>> > >     ..
>> > >   freq_comput:
>> > >     scale = arch_scale_freq_capacity(cpu);
>> > >     freq = scale * arch_scale_freq_ref(cpu);
>> > >   ----
>> > >
>> > This boils down to the question what that function, and the information it
>> > provides, represent really. The 'unknown' here simply says the CPU has been idle
>> > for a while and as such the frequency data is a bit stale and it does not
>> > represent the average freq the CPU is actually running at anymore, which is
>> > the intention here really. Or, that the given CPU is a non-housekeeping one.
>> > Either way I believe this is a useful information, instead of providing
>> > stale data with no indication on whether the frequency is really the 'current'
>> > one or not.
>> >
>> > If that is somehow undesirable we can discuss this further, though I'd rather
>> > avoid exposing an interface where the feedback provided is open to
>> > interpretation at all times.
>>
>> Would it make sense to identify that the frequency reporting is unknown due to
>> cpu being idle vs some other issue like being a non-housekeeping CPU? Would
>> returning a value of 0 make it easier for tools to represent that the CPU is
>> currently idle?
>That is an option.
>Another one would be to return an error for those cases. This would make it
>easier to distinguish between valid frequency &/| idle CPU vs tickless CPU
>(EINVAL vs ENOENT) ?
>

That seems like a good idea but I suspect it would be confusing to the end user.

If a user runs `cat /sys/devices/system/cpu/cpu2/cpuinfo_avg_freq` they would
get an error in some cases or get a number in some other iterations.

Thanks,
Vanshidhar

>---
>BR
>Beata
>>
>> Thanks,
>> Vanshidhar
>>
>> >
>> > ---
>> > Best Regards
>> > Beata
>> > > Thank you,
>> > > Sumit Gupta
>> > >
>> > > P.S. Will be on afk for next 2 weeks with no access to email. Please expect
>> > > a delay in response.
>> > >
>> > > > +               cpu = ref_cpu;
>> > > > +               goto retry;
>> > > > +       }
>> > > > +       /*
>> > > > +        * Reversed computation to the one used to determine
>> > > > +        * the arch_freq_scale value
>> > > > +        * (see amu_scale_freq_tick for details)
>> > > > +        */
>> > > > +       scale = arch_scale_freq_capacity(cpu);
>> > > > +       freq = scale * arch_scale_freq_ref(cpu);
>> > > > +       freq >>= SCHED_CAPACITY_SHIFT;
>> > > > +       return freq;
>> > > > +}
>> > > > +
>> > >
>> > > >   static void amu_fie_setup(const struct cpumask *cpus)
>> > > >   {
>> > > >          int cpu;
>> > > > --
>> > > > 2.25.1
>> > > >