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

[Beata Michalska <beata.michalska@arm.com>]

On Thu, Oct 03, 2024 at 02:54:22PM -0700, Vanshidhar Konda wrote:
> 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@arm.com>
> > > > > > Signed-off-by: Beata Michalska <beata.michalska@arm.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.
>
That is a fair point but I am not entirely convinced using '0' instead makes
things any more clearer as this is in no way a valid CPU frequency.
As long as we document the expected behaviour keeping the interface well
defined,  both options should be fine I guess.

@Viresh: what is your opinion on that one ?

---
BR
Beata
> 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
> > > > > >