linux-kernel - Re: [PATCH v7 1/4] cpufreq: Introduce an optional cpuinfo_avg

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Message-ID: <CAJZ5v0gCRKzaFrwkoBpLHQUxoP_+jAyhMiCkLQaBUpduk9yxtA@mail.gmail.com>
Date: Mon, 4 Nov 2024 14:26:04 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Beata Michalska <beata.michalska@....com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Viresh Kumar <viresh.kumar@...aro.org>, 
	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, sumitg@...dia.com, 
	yang@...amperecomputing.com, vanshikonda@...amperecomputing.com, 
	lihuisong@...wei.com, zhanjie9@...ilicon.com
Subject: Re: [PATCH v7 1/4] cpufreq: Introduce an optional cpuinfo_avg_freq
 sysfs entry

On Mon, Nov 4, 2024 at 9:01 AM Beata Michalska <beata.michalska@....com> wrote:
>
> On Tue, Oct 29, 2024 at 12:31:11PM +0100, Rafael J. Wysocki wrote:
> > On Tue, Oct 29, 2024 at 8:04 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > >
> > > Apologies for the delay from my side. September was mostly holidays
> > > for me and then I was stuck with other stuff plus email backlog and
> > > this series was always a painful point to return to :(
> > >
> > > On 13-09-24, 14:29, Beata Michalska wrote:
> > > > Currently the CPUFreq core exposes two sysfs attributes that can be used
> > > > to query current frequency of a given CPU(s): namely cpuinfo_cur_freq
> > > > and scaling_cur_freq. Both provide slightly different view on the
> > > > subject and they do come with their own drawbacks.
> > > >
> > > > cpuinfo_cur_freq provides higher precision though at a cost of being
> > > > rather expensive. Moreover, the information retrieved via this attribute
> > > > is somewhat short lived as frequency can change at any point of time
> > > > making it difficult to reason from.
> > > >
> > > > scaling_cur_freq, on the other hand, tends to be less accurate but then
> > > > the actual level of precision (and source of information) varies between
> > > > architectures making it a bit ambiguous.
> > > >
> > > > The new attribute, cpuinfo_avg_freq, is intended to provide more stable,
> > > > distinct interface, exposing an average frequency of a given CPU(s), as
> > > > reported by the hardware, over a time frame spanning no more than a few
> > > > milliseconds. As it requires appropriate hardware support, this
> > > > interface is optional.
> > >
> > > From what I recall, the plan is to:
> > > - keep cpuinfo_cur_freq as it is, not expose for x86 and call ->get()
> > >   for ARM.
> >
> > Yes.
> That one indeed remains unchanged.
> >
> > > - introduce cpuinfo_avg_freq() and make it return frequency from hw
> > >   counters for both ARM and Intel and others who provide the API.
> >
> > Yes.
> Will add changes for Intel as well.
> >
> > > - update scaling_cur_freq() to only return the requested frequency or
> > >   error in case of X86
> >
> > Yes.
> >
> > Preferably, -ENOTSUPP for "setpolicy" drivers without the .get() callback.
> Right, my impression was that we want to leave that one as is.

For now, yes please.

> Will add appropriate changes.

In the future.

> > >   and update documentation to reflect the same.
> > >   Right now or after some time ? How much time ?
> >
> > After some time, I think at least two cycles, so people have the time
> > to switch over, but much more may be necessary if someone is stuck
> > with RHEL or similar user space.
> >
> > Anyway, x86 will be the only one affected and there may be a Kconfig
> > option even to allow it to be changed at the kernel build time.
> >
> So just for my clarification we want a config switch to control what
> scaling_cur_freq is to actually provide.

Something like:

    if (IS_ENABLED(CONFIG_CPUFREQ_CUR_FREQ_FROM_ARCH))) {
        freq = arch_freq_get_on_cpu(policy->cpu);
        if (freq)
            return sysfs_emit(buf, "%u\n", freq);
    }

    if (cpufreq_driver->setpolicy) {
        if (cpufreq_driver->get))
            return sysfs_emit(buf, "%u\n", cpufreq_driver->get(policy->cpu));

        return -ENOTSUPP;
    }

    return sysfs_emit(buf, "%u\n", policy->cur);

And select CONFIG_CPUFREQ_CUR_FREQ_FROM_ARCH on x86 for now.

> It will keep the current behaviour as
> default until we are ready to flip it and ultimately drop that temporary config
> option ?

Yes.

> > The documentation for cpuinfo_avg_freq() needs to be added along with it.
> That one is already provided unless you have smth else on mind ?
> Like updating scaling_cur_freq to reference the new sysfs attribute ?

I haven't checked super-thoroughly, but generally all documentation
needs to be consistent.