linux-kernel - Re: [PATCH v1] cpufreq: Add policy

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Message-ID: <CAG2KctqdJzhaC4pRG9rAgteVKHtKsA8Y7=_MHEUCCeBBhoejWQ@mail.gmail.com>
Date: Thu, 13 Nov 2025 19:41:08 -0800
From: Samuel Wu <wusamuel@...gle.com>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	kernel-team@...roid.com, linux-pm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v1] cpufreq: Add policy_frequency trace event

On Wed, Nov 12, 2025 at 10:45 PM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 12-11-25, 15:51, Samuel Wu wrote:
> > The existing cpu_frequency trace_event can be verbose, emitting an event
> > for every CPU in the policy even when their frequencies are identical.
> >
> > This patch adds a new policy_frequency trace event, which provides a
> > more efficient alternative to cpu_frequency trace event. This option
> > allows users who only need frequency at a policy level more concise logs
> > with simpler analysis.
> >
> > Signed-off-by: Samuel Wu <wusamuel@...gle.com>
> > ---
> >  drivers/cpufreq/cpufreq.c    |  2 ++
> >  include/trace/events/power.h | 21 +++++++++++++++++++++
> >  2 files changed, 23 insertions(+)
> >
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > index 4472bb1ec83c..b65534a4fd9a 100644
> > --- a/drivers/cpufreq/cpufreq.c
> > +++ b/drivers/cpufreq/cpufreq.c
> > @@ -345,6 +345,7 @@ static void cpufreq_notify_transition(struct cpufreq_policy *policy,
> >               pr_debug("FREQ: %u - CPUs: %*pbl\n", freqs->new,
> >                        cpumask_pr_args(policy->cpus));
> >
> > +             trace_policy_frequency(freqs->new, policy->cpu);
> >               for_each_cpu(cpu, policy->cpus)
> >                       trace_cpu_frequency(freqs->new, cpu);
>
> I don't see much value in almost duplicate trace events. If we feel that a
> per-policy event is a better fit (which makes sens), then we can just drop the
> trace_cpu_frequency() events and print policy->cpus (or related_cpus)
> information along with the per-policy events.

Thank you for the feedback Viresh. Fair enough, I've done some testing
and a single trace event should work and would be cleaner. Please let
me know what you think of this proposal for v2.

We can append a bitmask of policy->cpus field to
trace_cpu_frequency(). This way we maintain backwards compatibility:
trace_cpu_frequency() is not removed, and its pre-existing fields are
not disturbed.

Call flow wise, we can delete all the for_each_cpu() loops, and we
still retain the benefits of the trace emitting once per policy
instead of once per cpu.