Re: [RFC PATCH 2/4] cpuidle: Add Active Stats calls tracking idle entry/exit

From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Lukasz Luba <lukasz.luba@arm.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	Linux PM <linux-pm@vger.kernel.org>,
	Amit Kucheria <amitk@kernel.org>,
	"Zhang, Rui" <rui.zhang@intel.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Chris Redpath <Chris.Redpath@arm.com>,
	Beata.Michalska@arm.com, Viresh Kumar <viresh.kumar@linaro.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Amit Kachhap <amit.kachhap@gmail.com>
Subject: Re: [RFC PATCH 2/4] cpuidle: Add Active Stats calls tracking idle entry/exit
Date: Tue, 22 Jun 2021 16:44:11 +0200	[thread overview]
Message-ID: <CAJZ5v0jExEJRM=aJhEpKoVjvppDz_x+pYG2-HSQUuehccwnVTQ@mail.gmail.com> (raw)
In-Reply-To: <2f7d855c-5232-ddbe-8403-db3596dcebc5@arm.com>

On Tue, Jun 22, 2021 at 3:59 PM Lukasz Luba <lukasz.luba@arm.com> wrote:
>
>
>
> On 6/22/21 1:33 PM, Rafael J. Wysocki wrote:
> > On Tue, Jun 22, 2021 at 9:59 AM Lukasz Luba <lukasz.luba@arm.com> wrote:
> >>
> >> The Active Stats framework tracks and accounts the activity of the CPU
> >> for each performance level. It accounts the real residency,
> >
> > No, it doesn't.  It just measures the time between the entry and exit
> > and that's not the real residency (because it doesn't take the exit
> > latency into account, for example).
>
> It's 'just' a 'model' and as other models has limitations, but it's
> better than existing one, which IPA has to use:
> cpu_util + currect_freq_at_sampling_time

But the idle duration is already measured by cpuidle as
last_residency_ns.  Why does it need to be measured once more in
addition to that?

> >
> >> when the CPU was not idle, at a given performance level. This patch adds needed calls
> >> which provide the CPU idle entry/exit events to the Active Stats
> >> framework.
> >
> > And it adds overhead to overhead-sensitive code.
> >
> > AFAICS, some users of that code will not really get the benefit, so
> > adding the overhead to it is questionable.
> >
> > First, why is the existing instrumentation in the idle loop insufficient?
>
> The instrumentation (tracing) cannot be used at run time AFAIK. I need
> this idle + freq information combined in a running platform, not for
> post-processing (like we have in LISA). The thermal governor IPA would
> use them for used power estimation.

What about snapshotting last_residency_ns in the CPU wakeup path?

> >
> > Second, why do you need to add locking to this code?
>
> The idle entry/exit updates the CPU's accounting data structure.
> There is a reader of those data structures: thermal governor,
> run from different CPU, which is the reason why I put locking for them.

So please consider doing it in a lockless manner and avoid running
this code when it is not needed in the first place.