linux-kernel - Re: [PATCH v2 2/5] cpufreq: intel_pstate: Always return last EPP value from sysfs

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Message-ID: <61ea43fce7dd8700d94f12236a86ffec6f76a898.camel@gmail.com>
Date:   Tue, 25 Aug 2020 09:20:33 +0300
From:   Artem Bityutskiy <dedekind1@...il.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>
Cc:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Doug Smythies <dsmythies@...us.net>
Subject: Re: [PATCH v2 2/5] cpufreq: intel_pstate: Always return last EPP
 value from sysfs

On Mon, 2020-08-24 at 19:42 +0200, Rafael J. Wysocki wrote:
> From: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
> 
> Make the energy_performance_preference policy attribute in sysfs
> always return the last EPP value written to it instead of the one
> currently in the HWP Request MSR to avoid possible confusion when
> the performance scaling algorithm is used in the active mode with
> HWP enabled (in which case the EPP is forced to 0 regardless of
> what value it has been set to via sysfs).

Why is this a good idea, I wonder. If there was a prior discussion,
please, point to it.

The general approach to changing settings via sysfs is often like this:

1. Write new value.
2. Read it back and verify that it is the same. Because there is no
better way to verify that the kernel "accepted" the value.

Let's say I write 'balanced' to energy_performance_preference. I read
it back, and it contains 'balanced', so I am happy, I trust the kernel
changed EPP to "balanced".

If the kernel, in fact, uses something else, I want to know about it
and have my script fail. Why caching the value and making my script
_think_ it succeeded is a good idea.

In other words, in my usage scenarios at list, I prefer kernel telling
the true EPP value, not some "cached, but not used" value.

Artem.