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

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Message-ID: <CAJZ5v0gf9WCwm8Bdzo02Jfbz5vgdH8c_T9y89SaVvTNXH7p4hA@mail.gmail.com>
Date:   Tue, 25 Aug 2020 17:14:39 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Artem Bityutskiy <dedekind1@...il.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        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 Tue, Aug 25, 2020 at 5:06 PM Srinivas Pandruvada
<srinivas.pandruvada@...ux.intel.com> wrote:
>
> On Tue, 2020-08-25 at 16:51 +0200, Rafael J. Wysocki wrote:
> > On Tue, Aug 25, 2020 at 8:20 AM Artem Bityutskiy <dedekind1@...il.com
> > > wrote:
> > > 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.
> >
> > If the write is successful (ie. no errors returned and the value
> > returned is equal to the number of written characters), the kernel
> > *has* accepted the written value, but it may not have taken effect.
> > These are two different things.
> >
> > The written value may take an effect immediately or it may take an
> > effect later, depending on the current configuration etc.  If you
> > don't see the effect of it immediately, it doesn't matter that there
> > was a failure of some sort.
> >
> > > 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 do you want it to fail then?
> >
> > > Why caching the value and making my script _think_ it succeeded is
> > > a good idea.
> >
> > Because when you change the scaling algorithm or the driver's
> > operation mode, the value you have written will take effect.
> >
> > In this particular case it is explained in the driver documentation
> > that the performance scaling algorithm in the active mode overrides
> > the sysfs value and that's the only case when it can be overridden.
> > So whatever you write to this attribute will not take effect
> > immediately anyway, but it may take an effect later.
>
> In some cases without even changing active/passive this is happening
> when there was some error previously. For example:
>
> #cat energy_performance_preference
> 127
> [root@...pl-perf-test-skx-i9 cpufreq]# rdmsr -p 1 0x774
> 8000ff00
>
> I think we should show reality. In mode change can be a special case
> and use the stored value to restore in new mode.

OK, so I'll make it fail on attempts to change the EPP from 0
(performance) in the active mode with the performance "governor".

Cheers!