linux-kernel - Re: problem in changing from active to passive mode

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Message-ID: <CAJZ5v0iM2=VAymcmBXKVL0SVt-P6iswoYqXAzuzBuQpUhKnZjA@mail.gmail.com>
Date:   Thu, 28 Oct 2021 20:43:03 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Julia Lawall <julia.lawall@...ia.fr>
Cc:     Doug Smythies <dsmythies@...us.net>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: problem in changing from active to passive mode

On Thu, Oct 28, 2021 at 8:16 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Thu, Oct 28, 2021 at 7:57 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > On Thu, Oct 28, 2021 at 7:29 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> > >
> > > On Thu, Oct 28, 2021 at 7:10 PM Julia Lawall <julia.lawall@...ia.fr> wrote:
> > > >
> > > > > Now, for your graph 3, are you saying this pseudo
> > > > > code of the process is repeatable?:
> > > > >
> > > > > Power up the system, booting kernel 5.9
> > > > > switch to passive/schedutil.
> > > > > wait X minutes for system to settle
> > > > > do benchmark, result ~13 seconds
> > > > > re-boot to kernel 5.15-RC
> > > > > switch to passive/schedutil.
> > > > > wait X minutes for system to settle
> > > > > do benchmark, result ~40 seconds
> > > > > re-boot to kernel 5.9
> > > > > switch to passive/schedutil.
> > > > > wait X minutes for system to settle
> > > > > do benchmark, result ~28 seconds
> > > >
> > > > In the first boot of 5.9, the des (desired?) field of the HWP_REQUEST
> > > > register is 0 and in the second boot (after booting 5.15 and entering
> > > > passive mode) it is 10.  I don't know though if this is a bug or a
> > > > feature...

I think I didn't understand you correctly, sorry about that.

In 5.15-rc (starting in 5.11-rc) the desired perf field in HWP_REQUEST
is used in the passive mode, so that is expected.

However, it may not be reset to 0 when going back from the passive to
the active mode.

> > > It looks like a bug.
> > >
> > > I think that the desired value is not cleared on driver exit which
> > > should happen.  Let me see if I can do a quick patch for that.
> >
> > Please check the behavior with the attached patch applied.
>
> Well, actually, the previous one won't do anything, because the
> desired perf field is already cleared in this function before writing
> the MSR, so please try the one attached to this message instead.

So with the last patch applied, can you please check if you get
desired=0 with 5.15-rc when switching driver modes from passive to
active?  FWIW, this works for me here.

In any case, the desired perf value in HWP_REQUEST is expected to be
reset to 0 on system restart.