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

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Date:   Thu, 28 Oct 2021 22:18:35 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
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, 28 Oct 2021, Rafael J. Wysocki wrote:

> On Thu, Oct 28, 2021 at 9:25 PM Julia Lawall <julia.lawall@...ia.fr> wrote:
> >
> >
> >
> > On Thu, 28 Oct 2021, Rafael J. Wysocki wrote:
> >
> > > On Thu, Oct 28, 2021 at 9:13 PM Julia Lawall <julia.lawall@...ia.fr> wrote:
> > > >
> > > >
> > > >
> > > > On Thu, 28 Oct 2021, Rafael J. Wysocki 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...
> > > > > > >
> > > > > > > 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.
> > > > >
> > > >
> > > > Turbostat still shows 10:
> > > >
> > > > cpu0: MSR_HWP_CAPABILITIES: 0x070a1525 (high 37 guar 21 eff 10 low 7)
> > > > cpu0: MSR_HWP_REQUEST: 0x000a2525 (min 37 max 37 des 10 epp 0x0 window 0x0 pkg 0x0)
> > > > cpu0: MSR_HWP_REQUEST_PKG: 0x8000ff00 (min 0 max 255 des 0 epp 0x80 window 0x0)
> > > > cpu0: MSR_HWP_STATUS: 0x00000004 (No-Guaranteed_Perf_Change, No-Excursion_Min)
> > > > cpu1: MSR_PM_ENABLE: 0x00000001 (HWP)
> > > > cpu1: MSR_HWP_CAPABILITIES: 0x070a1525 (high 37 guar 21 eff 10 low 7)
> > > > cpu1: MSR_HWP_REQUEST: 0x000a2525 (min 37 max 37 des 10 epp 0x0 window 0x0 pkg 0x0)
> > > > cpu1: MSR_HWP_REQUEST_PKG: 0x8000ff00 (min 0 max 255 des 0 epp 0x80 window 0x0)
> > > > cpu1: MSR_HWP_STATUS: 0x00000004 (No-Guaranteed_Perf_Change, No-Excursion_Min)
> > > > cpu2: MSR_PM_ENABLE: 0x00000001 (HWP)
> > > > cpu2: MSR_HWP_CAPABILITIES: 0x070a1525 (high 37 guar 21 eff 10 low 7)
> > > > cpu2: MSR_HWP_REQUEST: 0x000a2525 (min 37 max 37 des 10 epp 0x0 window 0x0 pkg 0x0)
> > > > cpu2: MSR_HWP_REQUEST_PKG: 0x8000ff00 (min 0 max 255 des 0 epp 0x80 window 0x0)
> > > > cpu2: MSR_HWP_STATUS: 0x00000004 (No-Guaranteed_Perf_Change, No-Excursion_Min)
> > > > cpu3: MSR_PM_ENABLE: 0x00000001 (HWP)
> > > > cpu3: MSR_HWP_CAPABILITIES: 0x070a1525 (high 37 guar 21 eff 10 low 7)
> > > > cpu3: MSR_HWP_REQUEST: 0x000a2525 (min 37 max 37 des 10 epp 0x0 window 0x0 pkg 0x0)
> > > > cpu3: MSR_HWP_REQUEST_PKG: 0x8000ff00 (min 0 max 255 des 0 epp 0x80 window 0x0)
> > > > cpu3: MSR_HWP_STATUS: 0x00000004 (No-Guaranteed_Perf_Change, No-Excursion_Min)
> > >
> > > Hmmm.
> > >
> > > Is this also the case if you go from "passive" to "active" on 5.15-rc
> > > w/ the patch applied?
> >
> > Sorry, I was wrong.  If I am in 5.15 and go from passive to active, the
> > des field indeed returns to 0.  If I use kexec
>
> Well, this means that the cpufreq driver cleanup is not carried out in
> the kexec path and the old desired value remains in the register.
>
> > to reboot from 5.15 passive into 5.9, then the des field remains 10.
>
> It looks like desired perf needs to be cleared explicitly in the active mode.
>
> Attached is a patch to do that, but please note that the 5.9 will need
> to be patched too to address this issue.

I'm not completely clear on what the new patch is doing and how I should
test it.  If I stay in 5.15, the original patch worked for clearing des
when going from passive to active.

julia