linux-kernel - Re: [PATCH] cpufreq: intel_pstate: Implement passive mode with HWP enabled

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Date:   Thu, 06 Aug 2020 13:39:53 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Doug Smythies <dsmythies@...us.net>
Cc:     "'Rafael J. Wysocki'" <rafael@...nel.org>,
        'Linux Documentation' <linux-doc@...r.kernel.org>,
        'LKML' <linux-kernel@...r.kernel.org>,
        'Peter Zijlstra' <peterz@...radead.org>,
        'Srinivas Pandruvada' <srinivas.pandruvada@...ux.intel.com>,
        'Giovanni Gherdovich' <ggherdovich@...e.cz>,
        'Francisco Jerez' <francisco.jerez.plata@...el.com>,
        'Linux PM' <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: intel_pstate: Implement passive mode with HWP enabled

On Thursday, August 6, 2020 7:54:47 AM CEST Doug Smythies wrote:
> On 2020.08.03 10:09 Rafael J. Wysocki wrote:
> > On Sunday, August 2, 2020 5:17:39 PM CEST Doug Smythies wrote:
> > > On 2020.07.19 04:43 Rafael J. Wysocki wrote:
> > > > On Fri, Jul 17, 2020 at 3:37 PM Doug Smythies <dsmythies@...us.net> wrote:
> > > > > On 2020.07.16 05:08 Rafael J. Wysocki wrote:
> > > > > > On Wed, Jul 15, 2020 at 10:39 PM Doug Smythies <dsmythies@...us.net> wrote:
> > > > > >> On 2020.07.14 11:16 Rafael J. Wysocki wrote:
> > > > > >> >
> > > > > >> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > > > >> ...
> > > > > >> > Since the passive mode hasn't worked with HWP at all, and it is not going to
> > > > > >> > the default for HWP systems anyway, I don't see any drawbacks related to making
> > > > > >> > this change, so I would consider this as 5.9 material unless there are any
> > > > > >> > serious objections.
> > > > > >>
> > > > > >> Good point.
> > > > >
> > > > > Actually, for those users that default to passive mode upon boot,
> > > > > this would mean they would find themselves using this.
> > > > > Also, it isn't obvious, from the typical "what driver and what governor"
> > > > > inquiry.
> > > >
> > > > So the change in behavior is that after this patch
> > > > intel_pstate=passive doesn't imply no_hwp any more.
> > > >
> > > > That's a very minor difference though and I'm not aware of any adverse
> > > > effects it can cause on HWP systems anyway.
> > >
> > > My point was, that it will now default to something where
> > > testing has not been completed.
> > >
> > > > The "what governor" is straightforward in the passive mode: that's
> > > > whatever cpufreq governor has been selected.
> > >
> > > I think you might have missed my point.
> > > From the normal methods of inquiry one does not know
> > > if HWP is being used or not. Why? Because with
> > > or without HWP one gets the same answers under:
> > >
> > > /sys/devices/system/cpu/cpu*/cpufreq/scaling_driver
> > > /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
> > 
> > Yes, but this is also the case in the active mode, isn't it?
> 
> Yes, fair enough.
> But we aren't changing what it means by default
> between kernel 5.8 and 5.9-rc1.

No, we aren't.

The only (expected) change is when booting with intel_pstate=passive and
without intel_pstate=no_hwp in the command line.

Which should be easy enough to address by adding intel_pstate=no_hwp to the
command line in 5.9-rc1 and later (to achieve the same behavior after a
fresh boot).

Cheers!