linux-kernel - RE: [PATCH v4 2/2] cpufreq: intel_pstate: Implement passive mode with HWP enabled

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Date:   Sun, 2 Aug 2020 07:14:36 -0700
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Srinivas Pandruvada'" <srinivas.pandruvada@...ux.intel.com>
Cc:     "'Linux Documentation'" <linux-doc@...r.kernel.org>,
        "'LKML'" <linux-kernel@...r.kernel.org>,
        "'Peter Zijlstra'" <peterz@...radead.org>,
        "'Giovanni Gherdovich'" <ggherdovich@...e.cz>,
        "'Francisco Jerez'" <francisco.jerez.plata@...el.com>,
        "'Linux PM'" <linux-pm@...r.kernel.org>,
        "'Rafael J. Wysocki'" <rjw@...ysocki.net>
Subject: RE: [PATCH v4 2/2] cpufreq: intel_pstate: Implement passive mode with HWP enabled

On 2020.08.01 16:41 Srinivas Pandruvada wrote:
> On Tue, 2020-07-28 at 17:13 +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > Allow intel_pstate to work in the passive mode with HWP enabled and
> > make it set the HWP minimum performance limit (HWP floor) to the
> > P-state value given by the target frequency supplied by the cpufreq
> > governor, so as to prevent the HWP algorithm and the CPU scheduler
> > from working against each other, at least when the schedutil governor
> > is in use, and update the intel_pstate documentation accordingly.
> >
> > Among other things, this allows utilization clamps to be taken
> > into account, at least to a certain extent, when intel_pstate is
> > in use and makes it more likely that sufficient capacity for
> > deadline tasks will be provided.
> >
> > After this change, the resulting behavior of an HWP system with
> > intel_pstate in the passive mode should be close to the behavior
> > of the analogous non-HWP system with intel_pstate in the passive
> > mode, except that in the frequency range below the base frequency
> > (ie. the frequency retured by the base_frequency cpufreq attribute
> > in sysfs on HWP systems) the HWP algorithm is allowed to go above
> > the floor P-state set by intel_pstate with or without hardware
> > coordination of P-states among CPUs in the same package.
> >
> Do you mean HWP.req.min will be below base_freq (unless user overrides
> it)?

No.

> With busy workload I see HWP req.min = HWP req.max.
> The base freq: 1.3GHz (ratio 0x0d), MAX 1C turbo: 3.9GHz (ratio: 0x27)
> When I monitor MSR 0x774 (HWP_REQ), I see
> 0x80002727

Yes, that is what I expect to see.

> 
> Normally msr 0x774
> 0x80002704

That would be "active" mode and the powersave governor, correct?.
And yes that is what I expect for your processor.
For mine, load or no load, decoded:
0x774: IA32_HWP_REQUEST:    CPU 0-5 :
    raw: 80002E08 : 80002E08 : 80002E08 : 80002E08 : 80002E08 : 80002E08 :
    min:        8 :        8 :        8 :        8 :        8 :        8 :
    max:       46 :       46 :       46 :       46 :       46 :       46 :
    des:        0 :        0 :        0 :        0 :        0 :        0 :
    epp:      128 :      128 :      128 :      128 :      128 :      128 :
    act:        0 :        0 :        0 :        0 :        0 :        0 :

This thread is about passive mode, and myself, I do not expect the last byte to be
4 (8 for mine) under load.

... Doug