linux-kernel - Re: [RFT][PATCH 0/2] cpufreq: intel_pstate: Handle _PPC updates on global turbo disable/enable

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Date:   Mon, 04 Mar 2019 15:04:07 -0800
From:   Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Chen Yu <yu.c.chen@...el.com>,
        Gabriele Mazzotta <gabriele.mzt@...il.com>
Subject: Re: [RFT][PATCH 0/2] cpufreq: intel_pstate: Handle _PPC updates on
 global turbo disable/enable

On Mon, 2019-03-04 at 22:57 +0100, Rafael J. Wysocki wrote:
> On Mon, Mar 4, 2019 at 7:06 PM Srinivas Pandruvada
> <srinivas.pandruvada@...ux.intel.com> wrote:
> > 
> > [...]
> > > > There are other methods like PL1 budget limit for such cases.
> > > > FW
> > > > can
> > > > just change the config TDP level.
> > > 
> > > OK, but that would be done without notification I suppose?
> > 
> > There is a notification via processor PCI device (B0D4). This is
> > passed
> > to user space to change the power limits. The new element is called
> > PPCC and it is exposed via sysfs.
> 
> What do you mean by "new element" and how exactly is it exposed?
This is part of DPTF processor ACPI object (INT3401 or B0D4). They are
exposed in sysfs 
E.g, /sys/bus/platform/devices/INT3401:00/power_limits/
There is a thermal uevent sent when they change. Both dptf daemon and
thermald listen and use to set rapl power-constraints including step
sizes for control. Someone can write a udev rule to do the same.

> 
> > Disabling turbo is not very interesting as there can be more turbo
> > than
> > non turbo. So you loose lots of performance. So instead you can
> > control
> > power in turbo region to give you more control. _PPC is even less
> > interesting as you can't control uncore power.
> 
> I guess that designers should know about that.  The kernel is on the
> receiving end here. :-)
I think they know. Hence you don't see this issue of enable/disable of
turbo by firmware quite often. This laptop here I guess released in
beginning of 2014 with Haswell.

> 
> > > 
[...]

> 
> I guess that you are talking about intel_pstate_update_max_freq()
> which acquires policy->rwsem.  If so, what exactly is the problem
> with
> it?
I was suggesting to use an API/define in cpufreq.h which does operation
on policy->rwsem for better abstraction. This is the first time it was
used outside core cpufreq.c. As more places it will be used in future,
common function will help debug, if in some path there is a bug in
aquire/release of semaphore. But you can ignore this.

Thanks,
Srinivas