linux-kernel - Re: [PATCH v2 2/4] powercap: idle_inject: Add prepare/complete callbacks

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Message-ID: <8d3a9644657c8f0b54dd272fe0a4d640a8a6dfb9.camel@linux.intel.com>
Date:   Thu, 22 Dec 2022 09:36:45 -0800
From:   srinivas pandruvada <srinivas.pandruvada@...ux.intel.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>, rafael@...nel.org
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        rui.zhang@...el.com, amitk@...nel.org
Subject: Re: [PATCH v2 2/4] powercap: idle_inject: Add prepare/complete
 callbacks

Hi Daniel,

On Thu, 2022-12-22 at 10:50 +0100, Daniel Lezcano wrote:
> 
> Hi Srinivas,
> 
> 
> On 21/12/2022 21:58, srinivas pandruvada wrote:
> > Hi Daniel,
> > 
> > On Wed, 2022-12-21 at 15:52 +0100, Daniel Lezcano wrote:
> > > 
> > > Hi Srinivas,
> > > 
> > > On 30/11/2022 00:34, Srinivas Pandruvada wrote:
> > > > The actual idle percentage can be less than the desired because
> > > > of
> > > > interrupts. Since the objective for CPU Idle injection is for
> > > > thermal
> > > > control, there should be some way to compensate for lost idle
> > > > percentage.
> > > > Some architectures provide interface to get actual idle percent
> > > > observed
> > > > by the hardware. So, the idle percent can be adjusted using the
> > > > hardware
> > > > feedback. For example, Intel CPUs provides package idle
> > > > counters,
> > > > which
> > > > is currently used by intel powerclamp driver to adjust idle
> > > > time.
> > > Can you provide an example in terms of timings?
> > > 
> > > I'm not getting how 'prepare' would do by returning a positive
> > > value
> > > to
> > > skip the play_idle_precise() and what will do 'complete' ?
> > > 
> > intel_powerclamp has a logic where if the current idle percentage
> > observed from hardware is more than the desired target inject
> > percent,
> > it skips calling play_idle().
> > 
> > For example if you want to inject 50% idle and system is naturally
> > idle
> > for 60%, there is no use of calling play_idle in the idle injection
> > framework to induce more idle. In this way a workload can run
> > immediately.
> > 
> > So trying to emulate the same logic by using powercap/idle_inject
> > framework. So prepare() callback in the intel_powerclamp driver
> > calls
> > the existing function to check if idle-inject should skip for this
> > time
> > or not.
> 
> The function 'prepare' has the 'cpu' parameter. How can it compare
> with 
> the desired idle duration as this information is not passed to the 
> callback ?
Good question.

Calling driver knows what idle_duration he set.
In my first version, I passed *idle_duration (with current
idle_duration set), so the caller can change this for the current
play_idle call if required for one time.

But in powerclamp case we either skip the whole play_idle or not. It
doesn't change idle duration. So didn't add.

But we can add this back.

Thanks,
Srinivas



> 
>