Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 6 Jun 2018 17:20:00 +0200
From:   Juri Lelli <juri.lelli@redhat.com>
To:     Quentin Perret <quentin.perret@arm.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@arm.com>, peterz@infradead.org,
        rjw@rjwysocki.net, gregkh@linuxfoundation.org,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        mingo@redhat.com, morten.rasmussen@arm.com, chris.redpath@arm.com,
        patrick.bellasi@arm.com, valentin.schneider@arm.com,
        vincent.guittot@linaro.org, thara.gopinath@linaro.org,
        viresh.kumar@linaro.org, tkjos@google.com, joelaf@google.com,
        smuckle@google.com, adharmap@quicinc.com, skannan@quicinc.com,
        pkondeti@codeaurora.org, edubezval@gmail.com,
        srinivas.pandruvada@linux.intel.com, currojerez@riseup.net,
        javi.merino@kernel.org
Subject: Re: [RFC PATCH v3 03/10] PM: Introduce an Energy Model management
 framework
Message-ID: <20180606152000.GB15894@localhost.localdomain>
References: <20180521142505.6522-1-quentin.perret@arm.com>
 <20180521142505.6522-4-quentin.perret@arm.com>
 <b47cb1f1-6ca1-7674-903f-724be43a57a0@arm.com>
 <20180606143739.GF10870@e108498-lin.cambridge.arm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180606143739.GF10870@e108498-lin.cambridge.arm.com>
User-Agent: Mutt/1.9.2 (2017-12-15)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 06/06/18 15:37, Quentin Perret wrote:
> Hi Dietmar,
> 
> On Wednesday 06 Jun 2018 at 15:12:15 (+0200), Dietmar Eggemann wrote:
> > > +static void fd_update_cs_table(struct em_cs_table *cs_table, int cpu)
> > > +{
> > > +	unsigned long cmax = arch_scale_cpu_capacity(NULL, cpu);
> > > +	int max_cap_state = cs_table->nr_cap_states - 1;
> > > +	unsigned long fmax = cs_table->state[max_cap_state].frequency;
> > > +	int i;
> > > +
> > > +	for (i = 0; i < cs_table->nr_cap_states; i++)
> > > +		cs_table->state[i].capacity = cmax *
> > > +					cs_table->state[i].frequency / fmax;
> > > +}
> > 
> > This has issues on a 32bit system. cs_table->state[i].capacity (unsigned
> > long) overflows with the frequency values stored in Hz.
> 
> Ah, thank you very much for pointing this out ! I haven't tried on a 32bit
> machine yet, my bad. I'll fix that for v4.
> 
> > 
> > Maybe something like this to cure it:
> > 
> > diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
> > index 6ad53f1cf7e6..c13b3eb8bf35 100644
> > --- a/kernel/power/energy_model.c
> > +++ b/kernel/power/energy_model.c
> > @@ -144,9 +144,11 @@ static void fd_update_cs_table(struct em_cs_table *cs_table, int cpu)
> >  	unsigned long fmax = cs_table->state[max_cap_state].frequency;
> >  	int i;
> >  
> > -	for (i = 0; i < cs_table->nr_cap_states; i++)
> > -		cs_table->state[i].capacity = cmax *
> > -					cs_table->state[i].frequency / fmax;
> > +	for (i = 0; i < cs_table->nr_cap_states; i++) {
> > +		u64 val = (u64)cmax * cs_table->state[i].frequency;
> > +		do_div(val, fmax);
> > +		cs_table->state[i].capacity = (unsigned long)val;
> > +	}
> >  }
> 
> Hmmm yes, that should work.
> 
> > 
> > This brings me to another question. Let's say there are multiple users of
> > the Energy Model in the system. Shouldn't the units of frequency and power
> > not standardized, maybe Mhz and mW?
> > The task scheduler doesn't care since it is only interested in power diffs
> > but other user might do.
> 
> So the good thing about specifying units is that we can probably assume
> ranges on the values. If the power is in mW, assuming that we're talking
> about a single CPU, it'll probably fit in 16 bits. 65W/core should be
> a reasonable upper-bound ?
> But there are also vendors who might not be happy with disclosing absolute
> values ... These are sometimes considered sensitive and only relative
> numbers are discussed publicly. Now, you can also argue that we already
> have units specified in IPA for ex, and that it doesn't really matter if
> a driver "lies" about the real value, as long as the ratios are correct.
> And I guess that anyone can do measurement on the hardware and get those
> values anyway. So specifying a unit (mW) for the power is probably a
> good idea.

Mmm, I remember we fought quite a bit while getting capacity-dmpis-mhz
binding accepted, and one of the musts was that the values were going to
be normalized. So, normalized power values again maybe?

Best,

- Juri