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From: Vincent Guittot <vincent.guittot@linaro.org>
To: Lukasz Luba <lukasz.luba@arm.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
	Chris Redpath <Chris.Redpath@arm.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Quentin Perret <qperret@google.com>,
	"open list:THERMAL" <linux-pm@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	segall@google.com, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	CCj.Yeh@mediatek.com
Subject: Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy
Date: Wed, 7 Jul 2021 11:37:29 +0200	[thread overview]
Message-ID: <CAKfTPtDk1ANfjR5h_EjErVfQ7=is3n9QOaKKxz81tMHtqUM7jA@mail.gmail.com> (raw)
In-Reply-To: <2f43b211-da86-9d48-4e41-1c63359865bb@arm.com>

On Wed, 7 Jul 2021 at 10:23, Lukasz Luba <lukasz.luba@arm.com> wrote:
>
>
>
> On 7/7/21 9:00 AM, Vincent Guittot wrote:
> > On Wed, 7 Jul 2021 at 09:49, Lukasz Luba <lukasz.luba@arm.com> wrote:
> >>
> >>
> >>
> >> On 7/7/21 8:07 AM, Vincent Guittot wrote:
> >>> On Fri, 25 Jun 2021 at 17:26, Lukasz Luba <lukasz.luba@arm.com> wrote:
> >>>>
> >>>> The Energy Aware Scheduler (EAS) tries to find best CPU for a waking up
> >>>> task. It probes many possibilities and compares the estimated energy values
> >>>> for different scenarios. For calculating those energy values it relies on
> >>>> Energy Model (EM) data and em_cpu_energy(). The precision which is used in
> >>>> EM data is in milli-Watts (or abstract scale), which sometimes is not
> >>>> sufficient. In some cases it might happen that two CPUs from different
> >>>> Performance Domains (PDs) get the same calculated value for a given task
> >>>> placement, but in more precised scale, they might differ. This rounding
> >>>> error has to be addressed. This patch prepares EAS code for better
> >>>> precision in the coming EM improvements.
> >>>
> >>> Could you explain why 32bits results are not enough and you need to
> >>> move to 64bits ?
> >>>
> >>> Right now the result is in the range [0..2^32[ mW. If you need more
> >>> precision and you want to return uW instead, you will have a result in
> >>> the range  [0..4kW[ which seems to be still enough
> >>>
> >>
> >> Currently we have the max value limit for 'power' in EM which is
> >> EM_MAX_POWER 0xffff (64k - 1). We allow to register such big power
> >> values ~64k mW (~64Watts) for an OPP. Then based on 'power' we
> >> pre-calculate 'cost' fields:
> >> cost[i] = power[i] * freq_max / freq[i]
> >> So, for max freq the cost == power. Let's use that in the example.
> >>
> >> Then the em_cpu_energy() calculates as follow:
> >> cost * sum_util / scale_cpu
> >> We are interested in the first part - the value of multiplication.
> >
> > But all these are internal computations of the energy model. At the
> > end, the computed energy that is returned by compute_energy() and
> > em_cpu_energy(), fits in a long
>
> Let's take a look at existing *10000 precision for x CPUs:
> cost * sum_util / scale_cpu =
> (64k *10000) * (x * 800) / 1024
> which is:
> x * ~500mln
>
> So to be close to overflowing u32 the 'x' has to be > (?=) 8
> (depends on sum_util).

Sorry but I don't get your point.
This patch is about the return type of compute_energy() and
em_cpu_energy(). And even if we decide to return uW instead of mW,
there is still a lot of margin.

It's not because you need u64 for computing intermediate value that
you must returns u64

>
> >
> >>
> >> The sum_util values that we can see for x CPUs which have scale_cap=1024
> >> can be close to 800, let's use it in the example:
> >> cost * sum_util = 64k * (x * 800), where
> >> x=4: ~200mln
> >> x=8: ~400mln
> >> x=16: ~800mln
> >> x=64: ~3200mln (last one which would fit in u32)
> >>
> >> When we increase the precision by even 100, then the above values won't
> >> fit in the u32. Even a max cost of e.g. 10k mW and 100 precision has
> >> issues:
> >> cost * sum_util = (10k *100) * (x * 800), where
> >> x=4: ~3200mln
> >> x=8: ~6400mln
> >>
> >> For *1000 precision even a power of 1Watt becomes an issue:
> >> cost * sum_util = (1k *1000) * (x * 800), where
> >> x=4: ~3200mln
> >> x=8: ~6400mln
> >>
> >> That's why to make the code safe for bigger power values, I had to use
> >> the u64 on 32bit machines.

  reply	other threads:[~2021-07-07  9:37 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-25 15:26 [PATCH 0/3] Improve EAS energy estimation and increase precision Lukasz Luba
2021-06-25 15:26 ` [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy Lukasz Luba
2021-06-30 17:01   ` Rafael J. Wysocki
2021-06-30 17:28     ` Lukasz Luba
2021-07-02 19:07       ` Lukasz Luba
2021-07-07  7:07   ` Vincent Guittot
2021-07-07  7:49     ` Lukasz Luba
2021-07-07  8:00       ` Vincent Guittot
2021-07-07  8:23         ` Lukasz Luba
2021-07-07  9:37           ` Vincent Guittot [this message]
2021-07-07  9:48             ` Lukasz Luba
2021-07-07  9:56               ` Vincent Guittot
2021-07-07 10:06                 ` Lukasz Luba
2021-07-07 10:11                   ` Vincent Guittot
2021-07-07 10:29                     ` Lukasz Luba
2021-07-07 10:32                       ` Vincent Guittot
2021-07-07 10:41                         ` Lukasz Luba
2021-07-07 10:50                           ` Vincent Guittot
2021-07-07 11:02                             ` Lukasz Luba
2021-07-07 13:53                               ` Vincent Guittot
2021-07-07 14:25                                 ` Lukasz Luba
2021-07-07  9:45           ` Dietmar Eggemann
2021-07-07  9:54             ` Lukasz Luba
2021-06-25 15:26 ` [PATCH 2/3] PM: EM: Make em_cpu_energy() able to return bigger values Lukasz Luba
2021-07-05 12:44   ` Dietmar Eggemann
2021-07-06 19:44     ` Lukasz Luba
2021-07-07  7:07   ` Peter Zijlstra
2021-07-07  8:09     ` Lukasz Luba
2021-07-07 10:01       ` Peter Zijlstra
2021-07-07 10:23         ` Lukasz Luba
2021-06-25 15:26 ` [PATCH 3/3] PM: EM: Increase energy calculation precision Lukasz Luba
2021-07-05 12:45   ` Dietmar Eggemann
2021-07-06 19:51     ` Lukasz Luba

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