linux-kernel - Re: [RFC 1/1] Weighted approach to gather and use history in TEO governor

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Date:   Wed, 13 May 2020 16:49:47 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Pratik Sampat <psampat@...ux.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Doug Smythies <dsmythies@...us.net>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@...ux.ibm.com>,
        pratik.sampat@...ibm.com, pratik.r.sampat@...il.com
Subject: Re: [RFC 1/1] Weighted approach to gather and use history in TEO governor

On Wed, May 13, 2020 at 7:31 AM Pratik Sampat <psampat@...ux.ibm.com> wrote:
>
> Thanks for your comment.
>
>
> On 12/05/20 11:07 pm, Peter Zijlstra wrote:
> > Just a quick note..
> >
> > On Mon, May 11, 2020 at 07:40:55PM +0530, Pratik Rajesh Sampat wrote:
> >
> >> +    /*
> >> +     * Rearrange the weight distribution of the state, increase the weight
> >> +     * by the LEARNING RATE % for the idle state that was supposed to be
> >> +     * chosen and reduce by the same amount for rest of the states
> >> +     *
> >> +     * If the weights are greater than (100 - LEARNING_RATE) % or lesser
> >> +     * than LEARNING_RATE %, do not increase or decrease the confidence
> >> +     * respectively
> >> +     */
> >> +    for (i = 0; i < drv->state_count; i++) {
> >> +            unsigned int delta;
> >> +
> >> +            if (idx == -1)
> >> +                    break;
> >> +            if (i ==  idx) {
> >> +                    delta = (LEARNING_RATE * cpu_data->state_mat[last_idx][i]) / 100;
> > 100 is a crap number to divide by as a computer. We bio-puddings happend
> > to have 10 digits, so 100 makes sense to us, but it does not to our
> > binary friends.
> >
> >
> Absolutely! I just wrote the code exactly the way I did the Math on paper,
> definitely need to figure out an optimal way of doing things.

There is no particular reason to use percent in computations at all.
You may as well use 1/1024 parts instead (and then use shifts instead
of divisions).