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From:   Kalesh Singh <kaleshsingh@google.com>
Date:   Fri, 29 Oct 2021 09:39:59 -0700
Message-ID: <CAC_TJvfkGOMeZGDo8o=si=DyZfbF7TJ7XOj5dz+taCrt+h3foQ@mail.gmail.com>
Subject: Re: [PATCH] tracing/histogram: Optimize division by constants
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     surenb@google.com, hridya@google.com, namhyung@kernel.org,
        kernel-team@android.com, mhiramat@kernel.org, zanussi@kernel.org,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org
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On Thu, Oct 28, 2021 at 7:01 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Wed, 27 Oct 2021 20:28:54 -0700
> Kalesh Singh <kaleshsingh@google.com> wrote:
>
> > +/*
> > + * Returns the specific division function to use if the divisor
> > + * is constant. This avoids extra branches when the trigger is hit.
> > + */
> > +static hist_field_fn_t hist_field_get_div_fn(struct hist_field *divisor)
> > +{
> > +     u64 div;
> > +
> > +     if (divisor->flags & HIST_FIELD_FL_VAR_REF) {
> > +             struct hist_field *var;
> > +
> > +             var = find_var_field(divisor->var.hist_data, divisor->name);
> > +             div = var->constant;
> > +     } else
> > +             div = divisor->constant;
> > +
> > +     if (!div)
> > +             return div_by_zero;
>
> Do we really need a div_by_zero constant function? What about just
> erroring here and perhaps return -EDOM?

Good point. We can detect this if the divisor is a constant and print
an error message instead. It'll also warrant a small update to the
tests and documentation. I'll send out a new version addressing these.

Thanks,
Kalesh

>
> -- Steve
>
> > +
> > +     if (!(div & (div - 1)))
> > +             return div_by_power_of_two;
> > +
> > +     /* If the divisor is too large, do a regular division */
> > +     if (div > (1 << HIST_DIV_SHIFT))
> > +             return div_by_not_power_of_two;
> > +
> > +     divisor->div_multiplier = div64_u64((u64)(1 << HIST_DIV_SHIFT), div);
> > +     return div_by_mult_and_shift;
> > +}
> > +