linux-kernel - Re: [PATCH] tracing/histogram: Optimize division by constants

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Date:   Thu, 28 Oct 2021 22:01:34 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Kalesh Singh <kaleshsingh@...gle.com>
Cc:     surenb@...gle.com, hridya@...gle.com, namhyung@...nel.org,
        kernel-team@...roid.com, mhiramat@...nel.org, zanussi@...nel.org,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing/histogram: Optimize division by constants

On Wed, 27 Oct 2021 20:28:54 -0700
Kalesh Singh <kaleshsingh@...gle.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?

-- 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;
> +}
> +