linux-kernel - Re: [PATCH v13 12/32] x86,fs/resctrl: Support binary fixed point event counters

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Message-ID: <aQwHpFEfHGpoaVW9@agluck-desk3>
Date: Wed, 5 Nov 2025 18:27:48 -0800
From: "Luck, Tony" <tony.luck@...el.com>
To: Dave Martin <Dave.Martin@....com>
CC: Fenghua Yu <fenghuay@...dia.com>, Reinette Chatre
	<reinette.chatre@...el.com>, Maciej Wieczor-Retman
	<maciej.wieczor-retman@...el.com>, Peter Newman <peternewman@...gle.com>,
	James Morse <james.morse@....com>, Babu Moger <babu.moger@....com>, "Drew
 Fustini" <dfustini@...libre.com>, Chen Yu <yu.c.chen@...el.com>,
	<x86@...nel.org>, <linux-kernel@...r.kernel.org>, <patches@...ts.linux.dev>
Subject: Re: [PATCH v13 12/32] x86,fs/resctrl: Support binary fixed point
 event counters

On Wed, Nov 05, 2025 at 03:31:07PM -0800, Luck, Tony wrote:
> > > +
> > > +	if (!binary_bits) {
> > > +		seq_printf(m, "%llu.0\n", val);
> > > +		return;
> > > +	}

I can't completely escape a test for !binary_bits. Most of the
flow works ok (doing nothing, working towards frac == 0 when
it comes time for the snprintf()).

But the round-up code:

	frac += 1ull << (binary_bits - 1);

goes badly wrong if binary_bits == 0.

I could write it like this:


static void print_event_value(struct seq_file *m, unsigned int binary_bits, u64 val)
{
	char buf[decplaces[MAX_BINARY_BITS] + 1];
	unsigned long long frac = 0;

	if (binary_bits) {
		/* Mask off the integer part of the fixed-point value. */
		frac = val & GENMASK_ULL(binary_bits - 1, 0);

		/*
		 * Multiply by 10^{desired decimal places}. The integer part of
		 * the fixed point value is now almost what is needed.
		 */
		frac *= int_pow(10ull, decplaces[binary_bits]);

		/*
		 * Round to nearest by adding a value that would be a "1" in the
		 * binary_bits + 1 place.  Integer part of fixed point value is
		 * now the needed value.
		 */
		frac += 1ull << (binary_bits - 1);

		/*
		 * Extract the integer part of the value. This is the decimal
		 * representation of the original fixed-point fractional value.
		 */
		frac >>= binary_bits;
	}

	/*
	 * "frac" is now in the range [0 .. 10^decplaces).  I.e. string
	 * representation will fit into chosen number of decimal places.
	 */
	snprintf(buf, sizeof(buf), "%0*llu", decplaces[binary_bits], frac);

	seq_printf(m, "%llu.%s\n", val >> binary_bits, buf);
}

-Tony