linux-kernel - Re: [PATCH v4 6/8] tracing/histogram: Optimize division by a power of 2

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Message-ID: <20211026231557.1eedad9b@rorschach.local.home>
Date:   Tue, 26 Oct 2021 23:15:57 -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, Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>, Shuah Khan <shuah@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 6/8] tracing/histogram: Optimize division by a power
 of 2

On Tue, 26 Oct 2021 22:21:23 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> I'm sure there's an algorithm somewhere that can give as the real max.

You got me playing with this more ;-)

OK, I added the rounding in the wrong place. I found that we can make
the max_div to be the same as the shift! The bigger the shift, the
bigger the max!

	mult = (1 << shift) / div;
	max_div = (1 << shift)

But the rounding needs to be with the mult / shift:

	return (val * mult + ((1 << shift) - 1)) >> shift;


When val goes pass 1 << shift, then the error will be off by more than
one.

-- Steve