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Date:   Tue, 26 Oct 2021 23:15:57 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Kalesh Singh <kaleshsingh@google.com>
Cc:     surenb@google.com, hridya@google.com, namhyung@kernel.org,
        kernel-team@android.com, Jonathan Corbet <corbet@lwn.net>,
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        Masami Hiramatsu <mhiramat@kernel.org>,
        Tom Zanussi <zanussi@kernel.org>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v4 6/8] tracing/histogram: Optimize division by a power
 of 2
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On Tue, 26 Oct 2021 22:21:23 -0400
Steven Rostedt <rostedt@goodmis.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