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

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Message-ID: <20211026211511.403d76ca@rorschach.local.home>
Date:   Tue, 26 Oct 2021 21:15:11 -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 18:09:22 -0700
Kalesh Singh <kaleshsingh@...gle.com> wrote:

> >   delta = mult * div / 2^20
> >
> > That is if mult is a power of two, then there would be no rounding
> > errors, and the delta is zero, making the max infinite:

That should have been (as shown in the algorithm)

  delta = mult * div - 2 ^ 20

As mult is 2^20 / div; and the above should end up zero if there's no
rounding issues, as it would be:

 delta = (2^20 / div) * div - 2^20

-- Steve