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

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Date:   Tue, 26 Oct 2021 21:04:29 -0700
From:   Kalesh Singh <kaleshsingh@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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, Oct 26, 2021 at 8:16 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> 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!

Nice! :)
>
>         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.
Did you mean, val should be such that when we do the (val * mult) we
only get rounding errors less than (1 << shift)?

I think we also need to flip the delta now since we round down initially:

    delta =  (1 << shift) - (mult * div)

Thanks,
Kalesh
>
> -- Steve