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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAC_TJvdwqQAKrVs3w6NcQNBT+bAgdyqR+8Zt_An7R9AQSSthGA@mail.gmail.com>
Date:   Tue, 26 Oct 2021 18:31:21 -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 6:15 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> 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

Good catch. We're checking if we get back the exact value.

And IIUC max_div is an arbitrary value we decide on that's <= 2^shift?
Is there a rule of thumb for choosing this?

Thanks,
Kalesh
>
> -- Steve