Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20211025200852.3002369-1-kaleshsingh@google.com>
 <20211025200852.3002369-7-kaleshsingh@google.com> <20211026151451.7f3e09a4@gandalf.local.home>
 <CAC_TJveHgsPZw7p7BWOgQw6h8GNU_Pv_WUjNmw3AUq+wnSzk6Q@mail.gmail.com>
 <20211026201846.08990d1d@rorschach.local.home> <CAC_TJve-mKSojaXtukdFeQKvPz-8TQtS=pgGD0Z18Wt6yJi7dg@mail.gmail.com>
 <20211026211511.403d76ca@rorschach.local.home>
In-Reply-To: <20211026211511.403d76ca@rorschach.local.home>
From:   Kalesh Singh <kaleshsingh@google.com>
Date:   Tue, 26 Oct 2021 18:31:21 -0700
Message-ID: <CAC_TJvdwqQAKrVs3w6NcQNBT+bAgdyqR+8Zt_An7R9AQSSthGA@mail.gmail.com>
Subject: Re: [PATCH v4 6/8] tracing/histogram: Optimize division by a power of 2
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     surenb@google.com, hridya@google.com, namhyung@kernel.org,
        kernel-team@android.com, Jonathan Corbet <corbet@lwn.net>,
        Ingo Molnar <mingo@redhat.com>, Shuah Khan <shuah@kernel.org>,
        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
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, Oct 26, 2021 at 6:15 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Tue, 26 Oct 2021 18:09:22 -0700
> Kalesh Singh <kaleshsingh@google.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