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: <20211020013153.4106001-1-kaleshsingh@google.com>
 <20211020013153.4106001-3-kaleshsingh@google.com> <20211019222756.1fde436b@gandalf.local.home>
In-Reply-To: <20211019222756.1fde436b@gandalf.local.home>
From:   Kalesh Singh <kaleshsingh@google.com>
Date:   Wed, 20 Oct 2021 07:54:59 -0700
Message-ID: <CAC_TJvcZ4ndpQpsj4ANj9LpzSu6GfPSdxpVc0XShbi9u_bSUyw@mail.gmail.com>
Subject: Re: [PATCH v2 2/5] tracing: Add division and multiplication support
 for hist triggers
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Suren Baghdasaryan <surenb@google.com>,
        Hridya Valsaraju <hridya@google.com>,
        Namhyung Kim <namhyung@kernel.org>,
        "Cc: Android Kernel" <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>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, Oct 19, 2021 at 7:28 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Tue, 19 Oct 2021 18:31:39 -0700
> Kalesh Singh <kaleshsingh@google.com> wrote:
>
> > +static u64 hist_field_div(struct hist_field *hist_field,
> > +                        struct tracing_map_elt *elt,
> > +                        struct trace_buffer *buffer,
> > +                        struct ring_buffer_event *rbe,
> > +                        void *event)
> > +{
> > +     struct hist_field *operand1 = hist_field->operands[0];
> > +     struct hist_field *operand2 = hist_field->operands[1];
> > +
> > +     u64 val1 = operand1->fn(operand1, elt, buffer, rbe, event);
> > +     u64 val2 = operand2->fn(operand2, elt, buffer, rbe, event);
> > +
> > +     /* Return -1 for the undefined case */
> > +     if (!val2)
> > +             return -1;
> > +
> > +     return div64_u64(val1, val2);
> > +}
> > +
>
> I wonder if you should add a shift operator as well?
>
> I mean, if for some reason you want to divide by a power of two, then why
> us the division. Especially if this is on a 32 bit machine.
>
> Of course, the parsing could detect that. If the divisor is a constant. Or
> we could even optimize the above with:
>
>         if (!val2)
>                 return -1;
>
>         if (!(val2 & (val2 - 1))
>                 return val1 >> __ffs64(val2);
>
> Which should be faster than a divide, and even if it isn't a power of two,
> the subtract and & should be in the noise compared to the divide.
>
> Note, the above can be added to this. I'm not suggesting changing this
> patch.

Is it worth adding something like this for the multiplication case as well?

- Kalesh

>
> -- Steve