linux-kernel - Re: [PATCH v2 2/5] tracing: Add division and multiplication support for hist triggers

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Message-ID: <CAC_TJvcZ4ndpQpsj4ANj9LpzSu6GfPSdxpVc0XShbi9u_bSUyw@mail.gmail.com>
Date:   Wed, 20 Oct 2021 07:54:59 -0700
From:   Kalesh Singh <kaleshsingh@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>,
        Hridya Valsaraju <hridya@...gle.com>,
        Namhyung Kim <namhyung@...nel.org>,
        "Cc: Android Kernel" <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>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v2 2/5] tracing: Add division and multiplication support
 for hist triggers

On Tue, Oct 19, 2021 at 7:28 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Tue, 19 Oct 2021 18:31:39 -0700
> Kalesh Singh <kaleshsingh@...gle.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