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

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Date:   Tue, 19 Oct 2021 16:45:43 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Kalesh Singh <kaleshsingh@...gle.com>
Cc:     surenb@...gle.com, hridya@...gle.com, namhyung@...nel.org,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>, Shuah Khan <shuah@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/5] tracing: Add division and multiplication support
 for hist triggers

On Wed, 15 Sep 2021 19:52:46 +0000
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 val1 / val2;

This wont work on x86 32 bit machines, as u64 division will trigger
floating point arithmetic by the compiler, and cause a fault.

You'll need to use one of the div64() helpers.

-- Steve


> +}
> +