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

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.

-- Steve