Re: [PATCH v2 3/5] tracing: Fix operator precedence for hist triggers expression

On Wed, Oct 20, 2021 at 8:48 AM Steven Rostedt [off-list ref] wrote:

On Tue, 19 Oct 2021 18:31:40 -0700
Kalesh Singh [off-list ref] wrote:

quoted

@@ -2391,60 +2460,61 @@ static int check_expr_operands(struct trace_array *tr,
 static struct hist_field *parse_expr(struct hist_trigger_data *hist_data,
                                   struct trace_event_file *file,
                                   char *str, unsigned long flags,
-                                  char *var_name, unsigned int level)
+                                  char *var_name, unsigned int *n_subexprs)
 {
      struct hist_field *operand1 = NULL, *operand2 = NULL, *expr = NULL;
      unsigned long operand_flags;
      int field_op, ret = -EINVAL;
      char *sep, *operand1_str;

-     if (level > 3) {
+     if (*n_subexprs > 3) {

Why limit the sub expressions, and not just keep the limit of the level of
recursion. We allow 3 levels of recursion, but we could have more than 3
sub expressions.


If we have:  a * b + c / d - e * f / h

It would break down into:
              -
       +            /
   *       /     *     h
 a   b   c  d  e  f


Which I believe is 6 "sub expressions", but never goes more than three deep
in recursion:

   "a * b + c / d - e * f / h"

Step 1:

  op = "-"
  operand1 = "a * b + c / d"
  operand2 = "e * f / h"

Process operand1: (recursion level 1)

  op = "+"
  operand1a = "a * b"
  operand2a = "c / d"

Process operand1a: (recursion level 2)

  op = "*"
  operand1b = "a"
  operand2b = "b"

return;

Process operand1b: (recursion level 2)

  op = "/"
  operand1b = "c"
  operand2b = "d"

return;

return;

Process operand2: (recursion level 1)

  op = "/"
  operand1c = "e * f"
  operand2c = "h"

Process operand1c: (recursion level 2)

  op = "*"
  operand1c = "e"
  operand2c = "f"

return;

return;

quoted

+
+     /* LHS of string is an expression e.g. a+b in a+b+c */
+     operand1 = parse_expr(hist_data, file, operand1_str, operand_flags, NULL, n_subexprs);
      if (IS_ERR(operand1)) {
              ret = PTR_ERR(operand1);
              operand1 = NULL;

I wonder if we should look for optimizations, in case of operand1 and
operand2 are both constants?

Just perform the function, and convert it into a constant as well.

I think we achieve something like this by propagating up the
HIST_FIELD_FL_CONST flag. Thanks for the suggestions. I'll update this
in the next version.

Thanks,
Kalesh

-- Steve