tracing/histogram: Optimize division by constants

If the divisor is a constant use specific division functions to
avoid extra branches when the trigger is hit.

If the divisor constant but not a power of 2, the division can be
replaced with a multiplication and shift in the following case:

Let X = dividend and Y = divisor.

Choose Z = some power of 2. If Y <= Z, then:
    X / Y = (X * (Z / Y)) / Z

(Z / Y) is a constant (mult) which is calculated at parse time, so:
    X / Y = (X * mult) / Z

The division by Z can be replaced by a shift since Z is a power of 2:
    X / Y = (X * mult) >> shift

As long, as X < Z the results will not be off by more than 1.

Link: https://lkml.kernel.org/r/20211029232410.3494196-1-kaleshsingh@google.com
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 682870d..8ff572a 100644 (file)
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -68,7 +68,8 @@
        C(INVALID_SORT_FIELD,   "Sort field must be a key or a val"),   \
        C(INVALID_STR_OPERAND,  "String type can not be an operand in expression"), \
        C(EXPECT_NUMBER,        "Expecting numeric literal"),           \
-       C(UNARY_MINUS_SUBEXPR,  "Unary minus not supported in sub-expressions"),
+       C(UNARY_MINUS_SUBEXPR,  "Unary minus not supported in sub-expressions"), \
+       C(DIVISION_BY_ZERO,     "Division by zero"),
 
 #undef C
 #define C(a, b)                HIST_ERR_##a
@@ -92,6 +93,7 @@ typedef u64 (*hist_field_fn_t) (struct hist_field *field,
 #define HIST_FIELDS_MAX                (TRACING_MAP_FIELDS_MAX + TRACING_MAP_VARS_MAX)
 #define HIST_ACTIONS_MAX       8
 #define HIST_CONST_DIGITS_MAX  21
+#define HIST_DIV_SHIFT         20  /* For optimizing division by constants */
 
 enum field_op_id {
        FIELD_OP_NONE,
@@ -160,6 +162,8 @@ struct hist_field {
 
        /* Numeric literals are represented as u64 */
        u64                             constant;
+       /* Used to optimize division by constants */
+       u64                             div_multiplier;
 };
 
 static u64 hist_field_none(struct hist_field *field,
@@ -311,6 +315,68 @@ static u64 hist_field_div(struct hist_field *hist_field,
        return div64_u64(val1, val2);
 }
 
+static u64 div_by_power_of_two(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);
+
+       return val1 >> __ffs64(operand2->constant);
+}
+
+static u64 div_by_not_power_of_two(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);
+
+       return div64_u64(val1, operand2->constant);
+}
+
+static u64 div_by_mult_and_shift(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);
+
+       /*
+        * If the divisor is a constant, do a multiplication and shift instead.
+        *
+        * Choose Z = some power of 2. If Y <= Z, then:
+        *     X / Y = (X * (Z / Y)) / Z
+        *
+        * (Z / Y) is a constant (mult) which is calculated at parse time, so:
+        *     X / Y = (X * mult) / Z
+        *
+        * The division by Z can be replaced by a shift since Z is a power of 2:
+        *     X / Y = (X * mult) >> HIST_DIV_SHIFT
+        *
+        * As long, as X < Z the results will not be off by more than 1.
+        */
+       if (val1 < (1 << HIST_DIV_SHIFT)) {
+               u64 mult = operand2->div_multiplier;
+
+               return (val1 * mult + ((1 << HIST_DIV_SHIFT) - 1)) >> HIST_DIV_SHIFT;
+       }
+
+       return div64_u64(val1, operand2->constant);
+}
+
 static u64 hist_field_mult(struct hist_field *hist_field,
                           struct tracing_map_elt *elt,
                           struct trace_buffer *buffer,
@@ -573,6 +639,25 @@ struct snapshot_context {
        void                    *key;
 };
 
+/*
+ * Returns the specific division function to use if the divisor
+ * is constant. This avoids extra branches when the trigger is hit.
+ */
+static hist_field_fn_t hist_field_get_div_fn(struct hist_field *divisor)
+{
+       u64 div = divisor->constant;
+
+       if (!(div & (div - 1)))
+               return div_by_power_of_two;
+
+       /* If the divisor is too large, do a regular division */
+       if (div > (1 << HIST_DIV_SHIFT))
+               return div_by_not_power_of_two;
+
+       divisor->div_multiplier = div64_u64((u64)(1 << HIST_DIV_SHIFT), div);
+       return div_by_mult_and_shift;
+}
+
 static void track_data_free(struct track_data *track_data)
 {
        struct hist_elt_data *elt_data;
@@ -2575,6 +2660,24 @@ static struct hist_field *parse_expr(struct hist_trigger_data *hist_data,
        expr->operands[0] = operand1;
        expr->operands[1] = operand2;
 
+       if (field_op == FIELD_OP_DIV &&
+                       operand2_flags & HIST_FIELD_FL_CONST) {
+               u64 divisor = var2 ? var2->constant : operand2->constant;
+
+               if (!divisor) {
+                       hist_err(file->tr, HIST_ERR_DIVISION_BY_ZERO, errpos(str));
+                       ret = -EDOM;
+                       goto free;
+               }
+
+               /*
+                * Copy the divisor here so we don't have to look it up
+                * later if this is a var ref
+                */
+               operand2->constant = divisor;
+               op_fn = hist_field_get_div_fn(operand2);
+       }
+
        if (combine_consts) {
                if (var1)
                        expr->operands[0] = var1;