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.
Signed-off-by: Kalesh Singh kaleshsingh@google.com Suggested-by: Steven Rostedt rostedt@goodmis.org ---
Changes in v2: - Return -EDOM if divisor is a constant and zero, per Steve
kernel/trace/trace_events_hist.c | 117 ++++++++++++++++++++++++++++++- 1 file changed, 116 insertions(+), 1 deletion(-)
diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c index 364cb3091789..1084aa41f047 100644 --- 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 +315,72 @@ 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); + u64 val2 = operand2->fn(operand2, elt, buffer, rbe, event); + + return val1 >> __ffs64(val2); +} + +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); + u64 val2 = operand2->fn(operand2, elt, buffer, rbe, event); + + return div64_u64(val1, val2); +} + +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; + } else { + u64 val2 = operand2->fn(operand2, elt, buffer, rbe, event); + + return div64_u64(val1, val2); + } +} + static u64 hist_field_mult(struct hist_field *hist_field, struct tracing_map_elt *elt, struct trace_buffer *buffer, @@ -573,6 +643,37 @@ struct snapshot_context { void *key; };
+ +static struct hist_field *find_var_field(struct hist_trigger_data *hist_data, + const char *var_name); + +/* + * 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; + + if (divisor->flags & HIST_FIELD_FL_VAR_REF) { + struct hist_field *var; + + var = find_var_field(divisor->var.hist_data, divisor->name); + div = var->constant; + } else + 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 +2676,20 @@ 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; + } + + op_fn = hist_field_get_div_fn(operand2); + } + if (combine_consts) { if (var1) expr->operands[0] = var1;