[for-next][PATCH 10/14] tracing/histogram: Optimize division by constants

Steven Rostedt
2021-11-02T20:12:39Z

From: Kalesh Singh <kaleshsingh@google.com>

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>
---
kernel/trace/trace_events_hist.c | 105 ++++++++++++++++++++++++++++++-
1 file changed, 104 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 682870d004c4..8ff572a31fd3 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 +315,68 @@ 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;
--
2.33.0