Commit a8e4e880 authored by Ian Rogers's avatar Ian Rogers Committed by Arnaldo Carvalho de Melo
Browse files

perf metric: Don't compute unused events 

For a metric like:

  EVENT1 if #smt_on else EVENT2

currently EVENT1 and EVENT2 will be measured and then when the metric is
reported EVENT1 or EVENT2 will be printed depending on the value from
smt_on() during the expr parsing. Computing both events is unnecessary and
can lead to multiplexing as discussed in this thread:

  https://lore.kernel.org/lkml/20201110100346.2527031-1-irogers@google.com/



If the input is constant to certain operators like:

 IDS1 if CONST else IDS2

then the result will be either IDS1 or IDS2 depending on CONST (which
may be evaluated from an entire expression), and so IDS1 or IDS2 may
be discarded avoiding events from being programmed.

Signed-off-by: default avatarIan Rogers <irogers@google.com>
Tested-by: default avatarJohn Garry <john.garry@huawei.com>
Acked-by: default avatarJiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Clarke <pc@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sandeep Dasgupta <sdasgup@google.com>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20210923074616.674826-13-irogers@google.com


Signed-off-by: default avatarArnaldo Carvalho de Melo <acme@redhat.com>
parent 970f7afe

tools/perf/tests/expr.c

+11 −0
Original line number Diff line number Diff line 
// SPDX-License-Identifier: GPL-2.0

#include "util/debug.h"

#include "util/expr.h"

#include "util/smt.h"

#include "tests.h"

#include <stdlib.h>

#include <string.h>
@@ -132,6 +133,16 @@ int test__expr(struct test *t __maybe_unused, int subtest __maybe_unused) 
	TEST_ASSERT_VAL("find ids", hashmap__find(ctx->ids, "EVENT2,param=3/",

						    (void **)&val_ptr));



	/* Only EVENT1 or EVENT2 need be measured depending on the value of smt_on. */

	expr__ctx_clear(ctx);

	TEST_ASSERT_VAL("find ids",

			expr__find_ids("EVENT1 if #smt_on else EVENT2",

				NULL, ctx, 0) == 0);

	TEST_ASSERT_VAL("find ids", hashmap__size(ctx->ids) == 1);

	TEST_ASSERT_VAL("find ids", hashmap__find(ctx->ids,

						  smt_on() ? "EVENT1" : "EVENT2",

						  (void **)&val_ptr));



	expr__ctx_free(ctx);



	return 0;

tools/perf/util/expr.y

+23 −7
Original line number Diff line number Diff line
@@ -123,14 +123,30 @@ start: if_expr 


if_expr: expr IF expr ELSE expr

{

	if (!compute_ids) {

		$$.ids = NULL;

	if (fpclassify($3.val) == FP_ZERO) {

		/*

		 * The IF expression evaluated to 0 so treat as false, take the

		 * ELSE and discard everything else.

		 */

		$$.val = $5.val;

		} else {

		$$.ids = $5.ids;

		ids__free($1.ids);

		ids__free($3.ids);

	} else if (!compute_ids || is_const($3.val)) {

		/*

		 * If ids aren't computed then treat the expression as true. If

		 * ids are being computed and the IF expr is a non-zero

		 * constant, then also evaluate the true case.

		 */

		$$.val = $1.val;

		}

		$$.ids = $1.ids;

		ids__free($3.ids);

		ids__free($5.ids);

	} else {

		/*

		 * Value is either the LHS or RHS and we need the IF expression

		 * to compute it.

		 */

		$$ = union_expr($1, union_expr($3, $5));

	}

}