linux-kernel - [PATCH] perf synthetic-events: save 4kb from 2 stack frames

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Date:   Thu, 26 Mar 2020 23:36:51 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org
Cc:     Stephane Eranian <eranian@...gle.com>,
        Ian Rogers <irogers@...gle.com>
Subject: [PATCH] perf synthetic-events: save 4kb from 2 stack frames

Reduce the scope of PATH_MAX sized char buffers so that the compiler can
overlap their usage.

perf_event__synthesize_mmap_events before 'sub $0x45b8,%rsp' after
'sub $0x35b8,%rsp'.

perf_event__get_comm_ids before 'sub $0x2028,%rsp' after 'sub $0x1028,%rsp'.

Signed-off-by: Ian Rogers <irogers@...gle.com>
---
 tools/perf/util/synthetic-events.c | 46 ++++++++++++++++++------------
 1 file changed, 27 insertions(+), 19 deletions(-)

diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c
index 3f28af39f9c6..9ff54707bb30 100644
--- a/tools/perf/util/synthetic-events.c
+++ b/tools/perf/util/synthetic-events.c
@@ -70,7 +70,6 @@ int perf_tool__process_synth_event(struct perf_tool *tool,
 static int perf_event__get_comm_ids(pid_t pid, char *comm, size_t len,
 				    pid_t *tgid, pid_t *ppid)
 {
-	char filename[PATH_MAX];
 	char bf[4096];
 	int fd;
 	size_t size = 0;
@@ -80,12 +79,16 @@ static int perf_event__get_comm_ids(pid_t pid, char *comm, size_t len,
 	*tgid = -1;
 	*ppid = -1;
 
-	snprintf(filename, sizeof(filename), "/proc/%d/status", pid);
+	{
+		char filename[PATH_MAX];
 
-	fd = open(filename, O_RDONLY);
-	if (fd < 0) {
-		pr_debug("couldn't open %s\n", filename);
-		return -1;
+		snprintf(filename, sizeof(filename), "/proc/%d/status", pid);
+
+		fd = open(filename, O_RDONLY);
+		if (fd < 0) {
+			pr_debug("couldn't open %s\n", filename);
+			return -1;
+		}
 	}
 
 	n = read(fd, bf, sizeof(bf) - 1);
@@ -280,7 +283,6 @@ int perf_event__synthesize_mmap_events(struct perf_tool *tool,
 				       struct machine *machine,
 				       bool mmap_data)
 {
-	char filename[PATH_MAX];
 	FILE *fp;
 	unsigned long long t;
 	bool truncation = false;
@@ -292,18 +294,22 @@ int perf_event__synthesize_mmap_events(struct perf_tool *tool,
 	if (machine__is_default_guest(machine))
 		return 0;
 
-	snprintf(filename, sizeof(filename), "%s/proc/%d/task/%d/maps",
-		 machine->root_dir, pid, pid);
+#define FILENAME_FMT_STRING "%s/proc/%d/task/%d/maps"
+	{
+		char filename[PATH_MAX];
 
-	fp = fopen(filename, "r");
-	if (fp == NULL) {
-		/*
-		 * We raced with a task exiting - just return:
-		 */
-		pr_debug("couldn't open %s\n", filename);
-		return -1;
-	}
+		snprintf(filename, sizeof(filename), FILENAME_FMT_STRING,
+			machine->root_dir, pid, pid);
 
+		fp = fopen(filename, "r");
+		if (fp == NULL) {
+			/*
+			 * We raced with a task exiting - just return:
+			 */
+			pr_debug("couldn't open %s\n", filename);
+			return -1;
+		}
+	}
 	event->header.type = PERF_RECORD_MMAP2;
 	t = rdclock();
 
@@ -320,10 +326,10 @@ int perf_event__synthesize_mmap_events(struct perf_tool *tool,
 			break;
 
 		if ((rdclock() - t) > timeout) {
-			pr_warning("Reading %s time out. "
+			pr_warning("Reading " FILENAME_FMT_STRING " time out. "
 				   "You may want to increase "
 				   "the time limit by --proc-map-timeout\n",
-				   filename);
+				   machine->root_dir, pid, pid);
 			truncation = true;
 			goto out;
 		}
@@ -412,6 +418,8 @@ int perf_event__synthesize_mmap_events(struct perf_tool *tool,
 
 	fclose(fp);
 	return rc;
+
+#undef FILENAME_FMT_STRING
 }
 
 int perf_event__synthesize_modules(struct perf_tool *tool, perf_event__handler_t process,
-- 
2.25.1.696.g5e7596f4ac-goog