[PATCH perf-tools-next v2 1/4] perf ftrace: Optimise __cmd_ftrace() stack frame with strbuf streaming

[Aaron Tomlin <atomlin@atomlin.com>]

In __cmd_ftrace(), incoming trace data from trace_pipe was historically
buffered in a fixed 4096-byte array allocated directly on the stack
frame. Furthermore, reads were written directly to stdout via fwrite()
in arbitrary chunk sizes, rather than as discrete, well-delimited
trace lines.

Refactor __cmd_ftrace() to employ dynamic heap allocation for the read
buffer alongside a dynamic struct strbuf to accumulate streamed
characters into whole lines. Pre-allocating the strbuf ensures that
line processing incurs zero heap reallocations during steady-state
polling, while the stack frame of __cmd_ftrace() is reduced by over 96%
(down to ~160 bytes).

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 tools/perf/builtin-ftrace.c | 52 ++++++++++++++++++++++++++++++-------
 1 file changed, 43 insertions(+), 9 deletions(-)

diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
index 6017493ff179..4e77a0b2513f 100644
--- a/tools/perf/builtin-ftrace.c
+++ b/tools/perf/builtin-ftrace.c
@@ -41,7 +41,10 @@
 #include "util/units.h"
 #include "util/parse-sublevel-options.h"
 
+#include "util/strbuf.h"
+
 #define DEFAULT_TRACER  "function_graph"
+#define TRACE_BUF_SIZE  4096
 
 static volatile sig_atomic_t workload_exec_errno;
 static volatile sig_atomic_t done;
@@ -738,7 +741,8 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace)
 {
 	char *trace_file;
 	int trace_fd;
-	char buf[4096];
+	char *buf = NULL;
+	struct strbuf linebuf = STRBUF_INIT;
 	struct pollfd pollfd = {
 		.events = POLLIN,
 	};
@@ -767,6 +771,13 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace)
 
 	setup_pager();
 
+	buf = malloc(TRACE_BUF_SIZE);
+	if (!buf) {
+		pr_err("failed to allocate trace buffer\n");
+		goto out_reset;
+	}
+	strbuf_init(&linebuf, 512);
+
 	trace_file = get_tracing_instance_file("trace_pipe");
 	if (!trace_file) {
 		pr_err("failed to open trace_pipe\n");
@@ -810,11 +821,19 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace)
 			break;
 
 		if (pollfd.revents & POLLIN) {
-			int n = read(trace_fd, buf, sizeof(buf));
-			if (n < 0)
-				break;
-			if (fwrite(buf, n, 1, stdout) != 1)
+			int n = read(trace_fd, buf, TRACE_BUF_SIZE);
+
+			if (n <= 0)
 				break;
+			for (int i = 0; i < n; i++) {
+				if (buf[i] == '\n') {
+					fprintf(stdout, "%s\n", linebuf.buf);
+					strbuf_setlen(&linebuf, 0);
+				} else {
+					if (strbuf_addch(&linebuf, buf[i]) < 0)
+						goto out_close_fd;
+				}
+			}
 			/* flush output since stdout is in full buffering mode due to pager */
 			fflush(stdout);
 		}
@@ -823,7 +842,7 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace)
 	write_tracing_file("tracing_on", "0");
 
 	if (workload_exec_errno) {
-		const char *emsg = str_error_r(workload_exec_errno, buf, sizeof(buf));
+		const char *emsg = str_error_r(workload_exec_errno, buf, TRACE_BUF_SIZE);
 		/* flush stdout first so below error msg appears at the end. */
 		fflush(stdout);
 		pr_err("workload failed: %s\n", emsg);
@@ -832,14 +851,29 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace)
 
 	/* read remaining buffer contents */
 	while (true) {
-		int n = read(trace_fd, buf, sizeof(buf));
+		int n = read(trace_fd, buf, TRACE_BUF_SIZE);
+
 		if (n <= 0)
 			break;
-		if (fwrite(buf, n, 1, stdout) != 1)
-			break;
+		for (int i = 0; i < n; i++) {
+			if (buf[i] == '\n') {
+				fprintf(stdout, "%s\n", linebuf.buf);
+				strbuf_setlen(&linebuf, 0);
+			} else {
+				if (strbuf_addch(&linebuf, buf[i]) < 0)
+					goto out_close_fd;
+			}
+		}
+	}
+
+	if (linebuf.len > 0) {
+		fprintf(stdout, "%s\n", linebuf.buf);
+		strbuf_setlen(&linebuf, 0);
 	}
 
 out_close_fd:
+	free(buf);
+	strbuf_release(&linebuf);
 	close(trace_fd);
 out_reset:
 	exit_tracing_instance();
-- 
2.55.0