[PATCH V3] rtla: Remove procps-ng dependency

[Daniel Bristot de Oliveira <bristot@kernel.org>]

Daniel Wagner reported to me that readproc.h got deprecated. Also,
while the procps-ng library was available on Fedora, it was not available
on RHEL, which is a piece of evidence that it was not that used.

rtla uses procps-ng only to find the PID of the tracers' workload.

I used the procps-ng library to avoid reinventing the wheel. But in this
case, reinventing the wheel took me less time than the time we already
took trying to work around problems.

Implement a function that reads /proc/ entries, checking if:
	- the entry is a directory
	- the directory name is composed only of digits (PID)
	- the directory contains the comm file
	- the comm file contains a comm that matches the tracers'
	  workload prefix.
	- then return true; otherwise, return false.

And use it instead of procps-ng.

Cc: John Kacur <jkacur@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Tao Zhou <tao.zhou@linux.dev>
Fixes: b1696371d865 ("rtla: Helper functions for rtla")
Reported-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
---
Changes from V2:
 - Add a space in char *t_name;
 - s/procps/procfs/ in set_comm_sched_attr() comment

 tools/tracing/rtla/Makefile    |   2 +-
 tools/tracing/rtla/README.txt  |   1 -
 tools/tracing/rtla/src/utils.c | 102 ++++++++++++++++++++++++++-------
 tools/tracing/rtla/src/utils.h |   3 +-
 4 files changed, 84 insertions(+), 24 deletions(-)

diff --git a/tools/tracing/rtla/Makefile b/tools/tracing/rtla/Makefile
index 5a3226e436ef..523f0a8c38c2 100644
--- a/tools/tracing/rtla/Makefile
+++ b/tools/tracing/rtla/Makefile
@@ -32,7 +32,7 @@ TRACEFS_HEADERS	:= $$($(PKG_CONFIG) --cflags libtracefs)
 
 CFLAGS	:=	-O -g -DVERSION=\"$(VERSION)\" $(FOPTS) $(MOPTS) $(WOPTS) $(TRACEFS_HEADERS)
 LDFLAGS	:=	-ggdb
-LIBS	:=	$$($(PKG_CONFIG) --libs libtracefs) -lprocps
+LIBS	:=	$$($(PKG_CONFIG) --libs libtracefs)
 
 SRC	:=	$(wildcard src/*.c)
 HDR	:=	$(wildcard src/*.h)
diff --git a/tools/tracing/rtla/README.txt b/tools/tracing/rtla/README.txt
index 0fbad2640b8c..4af3fd40f171 100644
--- a/tools/tracing/rtla/README.txt
+++ b/tools/tracing/rtla/README.txt
@@ -11,7 +11,6 @@ RTLA depends on the following libraries and tools:
 
  - libtracefs
  - libtraceevent
- - procps
 
 It also depends on python3-docutils to compile man pages.
 
diff --git a/tools/tracing/rtla/src/utils.c b/tools/tracing/rtla/src/utils.c
index 3bd6f64780cf..f2e9fc282da4 100644
--- a/tools/tracing/rtla/src/utils.c
+++ b/tools/tracing/rtla/src/utils.c
@@ -3,7 +3,7 @@
  * Copyright (C) 2021 Red Hat Inc, Daniel Bristot de Oliveira <bristot@kernel.org>
  */
 
-#include <proc/readproc.h>
+#include <dirent.h>
 #include <stdarg.h>
 #include <stdlib.h>
 #include <string.h>
@@ -262,43 +262,103 @@ int __set_sched_attr(int pid, struct sched_attr *attr)
 
 	return 0;
 }
+
 /*
- * set_comm_sched_attr - set sched params to threads starting with char *comm
+ * procfs_is_workload_pid - check if a procfs entry contains a comm_prefix* comm
+ *
+ * Check if the procfs entry is a directory of a process, and then check if the
+ * process has a comm with the prefix set in char *comm_prefix. As the
+ * current users of this function only check for kernel threads, there is no
+ * need to check for the threads for the process.
  *
- * This function uses procps to list the currently running threads and then
- * set the sched_attr *attr to the threads that start with char *comm. It is
+ * Return: True if the proc_entry contains a comm file with comm_prefix*.
+ * Otherwise returns false.
+ */
+static int procfs_is_workload_pid(const char *comm_prefix, struct dirent *proc_entry)
+{
+	char buffer[MAX_PATH];
+	int comm_fd, retval;
+	char *t_name;
+
+	if (proc_entry->d_type != DT_DIR)
+		return 0;
+
+	if (*proc_entry->d_name == '.')
+		return 0;
+
+	/* check if the string is a pid */
+	for (t_name = proc_entry->d_name; t_name; t_name++) {
+		if (!isdigit(*t_name))
+			break;
+	}
+
+	if (*t_name != '\0')
+		return 0;
+
+	snprintf(buffer, MAX_PATH, "/proc/%s/comm", proc_entry->d_name);
+	comm_fd = open(buffer, O_RDONLY);
+	if (comm_fd < 0)
+		return 0;
+
+	memset(buffer, 0, MAX_PATH);
+	retval = read(comm_fd, buffer, MAX_PATH);
+
+	close(comm_fd);
+
+	if (retval <= 0)
+		return 0;
+
+	retval = strncmp(comm_prefix, buffer, strlen(comm_prefix));
+	if (retval)
+		return 0;
+
+	/* comm already have \n */
+	debug_msg("Found workload pid:%s comm:%s", proc_entry->d_name, buffer);
+
+	return 1;
+}
+
+/*
+ * set_comm_sched_attr - set sched params to threads starting with char *comm_prefix
+ *
+ * This function uses /procfs to list the currently running threads and then set the
+ * sched_attr *attr to the threads that start with char *comm_prefix. It is
  * mainly used to set the priority to the kernel threads created by the
  * tracers.
  */
-int set_comm_sched_attr(const char *comm, struct sched_attr *attr)
+int set_comm_sched_attr(const char *comm_prefix, struct sched_attr *attr)
 {
-	int flags = PROC_FILLCOM | PROC_FILLSTAT;
-	PROCTAB *ptp;
-	proc_t task;
+	struct dirent *proc_entry;
+	DIR *procfs;
 	int retval;
 
-	ptp = openproc(flags);
-	if (!ptp) {
-		err_msg("error openproc()\n");
-		return -ENOENT;
+	if (strlen(comm_prefix) >= MAX_PATH) {
+		err_msg("Command prefix is too long: %d < strlen(%s)\n",
+			MAX_PATH, comm_prefix);
+		exit(EINVAL);
 	}
 
-	memset(&task, 0, sizeof(task));
+	procfs = opendir("/proc");
+
+	while ((proc_entry = readdir(procfs))) {
 
-	while (readproc(ptp, &task)) {
-		retval = strncmp(comm, task.cmd, strlen(comm));
-		if (retval)
+		retval = procfs_is_workload_pid(comm_prefix, proc_entry);
+		if (!retval)
 			continue;
-		retval = __set_sched_attr(task.tid, attr);
-		if (retval)
+
+		/* procfs_is_workload_pid confirmed it is a pid */
+		retval = __set_sched_attr(atoi(proc_entry->d_name), attr);
+		if (retval) {
+			err_msg("Error setting sched attributes for pid:%s\n", proc_entry->d_name);
 			goto out_err;
-	}
+		}
 
-	closeproc(ptp);
+		debug_msg("Set sched attributes for pid:%s\n", proc_entry->d_name);
+	}
 	return 0;
 
 out_err:
-	closeproc(ptp);
+	closedir(procfs);
 	return 1;
 }
 
diff --git a/tools/tracing/rtla/src/utils.h b/tools/tracing/rtla/src/utils.h
index fa08e374870a..5571afd3b549 100644
--- a/tools/tracing/rtla/src/utils.h
+++ b/tools/tracing/rtla/src/utils.h
@@ -6,6 +6,7 @@
  * '18446744073709551615\0'
  */
 #define BUFF_U64_STR_SIZE	24
+#define MAX_PATH		1024
 
 #define container_of(ptr, type, member)({			\
 	const typeof(((type *)0)->member) *__mptr = (ptr);	\
@@ -53,5 +54,5 @@ struct sched_attr {
 };
 
 int parse_prio(char *arg, struct sched_attr *sched_param);
-int set_comm_sched_attr(const char *comm, struct sched_attr *attr);
+int set_comm_sched_attr(const char *comm_prefix, struct sched_attr *attr);
 int set_cpu_dma_latency(int32_t latency);
-- 
2.32.0