[PATCH] udevd: throttle the forking of processes

If the system reaches a defined limit of processes in running state, udevd
starts to count its own processes in running state from its session (all
forked hotplug child processes, subprocesses and callouts) and throttles
further process forking if the limit is reached.
  
This should help setups with hundreds of events emitted hotplug events
in parallel with hundreds of processes in "R" state. which makes the machine
unresponsible.
  
I placed a 100% cpu time consuming program in /etc/hotplug.d/ which runs for 5
seconds. With this patch I can load "scsi_debug add_host=100" without any major
problem. Without the patch the box is completly unresponsible for many minutes.

1 files changed, 126 insertions, 9 deletions

diff --git a/udevd.c b/udevd.c
index a8328bbb81..53c7bf3df1 100644
--- a/udevd.c
+++ b/udevd.c
@@ -28,11 +28,13 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <ctype.h>
+#include <dirent.h>
+#include <fcntl.h>
 #include <sys/time.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/un.h>
-#include <fcntl.h>
 #include <sys/sysinfo.h>
 #include <sys/stat.h>
 
@@ -45,6 +47,7 @@
 
 /* global variables*/
 static int udevsendsock;
+static pid_t sid;
 
 static int pipefds[2];
 static long startup_time;
@@ -144,6 +147,8 @@ static void udev_run(struct hotplug_msg *msg)
 		/* child */
 		close(udevsendsock);
 		logging_close();
+
+		setpriority(PRIO_PROCESS, 0, UDEV_PRIORITY);
 		execve(udev_bin, argv, msg->envp);
 		dbg("exec of child failed");
 		_exit(1);
@@ -151,9 +156,6 @@ static void udev_run(struct hotplug_msg *msg)
 	case -1:
 		dbg("fork of child failed");
 		run_queue_delete(msg);
-		/* note: we never managed to run, so we had no impact on 
-		 * running_with_devpath(), so don't bother setting run_exec_q
-		 */
 		break;
 	default:
 		/* get SIGCHLD in main loop */
@@ -162,6 +164,100 @@ static void udev_run(struct hotplug_msg *msg)
 	}
 }
 
+static int running_processes(void)
+{
+	int f;
+	static char buf[4096];
+	int len;
+	int running;
+	const char *pos;
+
+	f = open("/proc/stat", O_RDONLY);
+	if (f == -1)
+		return -1;
+
+	len = read(f, buf, sizeof(buf));
+	close(f);
+
+	if (len <= 0)
+		return -1;
+	else
+		buf[len] = '\0';
+
+	pos = strstr(buf, "procs_running ");
+	if (pos == NULL)
+		return -1;
+
+	if (sscanf(pos, "procs_running %u", &running) != 1)
+		return -1;
+
+	return running;
+}
+
+/* return the number of process es in our session, count only until limit */
+static int running_processes_in_session(pid_t session, int limit)
+{
+	DIR *dir;
+	struct dirent *dent;
+	int running = 0;
+
+	dir = opendir("/proc");
+	if (!dir)
+		return -1;
+
+	/* read process info from /proc */
+	for (dent = readdir(dir); dent != NULL; dent = readdir(dir)) {
+		int f;
+		char procdir[64];
+		char line[256];
+		const char *pos;
+		char state;
+		pid_t ppid, pgrp, sess;
+		int len;
+
+		if (!isdigit(dent->d_name[0]))
+			continue;
+
+		snprintf(procdir, sizeof(procdir), "/proc/%s/stat", dent->d_name);
+		procdir[sizeof(procdir)-1] = '\0';
+
+		f = open(procdir, O_RDONLY);
+		if (f == -1)
+			continue;
+
+		len = read(f, line, sizeof(line));
+		close(f);
+
+		if (len <= 0)
+			continue;
+		else
+			line[len] = '\0';
+
+		/* skip ugly program name */
+		pos = strrchr(line, ')') + 2;
+		if (pos == NULL)
+			continue;
+
+		if (sscanf(pos, "%c %d %d %d ", &state, &ppid, &pgrp, &sess) != 4)
+			continue;
+
+		/* count only processes in our session */
+		if (sess != session)
+			continue;
+
+		/* count only running, no sleeping processes */
+		if (state != 'R')
+			continue;
+
+		running++;
+		if (limit > 0 && running >= limit)
+			break;
+	}
+	closedir(dir);
+
+	return running;
+}
+
 static int compare_devpath(const char *running, const char *waiting)
 {
 	int i;
@@ -218,13 +314,30 @@ static void exec_queue_manager(void)
 	struct hotplug_msg *loop_msg;
 	struct hotplug_msg *tmp_msg;
 	struct hotplug_msg *msg;
+	int running;
+
+	running = running_processes();
+	dbg("%d processes runnning on system", running);
+	if (running < 0)
+		running = THROTTLE_MAX_RUNNING_CHILDS;
 
 	list_for_each_entry_safe(loop_msg, tmp_msg, &exec_list, list) {
+		/* check running processes in our session and possibly throttle */
+		if (running >= THROTTLE_MAX_RUNNING_CHILDS) {
+			running = running_processes_in_session(sid, THROTTLE_MAX_RUNNING_CHILDS+10);
+			dbg("%d processes running in session", running);
+			if (running >= THROTTLE_MAX_RUNNING_CHILDS) {
+				dbg("delay seq %llu, cause too many processes already running", loop_msg->seqnum);
+				return;
+			}
+		}
+
 		msg = running_with_devpath(loop_msg);
 		if (!msg) {
 			/* move event to run list */
 			list_move_tail(&loop_msg->list, &running_list);
 			udev_run(loop_msg);
+			running++;
 			dbg("moved seq %llu to running list", loop_msg->seqnum);
 		} else {
 			dbg("delay seq %llu (%s), cause seq %llu (%s) is still running",
@@ -529,10 +642,17 @@ int main(int argc, char *argv[], char *envp[])
 		}
 	}
 
+	/* become session leader */
+	sid = setsid();
+	dbg("our session is %d", sid);
+
 	/* make sure we don't lock any path */
 	chdir("/");
 	umask(umask(077) | 022);
 
+	/*set a reasonable scheduling priority for the daemon */
+	setpriority(PRIO_PROCESS, 0, UDEVD_PRIORITY);
+
 	/* Set fds to dev/null */
 	fd = open( "/dev/null", O_RDWR );
 	if (fd >= 0)  {
@@ -544,9 +664,6 @@ int main(int argc, char *argv[], char *envp[])
 	} else
 		dbg("error opening /dev/null %s", strerror(errno));
 
-	/* become session leader */
-	setsid();
-
 	/* setup signal handler pipe */
 	retval = pipe(pipefds);
 	if (retval < 0) {
@@ -597,14 +714,14 @@ int main(int argc, char *argv[], char *envp[])
 	else
 		udev_bin = UDEV_BIN;
 
-	/* possible set of expected_seqnum number */
+	/* possible init of expected_seqnum value */
 	udevd_expected_seqnum = getenv("UDEVD_EXPECTED_SEQNUM");
 	if (udevd_expected_seqnum != NULL) {
 		expected_seqnum = strtoull(udevd_expected_seqnum, NULL, 10);
 		dbg("initialize expected_seqnum to %llu", expected_seqnum);
 	}
 
-	/* get current time to provide shorter startup timeout */
+	/* get current time to provide shorter timeout on startup */
 	sysinfo(&info);
 	startup_time = info.uptime;