[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, 10 insertions, 0 deletions

diff --git a/udevd.h b/udevd.h
index 28f62cf528..ae1d20e5ec 100644
--- a/udevd.h
+++ b/udevd.h
@@ -28,10 +28,20 @@
 #define UDEVD_SOCK_PATH			"udevd"
 #define SEND_WAIT_MAX_SECONDS		3
 #define SEND_WAIT_LOOP_PER_SECOND	10
+
+#define UDEVD_PRIORITY			-4
+#define UDEV_PRIORITY			-2
+
+/* duration of initialization phase with shorter timeout */
 #define INIT_TIME_SEC			5
 #define EVENT_INIT_TIMEOUT_SEC		2
+
+/* timeout to wait for missing events */
 #define EVENT_TIMEOUT_SEC		10
 
+/* start to throttle forking if maximum number of running childs in our session is reached */
+#define THROTTLE_MAX_RUNNING_CHILDS	10
+
 /* environment buffer, should match the kernel's size in lib/kobject_uevent.h */
 #define HOTPLUG_BUFFER_SIZE		1024
 #define HOTPLUG_NUM_ENVP		32