path: root/Documentation/accounting/taskstats.txt
diff options
Diffstat (limited to 'Documentation/accounting/taskstats.txt')
1 files changed, 52 insertions, 12 deletions
diff --git a/Documentation/accounting/taskstats.txt b/Documentation/accounting/taskstats.txt
index efd8f605bcd..92ebf29e904 100644
--- a/Documentation/accounting/taskstats.txt
+++ b/Documentation/accounting/taskstats.txt
@@ -26,20 +26,28 @@ leader - a process is deemed alive as long as it has any task belonging to it.
-To get statistics during task's lifetime, userspace opens a unicast netlink
+To get statistics during a task's lifetime, userspace opens a unicast netlink
socket (NETLINK_GENERIC family) and sends commands specifying a pid or a tgid.
The response contains statistics for a task (if pid is specified) or the sum of
statistics for all tasks of the process (if tgid is specified).
-To obtain statistics for tasks which are exiting, userspace opens a multicast
-netlink socket. Each time a task exits, its per-pid statistics is always sent
-by the kernel to each listener on the multicast socket. In addition, if it is
-the last thread exiting its thread group, an additional record containing the
-per-tgid stats are also sent. The latter contains the sum of per-pid stats for
-all threads in the thread group, both past and present.
+To obtain statistics for tasks which are exiting, the userspace listener
+sends a register command and specifies a cpumask. Whenever a task exits on
+one of the cpus in the cpumask, its per-pid statistics are sent to the
+registered listener. Using cpumasks allows the data received by one listener
+to be limited and assists in flow control over the netlink interface and is
+explained in more detail below.
+If the exiting task is the last thread exiting its thread group,
+an additional record containing the per-tgid stats is also sent to userspace.
+The latter contains the sum of per-pid stats for all threads in the thread
+group, both past and present.
getdelays.c is a simple utility demonstrating usage of the taskstats interface
-for reporting delay accounting statistics.
+for reporting delay accounting statistics. Users can register cpumasks,
+send commands and process responses, listen for per-tid/tgid exit data,
+write the data received to a file and do basic flow control by increasing
+receive buffer sizes.
@@ -66,10 +74,20 @@ The messages are in the format
The taskstats payload is one of the following three kinds:
-1. Commands: Sent from user to kernel. The payload is one attribute, of type
-TASKSTATS_CMD_ATTR_PID/TGID, containing a u32 pid or tgid in the attribute
-payload. The pid/tgid denotes the task/process for which userspace wants
+1. Commands: Sent from user to kernel. Commands to get data on
+a pid/tgid consist of one attribute, of type TASKSTATS_CMD_ATTR_PID/TGID,
+containing a u32 pid or tgid in the attribute payload. The pid/tgid denotes
+the task/process for which userspace wants statistics.
+Commands to register/deregister interest in exit data from a set of cpus
+consist of one attribute, of type
+attribute payload. The cpumask is specified as an ascii string of
+comma-separated cpu ranges e.g. to listen to exit data from cpus 1,2,3,5,7,8
+the cpumask would be "1-3,5,7-8". If userspace forgets to deregister interest
+in cpus before closing the listening socket, the kernel cleans up its interest
+set over time. However, for the sake of efficiency, an explicit deregistration
+is advisable.
2. Response for a command: sent from the kernel in response to a userspace
command. The payload is a series of three attributes of type:
@@ -138,4 +156,26 @@ struct too much, requiring disparate userspace accounting utilities to
unnecessarily receive large structures whose fields are of no interest, then
extending the attributes structure would be worthwhile.
+Flow control for taskstats
+When the rate of task exits becomes large, a listener may not be able to keep
+up with the kernel's rate of sending per-tid/tgid exit data leading to data
+loss. This possibility gets compounded when the taskstats structure gets
+extended and the number of cpus grows large.
+To avoid losing statistics, userspace should do one or more of the following:
+- increase the receive buffer sizes for the netlink sockets opened by
+listeners to receive exit data.
+- create more listeners and reduce the number of cpus being listened to by
+each listener. In the extreme case, there could be one listener for each cpu.
+Users may also consider setting the cpu affinity of the listener to the subset
+of cpus to which it listens, especially if they are listening to just one cpu.
+Despite these measures, if the userspace receives ENOBUFS error messages
+indicated overflow of receive buffers, it should take measures to handle the
+loss of data.