2 files changed, 17 insertions, 29 deletions

diff --git a/Documentation/accounting/delay-accounting.txt b/Documentation/accounting/delay-accounting.txt
index f3dc0ca04fa4..be215e58423b 100644
--- a/Documentation/accounting/delay-accounting.txt
+++ b/Documentation/accounting/delay-accounting.txt
@@ -48,9 +48,10 @@ counter (say cpu_delay_total) for a task will give the delay
 experienced by the task waiting for the corresponding resource
 in that interval.
 
-When a task exits, records containing the per-task and per-process statistics
-are sent to userspace without requiring a command. More details are given in
-the taskstats interface description.
+When a task exits, records containing the per-task statistics
+are sent to userspace without requiring a command. If it is the last exiting
+task of a thread group, the per-tgid statistics are also sent. More details
+are given in the taskstats interface description.
 
 The getdelays.c userspace utility in this directory allows simple commands to
 be run and the corresponding delay statistics to be displayed. It also serves
@@ -107,9 +108,3 @@ IO	count	delay total
 	0	0
 MEM	count	delay total
 	0	0
-
-
-
-
-
-
diff --git a/Documentation/accounting/taskstats.txt b/Documentation/accounting/taskstats.txt
index acc6b4f37fc7..efd8f605bcd5 100644
--- a/Documentation/accounting/taskstats.txt
+++ b/Documentation/accounting/taskstats.txt
@@ -32,12 +32,11 @@ 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, two records are sent by the kernel to
-each listener on the multicast socket. The first the per-pid task's statistics
-and the second is the sum for all tasks of the process to which the task
-belongs (the task does not need to be the thread group leader). The need for
-per-tgid stats to be sent for each exiting task is explained in the per-tgid
-stats section below.
+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.
 
 getdelays.c is a simple utility demonstrating usage of the taskstats interface
 for reporting delay accounting statistics.
@@ -104,20 +103,14 @@ stats in userspace alone is inefficient and potentially inaccurate (due to lack
 of atomicity).
 
 However, maintaining per-process, in addition to per-task stats, within the
-kernel has space and time overheads. Hence the taskstats implementation
-dynamically sums up the per-task stats for each task belonging to a process
-whenever per-process stats are needed.
-
-Not maintaining per-tgid stats creates a problem when userspace is interested
-in getting these stats when the process dies i.e. the last thread of
-a process exits. It isn't possible to simply return some aggregated per-process
-statistic from the kernel.
-
-The approach taken by taskstats is to return the per-tgid stats *each* time
-a task exits, in addition to the per-pid stats for that task. Userspace can
-maintain task<->process mappings and use them to maintain the per-process stats
-in userspace, updating the aggregate appropriately as the tasks of a process
-exit.
+kernel has space and time overheads. To address this, the taskstats code
+accumalates each exiting task's statistics into a process-wide data structure.
+When the last task of a process exits, the process level data accumalated also
+gets sent to userspace (along with the per-task data).
+
+When a user queries to get per-tgid data, the sum of all other live threads in
+the group is added up and added to the accumalated total for previously exited
+threads of the same thread group.
 
 Extending taskstats
 -------------------