[PATCH] delay accounting taskstats interface send tgid once

Send per-tgid data only once during exit of a thread group instead of once
with each member thread exit.

Currently, when a thread exits, besides its per-tid data, the per-tgid data
of its thread group is also sent out, if its thread group is non-empty.
The per-tgid data sent consists of the sum of per-tid stats for all
*remaining* threads of the thread group.

This patch modifies this sending in two ways:

- the per-tgid data is sent only when the last thread of a thread group
  exits.  This cuts down heavily on the overhead of sending/receiving
  per-tgid data, especially when other exploiters of the taskstats
  interface aren't interested in per-tgid stats

- the semantics of the per-tgid data sent are changed.  Instead of being
  the sum of per-tid data for remaining threads, the value now sent is the
  true total accumalated statistics for all threads that are/were part of
  the thread group.

The patch also addresses a minor issue where failure of one accounting
subsystem to fill in the taskstats structure was causing the send of
taskstats to not be sent at all.

The patch has been tested for stability and run cerberus for over 4 hours
on an SMP.

[[email protected]: bugfixes]
Signed-off-by: Shailabh Nagar <[email protected]>
Signed-off-by: Balbir Singh <[email protected]>
Cc: Jay Lan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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
-
-
-
-
-
-

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.
+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).
 
-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.
+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
 -------------------