diff options
Diffstat (limited to 'Documentation/accounting/taskstats.txt')
| -rw-r--r-- | Documentation/accounting/taskstats.txt | 33 |
1 files changed, 13 insertions, 20 deletions
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 ------------------- |