[PATCH 0/7] [RFC] kernel: add a netlink interface to get information about processes

Andrey Vagin
2015-02-17T08:39:42Z

Here is a preview version. It provides restricted set of functionality.
I would like to collect feedback about this idea.

Currently we use the proc file system, where all information are
presented in text files, what is convenient for humans. But if we need
to get information about processes from code (e.g. in C), the procfs
doesn't look so cool.

>From code we would prefer to get information in binary format and to be
able to specify which information and for which tasks are required. Here
is a new interface with all these features, which is called task_diag.
In addition it's much faster than procfs.

task_diag is based on netlink sockets and looks like socket-diag, which
is used to get information about sockets.

A request is described by the task_diag_pid structure:

struct task_diag_pid {
__u64 show_flags; /* specify which information are required */
__u64 dump_stratagy; /* specify a group of processes */

__u32 pid;
};

A respone is a set of netlink messages. Each message describes one task.
All task properties are divided on groups. A message contains the
TASK_DIAG_MSG group and other groups if they have been requested in
show_flags. For example, if show_flags contains TASK_DIAG_SHOW_CRED, a
response will contain the TASK_DIAG_CRED group which is described by the
task_diag_creds structure.

struct task_diag_msg {
__u32 tgid;
__u32 pid;
__u32 ppid;
__u32 tpid;
__u32 sid;
__u32 pgid;
__u8 state;
char comm[TASK_DIAG_COMM_LEN];
};

Another good feature of task_diag is an ability to request information
for a few processes. Currently here are two stratgies
TASK_DIAG_DUMP_ALL - get information for all tasks
TASK_DIAG_DUMP_CHILDREN - get information for children of a specified
tasks

The task diag is much faster than the proc file system. We don't need to
create a new file descriptor for each task. We need to send a request
and get a response. It allows to get information for a few task in one
request-response iteration.

I have compared performance of procfs and task-diag for the
"ps ax -o pid,ppid" command.

A test stand contains 10348 processes.
$ ps ax -o pid,ppid | wc -l
10348

$ time ps ax -o pid,ppid > /dev/null

real 0m1.073s
user 0m0.086s
sys 0m0.903s

$ time ./task_diag_all > /dev/null

real 0m0.037s
user 0m0.004s
sys 0m0.020s

And here are statistics about syscalls which were called by each
command.
$ perf stat -e syscalls:sys_exit* -- ps ax -o pid,ppid 2>&1 | grep syscalls | sort -n -r | head -n 5
20,713 syscalls:sys_exit_open
20,710 syscalls:sys_exit_close
20,708 syscalls:sys_exit_read
10,348 syscalls:sys_exit_newstat
31 syscalls:sys_exit_write

$ perf stat -e syscalls:sys_exit* -- ./task_diag_all 2>&1 | grep syscalls | sort -n -r | head -n 5
114 syscalls:sys_exit_recvfrom
49 syscalls:sys_exit_write
8 syscalls:sys_exit_mmap
4 syscalls:sys_exit_mprotect
3 syscalls:sys_exit_newfstat

You can find the test program from this experiment in the last patch.

The idea of this functionality was suggested by Pavel Emelyanov
(xemul@), when he found that operations with /proc forms a significant
part of a checkpointing time.

Ten years ago here was attempt to add a netlink interface to access to /proc
information:
http://lwn.net/Articles/99600/

Signed-off-by: Andrey Vagin <avagin@openvz.org>

git repo: https://github.com/avagin/linux-task-diag

Andrey Vagin (7):
[RFC] kernel: add a netlink interface to get information about tasks
kernel: move next_tgid from fs/proc
task-diag: add ability to get information about all tasks
task-diag: add a new group to get process credentials
kernel: add ability to iterate children of a specified task
task_diag: add ability to dump children
selftest: check the task_diag functinonality

fs/proc/array.c | 58 +---
fs/proc/base.c | 43 ---
include/linux/proc_fs.h | 13 +
include/uapi/linux/taskdiag.h | 89 ++++++
init/Kconfig | 12 +
kernel/Makefile | 1 +
kernel/pid.c | 94 ++++++
kernel/taskdiag.c | 343 +++++++++++++++++++++
tools/testing/selftests/task_diag/Makefile | 16 +
tools/testing/selftests/task_diag/task_diag.c | 59 ++++
tools/testing/selftests/task_diag/task_diag_all.c | 82 +++++
tools/testing/selftests/task_diag/task_diag_comm.c | 195 ++++++++++++
tools/testing/selftests/task_diag/task_diag_comm.h | 47 +++
tools/testing/selftests/task_diag/taskdiag.h | 1 +
14 files changed, 967 insertions(+), 86 deletions(-)
create mode 100644 include/uapi/linux/taskdiag.h
create mode 100644 kernel/taskdiag.c
create mode 100644 tools/testing/selftests/task_diag/Makefile
create mode 100644 tools/testing/selftests/task_diag/task_diag.c
create mode 100644 tools/testing/selftests/task_diag/task_diag_all.c
create mode 100644 tools/testing/selftests/task_diag/task_diag_comm.c
create mode 100644 tools/testing/selftests/task_diag/task_diag_comm.h
create mode 120000 tools/testing/selftests/task_diag/taskdiag.h

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Roger Luethi <rl@hellgate.ch>
--
2.1.0