[PATCH 0/9] Task Watchers v2: Introduction

Matt Helsley
2006-11-03T04:22:57Z

This is version 2 of my Task Watchers patches.

Task watchers calls functions whenever a task forks, execs, changes its
[re][ug]id, or exits.

Task watchers is primarily useful to existing kernel code as a means of making
the code in fork and exit more readable. Kernel code uses these paths by
marking a function as a task watcher much like modules mark their init
functions with module_init(). This improves the readability of copy_process().

The first patch adds the basic infrastructure of task watchers: notification
function calls in the various paths and a table of function pointers to be
called. It uses an ELF section because parts of the table must be gathered
from all over the kernel code and using the linker is easier than resolving
and maintaining complex header interdependencies. An ELF table is also ideal
because its "readonly" nature means that no locking nor list traversal are
required.

Subsequent patches adapt existing parts of the kernel to use a task watcher
-- typically in the fork, clone, and exit paths:

FEATURE (notes) RELEVANT CONFIG VARIABLE
-----------------------------------------------------------------------
audit [ CONFIG_AUDIT ... ]
semundo [ CONFIG_SYSVIPC ]
cpusets [ CONFIG_CPUSETS ]
mempolicy [ CONFIG_NUMA ]
trace irqflags [ CONFIG_TRACE_IRQFLAGS ]
lockdep [ CONFIG_LOCKDEP ]
keys (for processes -- not for thread groups) [ CONFIG_KEYS ]
process events connector [ CONFIG_PROC_EVENTS ]

TODO:
Mark the task watcher table ELF section read-only. I've tried to "fix"
the .lds files to do this with no success. I'd really appreciate help
from folks familiar with writing linker scripts.

I'm working on three more patches that add support for creating a task
watcher from within a module using an ELF section. They haven't recieved
as much attention since I've been focusing on measuring the performance
impact of these patches.

Changes:
since v2 RFC:
Updated to 2.6.19-rc2-mm2
Compiled, booted, tested, and benchmarked
Testing
Booted with audit=1 profile=2
Enabled profiling tools
Enabled auditing
Ran random syscall test
IRQ trace and lockdep CONFIG=y not tested
Benchmarks
A clone benchmark (try to clone as fast as possible)
Unrealistic. Shows incremental cost of one task watcher
A fork benchmark (try to fork as fast as possible)
Unrealistic. Shows incremental cost of one task watcher
Kernbench
Closer to realistic.
Result summaries follow changelog
See patches for details
Fork and clone samples available on request (too large for email)
Fork and clone benchmark sources will be posted as replies to 00
v2:
Dropped use of notifier chains
Dropped per-task watchers
Can be implemented on top of this
Still requires notifier chains
Dropped taskstats conversion
Parts of taskstats had to move away from the regions of
copy_process() and do_exit() where task_watchers are notified
Used linker script mechanism suggested by Al Viro
Created one "list" of watchers per event as requested by Andrew Morton
No need to multiplex a single function call
Easier to static register/unregister watchers: 1 line of code
val param now used for:
WATCH_TASK_INIT: clone_flags
WATCH_TASK_CLONE: clone_flags
WATCH_TASK_EXIT: exit code
WATCH_TASK_*: <unused>
Renamed notify_watchers() to notify_task_watchers()
Replaced: if (err != 0) --> if (err)
Added patches converting more "features" to use task watchers
Added return code handling to WATCH_TASK_INIT
Return code handling elsewhere didn't seem appropriate
since there was generally no response necessary
Fixed process keys free to handle failure in fork as originally coded
in copy_process
Added process keys code to watch for [er][ug]id changes

v1:
Added ability to cause fork to fail with NOTIFY_STOP_MASK
Added WARN_ON() when watchers cause WATCH_TASK_FREE to stop early
Moved fork invocation
Moved exec invocation
Added current as argument to exec invocation
Moved exit code assignment
Added id change invocations
(70 insertions)
v0:
Based on Jes Sorensen's Task Notifiers patches (posted to LSE-Tech)

Benchmark result summaries (sorry, this part is 86 columns):
System: 4 1.7GHz ppc64 (Power 4+) processors, 30968600MB RAM, 2.6.19-rc2-mm2 kernel

Clone - Incremental worst-case costs measured in tasks/second and as a percentage of
expected rate
Patch
1 2 3 4 5 6 7 8 9
--------------------------------------------------------------------------------------
Incremental
Cost (tasks/s) -38.12 12.5 -84 25.2 -187.5 -0.5834 -11.36 -125.2 -64.05
Cost Err 122.3 17.84 67.11 61.03 41.8 34.64 45.53 58.28 53.18
Cost (%) -0.2 0.07 -0.5 0.1 -1 -0.004 -0.06 -0.7 -0.4
Cost Err (%) 0.7 0.1 0.4 0.3 0.2 0.2 0.2 0.3 0.3

Fork - Incremental worst-case costs measured in tasks/second and as a percentage of
expected rate
Patch
1 2 3 4 5 6 7 8 9
--------------------------------------------------------------------------------------
Incremental
Cost (tasks/s) -64.58 -35.74 -33.29 -25.8 -139.5 -7.311 -9.2 -131.4 -50.47
Cost Err 54.09 27.58 41.76 42.47 49.87 60.94 29.72 39.7 40.89
Cost (%) -0.3 -0.2 -0.2 -0.1 -0.8 -0.04 -0.05 -0.7 -0.3
Cost Err (%) 0.3 0.2 0.2 0.2 0.3 0.3 0.2 0.2 0.2

Kernbench Measurements
Patch Elapsed(s) User(s) System(s) CPU(%)
- 124.406 439.947 46.615 390.700 <-- baseline 2.6.19-rc2-mm2
1 124.353 439.935 46.334 390.400
2 124.234 439.700 46.503 390.800
3 124.248 439.830 46.258 390.700
4 124.357 439.753 46.582 390.600
5 124.333 439.787 46.491 390.700
6 124.532 439.732 46.497 389.900
7 124.359 439.756 46.457 390.300
8 124.272 439.643 46.320 390.500
9 124.400 439.787 46.485 390.300

Mean: 124.349 439.787 46.454 390.490
Stddev: 0.087641 0.095917 0.115309 0.272641

Kernbench - Incremental costs
Patch Elapsed(s) User(s) System(s) CPU(%)
1 -0.053 -0.012 -0.281 -0.3
2 -0.119 -0.235 0.169 0.4
3 0.014 0.130 -0.245 -0.1
4 0.109 -0.077 0.324 -0.1
5 -0.024 0.034 -0.091 0.1
6 0.199 -0.055 0.006 -0.8
7 -0.173 0.024 -0.040 0.4
8 -0.087 -0.113 -0.137 0.2
9 0.128 0.144 0.165 -0.2

Mean: 0.005875 -0.0185 0.018875 -0.0125
Stddev: 0.13094 0.12738 0.1877 0.39074

Andrew, please consider these patches for 2.6.20's -mm tree.

Cheers,
-Matt Helsley