Messages in this thread | | | Subject | Re: Task watchers v2 | From | Matt Helsley <> | Date | Mon, 18 Dec 2006 05:18:21 -0800 |
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On Mon, 2006-12-18 at 13:44 +0800, Zhang, Yanmin wrote: > On Thu, 2006-12-14 at 16:07 -0800, Matt Helsley wrote: > > plain text document attachment (task-watchers-v2) > > Associate function calls with significant events in a task's lifetime much like > > we handle kernel and module init/exit functions. This creates a table for each > > of the following events in the task_watchers_table ELF section: > > > > WATCH_TASK_INIT at the beginning of a fork/clone system call when the > > new task struct first becomes available. > > > > WATCH_TASK_CLONE just before returning successfully from a fork/clone. > > > > WATCH_TASK_EXEC just before successfully returning from the exec > > system call. > > > > WATCH_TASK_UID every time a task's real or effective user id changes. > > > > WATCH_TASK_GID every time a task's real or effective group id changes. > > > > WATCH_TASK_EXIT at the beginning of do_exit when a task is exiting > > for any reason. > > > > WATCH_TASK_FREE is called before critical task structures like > > the mm_struct become inaccessible and the task is subsequently freed. > > > > The next patch will add a debugfs interface for measuring fork and exit rates > > which can be used to calculate the overhead of the task watcher infrastructure. > > > > Subsequent patches will make use of task watchers to simplify fork, exit, > > and many of the system calls that set [er][ug]ids. > It's easier to get such watch capabilities by kprobe/systemtap. Why to > add new codes to kernel?
Good question! Disclaimer: Everything I know about kprobes I learned from Documentation/kprobes.txt
The task watchers patches have a few distinguishing capabilities yet lack capabilities important for kprobes -- so neither is a replacement for the other. Specifically:
- Task watchers are for use by the kernel for more than profiling and debugging. They need to work even when kernel debugging and instrumentation are disabled.
- Task watchers do not need to be dynamically enabled, disabled, or removed (though dynamic insertion would be nice -- I'm working on that). In fact I've been told that dynamically enabling, disabling, or removing them would incur unacceptable complexity and/or cost for an uninstrumented kernel.
- Task watchers don't require arch support. They use completely generic code. - Since they are written into the code task watchers don't need to modify instructions.
- Task watchers doesn't need to single-step an instruction
- Task watchers don't need to know about arch registers, calling conventions, etc. to work
- Task watchers don't need to have the same (possibly extensive) argument list as the function being "probed". This makes maintenance easier -- no need to keep the signature of the watchers in synch with the signature of the "probed" function.
- Task watchers don't require MODULES (2.6.20-rc1-mm1's arch/i386/Kconfig suggests this is true of kprobes).
- Task watchers don't need kernel symbols.
- Task watchers can affect flow control (see the patch hunks that change copy_process()) with their return value.
- Task watchers do not need to know the instruction address to be "probed".
- Task watchers can actually improve kernel performance slightly (up to 2% in extremely fork-heavy workloads for instance).
- Task watchers require local variables -- not necessarily arguments to the "probed" function.
- Task watchers don't care if preemption is enabled or disabled.
- Task watchers could sleep if they want to.
So to the best of my knowledge kprobes isn't a replacement for task watchers nor is task watchers capable of replacing kprobes.
Cheers, -Matt Helsley
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