LTTV 0.8.5

[lttv.git] / ltt / branches / poly / doc / developer / lttng-userspace-tracing.txt

Some thoughts about userspace tracing

Mathieu Desnoyers January 2006



* Goals

Fast and secure user space tracing.

Fast : 

- 5000ns for a system call is too long. Writing an event directly to memory
	takes 220ns.
- Still, we can afford a system call for buffer switch, which occurs less often.
- No locking, no signal disabling. Disabling signals require 2 system calls.
	Mutexes are implemented with a short spin lock, followed by a yield. Yet
	another system call. In addition, we have no way to know on which CPU we are
	running when in user mode. We can be preempted anywhere.
- No contention.
- No interrupt disabling : it doesn't exist in user mode.

Secure :

- A process shouldn't be able to corrupt the system's trace or another
	process'trace. It should be limited to its own memory space.



* Solution

- Signal handler concurrency

Using atomic space reservation in the buffer(s) will remove the requirement for
locking. This is the fast and safe way to deal with concurrency coming from
signal handlers.

- Start/stop tracing

Two possible solutions :

Either we export a read-only memory page from kernel to user space. That would
be somehow seen as a hack, as I have never even seen such interface anywhere
else. It may lead to problems related to exported types. The proper, but slow,
way to do it would be to have a system call that would return the tracing
status.

My suggestion is to go for a system call, but only call it :

- when the process starts
- when receiving a SIG_UPDTRACING

Two possibilities :

- one system call per information to get/one system call to get all information.
- one signal per information to get/one signal for "update" tracing info.

I would tend to adopt :

- One signal for "general tracing update"
	One signal handler would clearly be enough, more would be unnecessary
	overhead/pollution.
- One system call for all updates.
	We will need to have multiple parameters though. We have up to 6 parameters.

syscall get_tracing_info

first parameter : active traces mask (32 bits : 32 traces).


Concurrency

We must have per thread buffers. Then, no memory can be written by two threads
at once. It removes the need for locks (ok, atomic reservation was already doing
that) and removes false sharing.


Multiple traces

By having the number of active traces, we can allocate as much buffers as we
need. The only thing is that the buffers will only be allocated when receiving
the signal/starting the process and getting the number of traces actives.

It means that we must make sure to only update the data structures used by
tracing functions once the buffers are created.

When adding a new buffer, we should call the set_tracing_info syscall and give
the new buffers array to the kernel. It's an array of 32 pointers to user pages.
They will be used by the kernel to get the last pages when the thread dies.

If we remove a trace, the kernel should stop the tracing, and then get the last
buffer for this trace. What is important is to make sure no writers are still
trying to write in a memory region that get desallocated.

For that, we will keep an atomic variable "tracing_level", which tells how many
times we are nested in tracing code (program code/signal handlers) for a
specific trace.

We could do that trace removal in two operations :

- Send an update tracing signal to the process
	- the sig handler get the new tracing status, which tells that tracing is 
		disabled for the specific trace. It writes this status in the tracing
		control structure of the process.
	- If tracing_level is 0, well, it's fine : there are no potential writers in
		the removed trace. It's up to us to buffer switch the removed trace, and,
		after the control returns to us, set_tracing_info this page to NULL and
		delete this memory area.
	- Else (tracing_level > 0), flag the removed trace for later switch/delete.
	
	It then returns control to the process.

- If the tracing_level was > 0, there was one or more writers potentially
	accessing this memory area. When the control comes back to the writer, at the
	end of the write in a trace, if the trace is marked for switch/delete and the
	tracing_level is 0 (after the decrement of the writer itself), then the
	writer must buffer switch, set_tracing_info to NULL and then delete the
	memory area.


Filter

The update tracing info signal will make the thread get the new filter
information. Getting this information will also happen upon process creation.

parameter 2 for the get tracing info : array of 32 ints (32 bits).
Each integer is the filter mask for a trace. As there are up to 32 active
traces, we have 32 integers for filter.


Buffer switch

There could be a tracing_buffer_switch system call, that would give the page
start address as parameter. The job of the kernel is to steal this page,
possibly replacing it with a zeroed page (we don't care about the content of the
page after the syscall).

Process dying

The kernel should be aware of the current pages used for tracing in each thread.
If a thread dies unexpectedly, we want the kernel to get the last bits of
information before the thread crashes.

syscall set_tracing_info

parameter 1 : array of 32 user space pointers to current pages or NULL.


Memory protection

We want each process to be usable to make a trace unreadable, and each process
to have its own memory space.

Two possibilities :

Either we create one channel per process, or we have per cpu tracefiles for all
the processes, with the specification that data is written in a monotically
increasing time order and that no process share a 4k page with another process.

The problem with having only one tracefile per cpu is that we cannot safely
steal a process'buffer upon a schedule change because it may be currently
writing to it.

It leaves the one tracefile per thread as the only solution.

Another argument in favor of this solution is the possibility to have mixed
32-64 bits processes on the same machine. Dealing with types will be easier.


Corrupted trace

A corrupted tracefile will only affect one thread. The rest of the trace will
still be readable.


Facilities

Upon process creation or when receiving the signal of trace info update, when a
new trace appears, the thread should write the facility information into it. It
must then have a list of registered facilities, all done at the thread level.

We must decide if we allow a facility channel for each thread. The advantage is
that we have a readable channel in flight recorder mode, while the disadvantage
is to duplicate the number of channels, which may become quite high. To follow
the general design of a high throughput channel and a low throughput channel for
vital information, I suggest to have a separate channel for facilities, per
trace, per process.