Date: Fri, 26 Sep 2008 12:06:58 -0700 From: Dilip Chhetri <dchhetri@panasas.com> To: freebsd-threads@freebsd.org Subject: getting stack trace for other thread on the same process : libthr Message-ID: <48DD32D2.2060304@panasas.com>
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Question
--------
My program is linked with libthr in FreeBSD-7.0. The program has in
the order of 20 threads, and a designated monitoring thread at some
point wants to know what are other/stuck threads doing. This needs to be
done by printing stack backtrace for the thread to stdout.
I understand pthread_t structure has pointer to the target thread's
stack, but to get the trace I need to know value of stack-pointer
register and base-pointer register. I looked at the code and I don't
find any mechanism by which I could read the target threads register
context (because it all resides within kernel thread structure). Further
code study reveals that kernel_thread->td_frame contains the register
context for a thread, but is valid only when the thread is
executing/sleeping inside the kernel.
Is there anything I'm missing here ? Is there an easy way to traverse
stack for some thread with in the same process.
I considered/considering following approaches,
a) use PTRACE
ruled out, because you can't trace the process from within the
same process
b) somehow temporarily stop the target-thread and read td_frame by
traversing kernel data structure through /dev/kmem. After doing
stack traversal resume the target thread.
Detailed problem background
--------------------------
We have this process X with ~20 threads, each processing some
requests. One of them is designated as monitoring/dispatcher thread.
When a new request arrives, dispatcher thread tries to queue the task to
idle thread. But if all threads are busy processing requests, the
dispatcher thread is supposed to print the stack back trace for each of
the busy thread. This is our *debugging* mechanism to find potential
fault-points.
In FreeBSD-4.6.2, we hacked libc_r:pthread_t to achieve our goal. But
in FreeBSD-7.0, we decided to use libthr and hack doesn't seem to be easy.
Target setup
------------
* SMP : around 8 CPU
* process : it's going to be run as root and have around ~20 threads
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