Date: Fri, 26 Sep 2008 23:31:27 +0200 From: Tijl Coosemans <tijl@ulyssis.org> To: Dilip Chhetri <dchhetri@panasas.com> Cc: freebsd-threads@freebsd.org Subject: Re: getting stack trace for other thread on the same process : libthr Message-ID: <200809262331.29353.tijl@ulyssis.org> In-Reply-To: <48DD32D2.2060304@panasas.com> References: <48DD32D2.2060304@panasas.com>
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On Friday 26 September 2008 21:06:58 Dilip Chhetri wrote: > 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 You could try registering a signal handler for SIGUSR1 that prints a stack backtrace using the stack pointer in the sigcontext and then call pthread_kill(SIGUSR1) on whichever thread you want a backtrace of.
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