Messages in this thread | | | Date | Sun, 4 May 2008 09:48:39 -0400 | From | Mathieu Desnoyers <> | Subject | System call instrumentation |
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Hi Ingo,
I looked at the system call instrumentation present in LTTng lately. I tried different solutions, e.g. hooking a kernel-wide syscall trace in do_syscall_trace, but it appears that I ended up re-doing another syscall table, which consists of specialized functions which extracts the string and data structure parameters from user-space. Since code duplication is not exactly wanted, I think that the original approach taken in my patchset, which is to instrument the kernel code at the sys_* level (e.g. sys_open), which is the earliest level where the parameter information is made available to the kernel, is still the best way to go.
I would still identify the execution mode changes in the same way I do currently, which is by instrumenting do_syscall_trace, just to know as soon as possible when the mode has changed from user-space to kernel-space so we can do time accounting more accurately. I already have the patchset which adds the KERNEL_TRACE thread flag to every architectures. It's tested in assembly in the same way SYSCALL_TRACE is tested, but is activated globally by iterating on all the threads.
So, the currently proposed scheme for a system call would be (for the open() example)
shown as : kernel stack trace: event name (parameters)
do_syscall_trace() trace: kernel_arch_syscall_entry (syscall id, instruction pointer)
do_sys_open() trace: fs_open (fd, filename)
do_syscall_trace() kernel_arch_syscall_exit (return value)
If we take this open() example, filename is ready only in do_sys_open, which is called by sys_open and sys_openat. So the logical instrumentation site for this would really be do_sys_open(). The information about which system call has been done is made available in the kernel_arch_syscall_entry event. It is not present anymore at the do_sys_open level because this execution path can be called from more than one syscall.
What do you think of this approach ?
Mathieu
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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