• Ravi Bangoria's avatar
    uprobes: Support SDT markers having reference count (semaphore) · 1cc33161
    Ravi Bangoria authored
    Userspace Statically Defined Tracepoints[1] are dtrace style markers
    inside userspace applications. Applications like PostgreSQL, MySQL,
    Pthread, Perl, Python, Java, Ruby, Node.js, libvirt, QEMU, glib etc
    have these markers embedded in them. These markers are added by developer
    at important places in the code. Each marker source expands to a single
    nop instruction in the compiled code but there may be additional
    overhead for computing the marker arguments which expands to couple of
    instructions. In case the overhead is more, execution of it can be
    omitted by runtime if() condition when no one is tracing on the marker:
    
        if (reference_counter > 0) {
            Execute marker instructions;
        }
    
    Default value of reference counter is 0. Tracer has to increment the
    reference counter before tracing on a marker and decrement it when
    done with the tracing.
    
    Implement the reference counter logic in core uprobe. User will be
    able to use it from trace_uprobe as well as from kernel module. New
    trace_uprobe definition with reference counter will now be:
    
        <path>:<offset>[(ref_ctr_offset)]
    
    where ref_ctr_offset is an optional field. For kernel module, new
    variant of uprobe_register() has been introduced:
    
        uprobe_register_refctr(inode, offset, ref_ctr_offset, consumer)
    
    No new variant for uprobe_unregister() because it's assumed to have
    only one reference counter for one uprobe.
    
    [1] https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
    
    Note: 'reference counter' is called as 'semaphore' in original Dtrace
    (or Systemtap, bcc and even in ELF) documentation and code. But the
    term 'semaphore' is misleading in this context. This is just a counter
    used to hold number of tracers tracing on a marker. This is not really
    used for any synchronization. So we are calling it a 'reference counter'
    in kernel / perf code.
    
    Link: http://lkml.kernel.org/r/20180820044250.11659-2-ravi.bangoria@linux.ibm.comReviewed-by: default avatarMasami Hiramatsu <mhiramat@kernel.org>
    [Only trace_uprobe.c]
    Reviewed-by: default avatarOleg Nesterov <oleg@redhat.com>
    Reviewed-by: default avatarSong Liu <songliubraving@fb.com>
    Tested-by: default avatarSong Liu <songliubraving@fb.com>
    Signed-off-by: default avatarRavi Bangoria <ravi.bangoria@linux.ibm.com>
    Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    1cc33161
uprobes.c 55.8 KB