• Steven Rostedt's avatar
    tracing: Replace trace_event struct array with pointer array · e4a9ea5e
    Steven Rostedt authored
    
    
    Currently the trace_event structures are placed in the _ftrace_events
    section, and at link time, the linker makes one large array of all
    the trace_event structures. On boot up, this array is read (much like
    the initcall sections) and the events are processed.
    
    The problem is that there is no guarantee that gcc will place complex
    structures nicely together in an array format. Two structures in the
    same file may be placed awkwardly, because gcc has no clue that they
    are suppose to be in an array.
    
    A hack was used previous to force the alignment to 4, to pack the
    structures together. But this caused alignment issues with other
    architectures (sparc).
    
    Instead of packing the structures into an array, the structures' addresses
    are now put into the _ftrace_event section. As pointers are always the
    natural alignment, gcc should always pack them tightly together
    (otherwise initcall, extable, etc would also fail).
    
    By having the pointers to the structures in the section, we can still
    iterate the trace_events without causing unnecessary alignment problems
    with other architectures, or depending on the current behaviour of
    gcc that will likely change in the future just to tick us kernel developers
    off a little more.
    
    The _ftrace_event section is also moved into the .init.data section
    as it is now only needed at boot up.
    Suggested-by: default avatarDavid Miller <davem@davemloft.net>
    Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Acked-by: default avatarDavid S. Miller <davem@davemloft.net>
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
    e4a9ea5e
ftrace.h 21.3 KB