• Tom Zanussi's avatar
    tracing/filters: allow on-the-fly filter switching · 0a19e53c
    Tom Zanussi authored
    This patch allows event filters to be safely removed or switched
    on-the-fly while avoiding the use of rcu or the suspension of tracing of
    previous versions.
    
    It does it by adding a new filter_pred_none() predicate function which
    does nothing and by never deallocating either the predicates or any of
    the filter_pred members used in matching; the predicate lists are
    allocated and initialized during ftrace_event_calls initialization.
    
    Whenever a filter is removed or replaced, the filter_pred_* functions
    currently in use by the affected ftrace_event_call are immediately
    switched over to to the filter_pred_none() function, while the rest of
    the filter_pred members are left intact, allowing any currently
    executing filter_pred_* functions to finish up, using the values they're
    currently using.
    
    In the case of filter replacement, the new predicate values are copied
    into the old predicates after the above step, and the filter_pred_none()
    functions are replaced by the filter_pred_* functions for the new
    filter.  In this case, it is possible though very unlikely that a
    previous filter_pred_* is still running even after the
    filter_pred_none() switch and the switch to the new filter_pred_*.  In
    that case, however, because nothing has been deallocated in the
    filter_pred, the worst that can happen is that the old filter_pred_*
    function sees the new values and as a result produces either a false
    positive or a false negative, depending on the values it finds.
    
    So one downside to this method is that rarely, it can produce a bad
    match during the filter switch, but it should be possible to live with
    that, IMHO.
    
    The other downside is that at least in this patch the predicate lists
    are always pre-allocated, taking up memory from the start.  They could
    probably be allocated on first-use, and de-allocated when tracing is
    completely stopped - if this patch makes sense, I could create another
    one to do that later on.
    
    Oh, and it also places a restriction on the size of __arrays in events,
    currently set to 128, since they can't be larger than the now embedded
    str_val arrays in the filter_pred struct.
    Signed-off-by: default avatarTom Zanussi <tzanussi@gmail.com>
    Acked-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: paulmck@linux.vnet.ibm.com
    LKML-Reference: <1239610670.6660.49.camel@tropicana>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    0a19e53c
trace_events.c 16.1 KB