• Frederic Weisbecker's avatar
    tracing/events: provide string with undefined size support · 9cbf1176
    Frederic Weisbecker authored
    This patch provides the support for dynamic size strings on
    event tracing.
    
    The key concept is to use a structure with an ending char array field of
    undefined size and use such ability to allocate the minimal size on the
    ring buffer to make one or more string entries fit inside, as opposite
    to a fixed length strings with upper bound.
    
    The strings themselves are represented using fields which have an offset
    value from the beginning of the entry.
    
    This patch provides three new macros:
    
    __string(item, src)
    
    This one declares a string to the structure inside TP_STRUCT__entry.
    You need to provide the name of the string field and the source that will
    be copied inside.
    This will also add the dynamic size of the string needed for the ring
    buffer entry allocation.
    A stack allocated structure is used to temporarily store the offset
    of each strings, avoiding double calls to strlen() on each event
    insertion.
    
    __get_str(field)
    
    This one will give you a pointer to the string you have created. This
    is an abstract helper to resolve the absolute address given the field
    name which is a relative address from the beginning of the trace_structure.
    
    __assign_str(dst, src)
    
    Use this macro to automatically perform the string copy from src to
    dst. src must be a variable to assign and dst is the name of a __string
    field.
    
    Example on how to use it:
    
    TRACE_EVENT(my_event,
    	TP_PROTO(char *src1, char *src2),
    
    	TP_ARGS(src1, src2),
    	TP_STRUCT__entry(
    		__string(str1, src1)
    		__string(str2, src2)
    	),
    	TP_fast_assign(
    		__assign_str(str1, src1);
    		__assign_str(str2, src2);
    	),
    	TP_printk("%s %s", __get_str(src1), __get_str(src2))
    )
    
    Of course you can mix-up any __field or __array inside this
    TRACE_EVENT. The position of the __string or __assign_str
    doesn't matter.
    
    Changes in v2:
    
    Address the suggestion of Steven Rostedt: drop the opening_string() macro
    and redefine __ending_string() to get the size of the string to be copied
    instead of overwritting the whole ring buffer allocation.
    
    Changes in v3:
    
    Address other suggestions of Steven Rostedt and Peter Zijlstra with
    some changes: drop the __ending_string and the need to have only one
    string field.
    Use offsets instead of absolute addresses.
    
    [ Impact: allow more compact memory usage for string tracing ]
    Signed-off-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: Li Zefan <lizf@cn.fujitsu.com>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    9cbf1176
ftrace.h 14.7 KB