On Fri, 23 Feb 2024 14:50:49 -0500
Kent Overstreet [off-list ref] wrote:

Tangentially related though, what would make me really happy is if we
could create the string with in the TP__fast_assign() section. I have to
have a bunch of annoying wrappers right now because the string length
has to be known when we invoke the tracepoint.

You can use __string_len() to determine the string length in the tracepoint
(which is executed in the TP_fast_assign() section).

My clean up patches will make __assign_str_len() obsolete too (I'm working
on them now), and you can just use __assign_str().

I noticed that I don't have a string_len example in the sample code and I'm
actually writing it now.

// cutting out everything else:

TRACE_EVENT(foo_bar,

	TP_PROTO(const char *foo, int bar),

	TP_ARGS(foo, bar),

	TP_STRUCT__entry(
		__string_len(	lstr,	foo,	bar < strlen(foo) ? bar : strlen(foo) )
	),

	TP_fast_assign(
		__assign_str(lstr, foo);

// Note, the above is with my updates, without them, you need to duplicate the logic

//		__assign_str_len(lstr, foo, bar < strlen(foo) ? bar : strlen(foo));
	),

	TP_printk("%s", __get_str(lstr))
);


The above will allocate "bar < strlen(foo) ? bar : strlen(foo)" size on the
ring buffer. As the size is already stored, my clean up code uses that
instead of requiring duplicating the logic again.

-- Steve