On Thu, 13 Feb 2014 15:26:42 +0100
Frederic Weisbecker <fweis...@gmail.com> wrote:
> > Is this worth doing?
>
> It sounds worth yeah. I've run into this situation multiple times where I had
Is it?
> to pass 0 instead of nothing on a tracepoint.
What tracepoints?
The problem I have with a tracepoint that does not pass any information
what's so ever, is that it can be too tempting to use. Tracepoints do
not have zero overhead when not used, but a negligible one. Too many
tracepoints add up, and their foot print can start to be noticed.
Point being, why have a tracepoint if you are not recording any data?
Just do a function trace, or add a kprobe. That's easy enough.
But that said, see below.
>
> Now about the name, why not TRACE_EVENT_EMPTY?
Because that's an ugly name ;-)
Also a bit misleading, because it sounds like the there's no items
attached or something. It is too close to "list_empty()". My original
name was TRACE_EVENT_NOARGS(), which could work too.
Now another possible solution is to just introduce a trace_marker()
function that you could place anywhere. And then they could show up in
trace/events/markers/file-func-line/
We could do something like:
struct trace_marker {
char *file;
char *func;
int line;
struct static_key key;
};
#define trace_marker() __trace_marker(__FILE__,__func__, __LINE__)
static inline void __trace_marker(const char *file,
const char *func, int line)
{
static struct trace_marker marker
__attribute__((section("__trace_marker"))) = {
.file = file,
.func = func,
.line = line
};
if (static_key_false(&marker.key))
trace_marker_call(&marker);
}
As marker would be a static value, gcc should hard code the first
parameter to it and make the call. Basically something like:
mov $0x<marker-address>, %rdi
call trace_marker_call
If we really want to be efficient, we could extend jump labels for each
arch, and remove the call completely.
asm goto ("1:"
".byte NOP\n"
".pushsection __trace_marker_struct\n"
"2:.quad 1b\n"
".quad %file\n"
".quad %func\n'
".word %line\n"
".popsection\n"
".pushsection __trace_marker_ptrs\n"
".quad 2b\n"
".popsection\n"
: : file, func, line);
[ OK, I didn't follow the true format for asm syntax, but that's
because I'm just trying to relay an idea, not actually make working
code ]
Then the only thing that would be inserted in the code is a nop. We
could then replace that nop with a call to a trampoline (similar to
mcount) that can call a C function with the address that it came from
so that the function could do a look up to find the matching marker to
trace.
OK, this is probably a bit too much, but it is feasible. Most likely
not worth the pain.
-- Steve
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