Re: [patch 7/9] LTTng instrumentation - kernel

Ingo Molnar
2009-03-25T08:42:15Z

* Rusty Russell <rusty@rustcorp.com.au> wrote:

> On Wednesday 25 March 2009 05:03:13 Ingo Molnar wrote:
> >
> > (Rusty Cc:-ed - for the module.c tracepoints below)
>
> Thanks, tho they look fine and non-intrusive to me.

Thanks - i'll take this as an Acked-by :-)

( Mathieu, mind re-sending a variant that does them via
TRACE_EVENT(), against the tracing tree? That makes it useful not
just to LTTng but the default mainline kernel as well. Thanks! )

> > I believe that to have a complete picture of module usage, module
> > refcount get/put events should be included as well, beyond the basic
> > load/free events.
> >
> > These both have performance impact (a module get/put in a fastpath
> > hurts scalability), and are informative in terms of establishing the
> > module dependency graph.
>
> A module_get()/put() should not hurt scalability at all! I went
> to great and horrible lengths to ensure that was the case since
> the rewrite in 2.4.

i know, it uses percpu refcounts :) Still it can be somewhat
non-trivial in a fastpath:

static inline int try_module_get(struct module *module)
{
int ret = 1;

if (module) {
unsigned int cpu = get_cpu();
if (likely(module_is_live(module)))
local_inc(__module_ref_addr(module, cpu));
else
ret = 0;
put_cpu();
}
return ret;
}

So we want to reduce excessive uses of it.

[ And please forget i mentioned scalability - that bit is fine and
you were offended rightfully :) ]

This reminds me. Couldnt we now:

#ifdef CONFIG_SMP
char *refptr;
#else
local_t ref;
#endif

... unify these bits to just standardize on a per-cpu refptr all the
time, with Tejun's ueber-cool percpu changes in place?

> But a module dependency graph et. al. would be kind of cool.

Yeah. Can be in a separate patch as well - load/unload events are
meaningful in isolation as well.

Ingo