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

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Date:	Wed, 25 Mar 2009 09:40:54 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	ltt-dev@...ts.casi.polymtl.ca,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Hideo AOKI <haoki@...hat.com>,
	Takashi Nishiie <t-nishiie@...css.fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
Subject: Re: [patch 7/9] LTTng instrumentation - kernel


* Rusty Russell <rusty@...tcorp.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
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