linux-kernel - Re: [RFC PATCH 16/22 -v2] add get_monotonic

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Date:	Fri, 18 Jan 2008 23:23:52 -0500
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"Frank Ch. Eigler" <fche@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Bird <tim.bird@...sony.com>,
	Sam Ravnborg <sam@...nborg.org>,
	Steven Rostedt <srostedt@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Daniel Walker <dwalker@...sta.com>
Subject: Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles

Hi -

On Fri, Jan 18, 2008 at 10:55:27PM -0500, Steven Rostedt wrote:
> [...]
> > All this complexity is to be justified by keeping the raw prev/next
> > pointers from being sent to a naive tracer?  It seems to me way out of
> > proportion.
> 
> Damn, and I just blew away all my marker code for something like this ;-)

Sorry! :-)

> [...]
> We have in sched.c the following marker:
>  trace_mark(kernel_sched_scheduler, "prev %p next %p", prev, next);

Fine so far!

> Then Mathieu can add in some code somewhere (or a module, or something)
> 	ret = marker_probe_register("kernel_sched_scheduler",
> 				"prev %p next %p",
> 				pretty_print_sched_switch, NULL);

> static void pretty_print_sched_switch(const struct marker *mdata,
> 				void *private_data,
> 				const char *format, ...)
> {
> 	[...]
> 	trace_mark(kernel_pretty_print_sched_switch,
> 		"prev_pid %d next_pid %d prev_state %ld",
> 		prev->pid, next->pid, prev->state);
> }

That marker_probe_register call would need to be done only when the
embedded (k_p_p_s_s) marker is actually being used.  Otherwise we'd
lose all the savings of a dormant sched.c marker by always calling
into pretty_print_sched_switch(), whether or not the k_p_p_s_s marker
was active.

In any case, if the naive tracer agrees to become educated about some
of these markers in the form of intermediary functions like that, they
need not insist on a second hop through marker territory anyway:

 static void pretty_print_sched_switch(const struct marker *mdata,
 				void *private_data,
 				const char *format, ...)
 {
 	[...]
 	lttng_backend_trace(kernel_pretty_print_sched_switch,
 		            "prev_pid %d next_pid %d prev_state %ld",
               		    prev->pid, next->pid, prev->state);
 }


- FChE
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