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

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Message-ID: <Pine.LNX.4.58.0801182242440.16333@gandalf.stny.rr.com>
Date:	Fri, 18 Jan 2008 22:55:27 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Frank Ch. Eigler" <fche@...hat.com>
cc:	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



On Fri, 18 Jan 2008, Frank Ch. Eigler 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 ;-)

Actually, you just gave me a great idea that I think can help all of us.
OK, Mathieu may not be in total agreement, but I think this is the
ultimate compromise.

We have in sched.c the following marker:

 trace_mark(kernel_sched_scheduler, "prev %p next %p", prev, next);


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, ...)
{
	va_list ap;
	struct task_struct *prev;
	struct task_struct *next;

	va_start(ap, format);
	prev = va_arg(ap, typeof(prev));
	next = va_arg(ap, typeof(next));
	va_end;

	trace_mark(kernel_pretty_print_sched_switch,
		"prev_pid %d next_pid %d prev_state %ld",
		prev->pid, next->pid, prev->state);
}


Then LTTng on startup could arm the normal kernel_sched_switch code and
have the user see the nice one. All without adding any more goo or
overhead to the non tracing case, and keeping a few critical markers with
enough information to be useful to other tracers!

Thoughts?

-- Steve

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