linux-kernel - Re: [RFC PATCH 1/3] Unified trace buffer

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Date:	Thu, 25 Sep 2008 19:07:19 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Martin Bligh <mbligh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Martin Bligh <mbligh@...igh.org>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Tom Zanussi <zanussi@...cast.net>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH 1/3] Unified trace buffer

Steven Rostedt wrote:
> OK, let me rephrase my question.
>
> How and where do we record this?  Do we keep this information in some 
> global variable that we must compare to every time we add a new item in 
> the trace?
>
> Do we have the buffer register a call back to record this information?
>   

Something like (total pseudocode):

struct tsc_time_parameters {
	int version;		/* even - values OK; odd - values being updated */
	u64 tsc;
	u32 tsc_freq;
	u64 gtod;
};

DEFINE_PERCPU(struct tsc_time_parameters, tsc_params);

/* To be called after a tsc frequency change, before any new
   trace records are being emitted, in a context where we can call get_GTOD() */
void update_tsc_params(void)
{
	struct tsc_time_parameters *p = __get_percpu_var(tsc_params);

	p->version |= 1;
	wmb();

	p->tsc = get_tsc();
	p->tsc_freq = get_tsc_freq();
	p->gtod = get_GTOD();

	wmb();
	p->version++;
	wmb();
}

DEFINE_PERCPU(unsigned, current_tsc_version);
DEFINE_PERCPU(u64, prev_tsc);

/* may be called in any context */
u64 get_trace_timestamp_delta(void)
{
	const struct tsc_time_parameters *p = &__get_percpu_var(tsc_params);
	unsigned *current_version = &__get_cpu_var(current_tsc_version);
	u64 prev = __get_cpu_var(prev_tsc);
	u64 now, ret;

	/* check the current tsc_params version against the last one we emitted;
		if the version is odd, then we interrupted the parameters as they were
		being updated, so just emit a new delta with the old parameters */
	if (unlikely(*current_version != p->version && !(p->version & 1))) {
		/* XXX probably need a loop to deal with p->version changing under our feet */
		emit_tsc_freq_record(p);
		prev = p->tsc;
		__get_cpu_var(current_tsc_version) = p->version;
	}

	now = read_tsc();
	ret = now - prev;
	__get_cpu_var(prev_tsc) = now;

	return ret;
}


    J
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