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

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Date:	Wed, 09 Jan 2008 19:28:20 -0800
From:	Daniel Walker <dwalker@...sta.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	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>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles


On Wed, 2008-01-09 at 18:29 -0500, Steven Rostedt wrote:
> +cycle_t notrace get_monotonic_cycles(void)
> +{
> +       cycle_t cycle_now, cycle_delta, cycle_raw, cycle_last;
> +
> +       do {
> +               /*
> +                * cycle_raw and cycle_last can change on
> +                * another CPU and we need the delta calculation
> +                * of cycle_now and cycle_last happen atomic, as well
> +                * as the adding to cycle_raw. We don't need to grab
> +                * any locks, we just keep trying until get all the
> +                * calculations together in one state.
> +                *
> +                * In fact, we __cant__ grab any locks. This
> +                * function is called from the latency_tracer which can
> +                * be called anywhere. To grab any locks (including
> +                * seq_locks) we risk putting ourselves into a deadlock.
> +                */
> +               cycle_raw = clock->cycle_raw;
> +               cycle_last = clock->cycle_last;
> +
> +               /* read clocksource: */
> +               cycle_now = clocksource_read(clock);
> +
> +               /* calculate the delta since the last update_wall_time: */
> +               cycle_delta = (cycle_now - cycle_last) & clock->mask;
> +
> +       } while (cycle_raw != clock->cycle_raw ||
> +                cycle_last != clock->cycle_last);
> +
> +       return cycle_raw + cycle_delta;
> +}

The last I check this changed caused problems for me with the -rt
latency tracer.. I haven't tested this tree , but all things being equal
I would imagine the exists here also..

Daniel

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