Subject: Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles
From: Daniel Walker <dwalker@mvista.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Christoph Hellwig <hch@infradead.org>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       Gregory Haskins <ghaskins@novell.com>,
       Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
       Thomas Gleixner <tglx@linutronix.de>, Tim Bird <tim.bird@am.sony.com>,
       Sam Ravnborg <sam@ravnborg.org>, "Frank Ch. Eigler" <fche@redhat.com>,
       Steven Rostedt <srostedt@redhat.com>
In-Reply-To: <20080109233044.777564395@goodmis.org>
References: <20080109232914.676624725@goodmis.org>
	 <20080109233044.777564395@goodmis.org>
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Date: Wed, 09 Jan 2008 19:28:20 -0800
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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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