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From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
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>,
	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>
Subject: Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles
Date: Tue, 15 Jan 2008 16:46:36 -0500	[thread overview]
Message-ID: <20080115214636.GD17439@Krystal> (raw)
In-Reply-To: <20080109233044.777564395@goodmis.org>

* Steven Rostedt (rostedt@goodmis.org) wrote:
> The latency tracer needs a way to get an accurate time
> without grabbing any locks. Locks themselves might call
> the latency tracer and cause at best a slow down.
> 
> This patch adds get_monotonic_cycles that returns cycles
> from a reliable clock source in a monotonic fashion.
> 
> Signed-off-by: Steven Rostedt <srostedt@redhat.com>
> ---
>  include/linux/clocksource.h |    3 ++
>  kernel/time/timekeeping.c   |   48 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 51 insertions(+)
> 
> Index: linux-compile-i386.git/kernel/time/timekeeping.c
> ===================================================================
> --- linux-compile-i386.git.orig/kernel/time/timekeeping.c	2008-01-09 14:27:26.000000000 -0500
> +++ linux-compile-i386.git/kernel/time/timekeeping.c	2008-01-09 14:34:40.000000000 -0500
> @@ -103,6 +103,54 @@ static inline void __get_realtime_clock_
>  	timespec_add_ns(ts, nsecs);
>  }
>  
> +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.
> +		 */

I wonder what makes the compiler read the clock->cycle_raw and
clock->cycle_last variables twice ? I guess some memory barriers could
be welcome here ?

> +		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;
> +}
> +
> +unsigned long notrace cycles_to_usecs(cycle_t cycles)
> +{
> +	u64 ret = cyc2ns(clock, cycles);
> +
> +	ret += NSEC_PER_USEC/2; /* For rounding in do_div() */
> +	do_div(ret, NSEC_PER_USEC);
> +
> +	return ret;
> +}
> +
> +cycle_t notrace usecs_to_cycles(unsigned long usecs)
> +{
> +	return ns2cyc(clock, (u64)usecs * 1000);
> +}
> +
>  /**
>   * getnstimeofday - Returns the time of day in a timespec
>   * @ts:		pointer to the timespec to be set
> Index: linux-compile-i386.git/include/linux/clocksource.h
> ===================================================================
> --- linux-compile-i386.git.orig/include/linux/clocksource.h	2008-01-09 14:27:51.000000000 -0500
> +++ linux-compile-i386.git/include/linux/clocksource.h	2008-01-09 14:29:44.000000000 -0500
> @@ -273,6 +273,9 @@ extern int clocksource_register(struct c
>  extern struct clocksource* clocksource_get_next(void);
>  extern void clocksource_change_rating(struct clocksource *cs, int rating);
>  extern void clocksource_resume(void);
> +extern cycle_t get_monotonic_cycles(void);
> +extern unsigned long cycles_to_usecs(cycle_t cycles);
> +extern cycle_t usecs_to_cycles(unsigned long usecs);
>  
>  /* used to initialize clock */
>  extern struct clocksource clocksource_jiffies;
> 
> -- 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68

  parent reply	other threads:[~2008-01-15 21:46 UTC|newest]

Thread overview: 100+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-09 23:29 [RFC PATCH 00/22 -v2] mcount and latency tracing utility -v2 Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 01/22 -v2] Add basic support for gcc profiler instrumentation Steven Rostedt
2008-01-10 18:19   ` Jan Kiszka
2008-01-10 19:54     ` Steven Rostedt
2008-01-10 23:02     ` Steven Rostedt
2008-01-10 18:28   ` Sam Ravnborg
2008-01-10 19:10     ` Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 02/22 -v2] Annotate core code that should not be traced Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 03/22 -v2] x86_64: notrace annotations Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 04/22 -v2] add notrace annotations to vsyscall Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 05/22 -v2] add notrace annotations for NMI routines Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 06/22 -v2] mcount based trace in the form of a header file library Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 07/22 -v2] tracer add debugfs interface Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 08/22 -v2] mcount tracer output file Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 09/22 -v2] mcount tracer show task comm and pid Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 10/22 -v2] Add a symbol only trace output Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 11/22 -v2] Reset the tracer when started Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 12/22 -v2] separate out the percpu date into a percpu struct Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 13/22 -v2] handle accurate time keeping over long delays Steven Rostedt
2008-01-10  0:00   ` john stultz
2008-01-10  0:09     ` Steven Rostedt
2008-01-10 19:54     ` Tony Luck
2008-01-10 20:15       ` Steven Rostedt
2008-01-10 20:41         ` john stultz
2008-01-10 20:29       ` john stultz
2008-01-10 20:42         ` Mathieu Desnoyers
2008-01-10 21:25           ` john stultz
2008-01-10 22:00             ` Mathieu Desnoyers
2008-01-10 22:40               ` Steven Rostedt
2008-01-10 22:51               ` john stultz
2008-01-10 23:05                 ` john stultz
2008-01-10 21:33         ` [RFC PATCH 13/22 -v2] handle accurate time keeping over longdelays Luck, Tony
2008-01-10  0:19   ` [RFC PATCH 13/22 -v2] handle accurate time keeping over long delays john stultz
2008-01-10  0:25     ` Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 14/22 -v2] time keeping add cycle_raw for actual incrementation Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 15/22 -v2] initialize the clock source to jiffies clock Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 16/22 -v2] add get_monotonic_cycles Steven Rostedt
2008-01-10  3:28   ` Daniel Walker
2008-01-15 21:46   ` Mathieu Desnoyers [this message]
2008-01-15 22:01     ` Steven Rostedt
2008-01-15 22:03       ` Steven Rostedt
2008-01-15 22:08       ` Mathieu Desnoyers
2008-01-16  1:38         ` Steven Rostedt
2008-01-16  3:17           ` Mathieu Desnoyers
2008-01-16 13:17             ` Steven Rostedt
2008-01-16 14:56               ` Mathieu Desnoyers
2008-01-16 15:06                 ` Steven Rostedt
2008-01-16 15:28                   ` Mathieu Desnoyers
2008-01-16 15:58                     ` Steven Rostedt
2008-01-16 17:00                       ` Mathieu Desnoyers
2008-01-16 17:49                         ` Mathieu Desnoyers
2008-01-16 19:43                         ` Steven Rostedt
2008-01-16 20:17                           ` Mathieu Desnoyers
2008-01-16 20:45                             ` Tim Bird
2008-01-16 20:49                             ` Steven Rostedt
2008-01-17 20:08                             ` Steven Rostedt
2008-01-17 20:37                               ` Frank Ch. Eigler
2008-01-17 21:03                                 ` Steven Rostedt
2008-01-18 22:26                                   ` Mathieu Desnoyers
2008-01-18 22:49                                     ` Steven Rostedt
2008-01-18 23:19                                       ` Mathieu Desnoyers
2008-01-19  3:36                                         ` Frank Ch. Eigler
2008-01-19  3:55                                           ` Steven Rostedt
2008-01-19  4:23                                             ` Frank Ch. Eigler
2008-01-19 15:29                                               ` Mathieu Desnoyers
2008-01-19  3:32                                       ` Frank Ch. Eigler
2008-01-16 18:01                       ` Tim Bird
2008-01-16 22:36                 ` john stultz
2008-01-16 22:51                   ` john stultz
2008-01-16 23:33                     ` Steven Rostedt
2008-01-17  2:28                       ` john stultz
2008-01-17  2:40                         ` Mathieu Desnoyers
2008-01-17  2:50                           ` Mathieu Desnoyers
2008-01-17  3:02                             ` Steven Rostedt
2008-01-17  3:21                             ` Paul Mackerras
2008-01-17  3:39                               ` Steven Rostedt
2008-01-17  4:22                                 ` Mathieu Desnoyers
2008-01-17  4:25                                 ` Mathieu Desnoyers
2008-01-17  4:14                               ` Mathieu Desnoyers
2008-01-17 15:22                                 ` Steven Rostedt
2008-01-17 17:46                                 ` Linus Torvalds
2008-01-17  2:51                           ` Steven Rostedt
2008-01-16 23:39                     ` Mathieu Desnoyers
2008-01-16 23:50                       ` Steven Rostedt
2008-01-17  0:36                         ` Steven Rostedt
2008-01-17  0:33                       ` john stultz
2008-01-17  2:20                         ` Mathieu Desnoyers
2008-01-17  1:03                       ` Linus Torvalds
2008-01-17  1:35                         ` Mathieu Desnoyers
2008-01-17  2:20                       ` john stultz
2008-01-17  2:35                         ` Mathieu Desnoyers
2008-01-09 23:29 ` [RFC PATCH 17/22 -v2] Add timestamps to tracer Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 18/22 -v2] Sort trace by timestamp Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 19/22 -v2] speed up the output of the tracer Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 20/22 -v2] Add latency_trace format tor tracer Steven Rostedt
2008-01-10  3:41   ` Daniel Walker
2008-01-09 23:29 ` [RFC PATCH 21/22 -v2] Split out specific tracing functions Steven Rostedt
2008-01-09 23:29 ` [RFC PATCH 22/22 -v2] Trace irq disabled critical timings Steven Rostedt
2008-01-10  3:58   ` Daniel Walker
2008-01-10 14:45     ` Steven Rostedt

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