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From: Mathieu Desnoyers <compudj@krystal.dyndns.org>
To: john stultz <johnstul@us.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	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>,
	Paul Mackerras <paulus@samba.org>,
	Daniel Walker <dwalker@mvista.com>
Subject: Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles
Date: Wed, 16 Jan 2008 21:50:04 -0500	[thread overview]
Message-ID: <20080117025004.GE2322@Krystal> (raw)
In-Reply-To: <20080117024000.GD2322@Krystal>

* Mathieu Desnoyers (mathieu.desnoyers@polymtl.ca) wrote:
> * john stultz (johnstul@us.ibm.com) wrote:
> > 
> > On Wed, 2008-01-16 at 18:33 -0500, Steven Rostedt wrote:
> > > Thanks John for doing this!
> > > 
> > > (comments imbedded)
> > > 
> > > On Wed, 16 Jan 2008, john stultz wrote:
> > > > +	int num = !cs->base_num;
> > > > +	cycle_t offset = (now - cs->base[!num].cycle_base_last);
> > > > +	offset &= cs->mask;
> > > > +	cs->base[num].cycle_base = cs->base[!num].cycle_base + offset;
> > > > +	cs->base[num].cycle_base_last = now;
> > > 
> > > I would think that we would need some sort of barrier here. Otherwise,
> > > base_num could be updated before all the cycle_base. I'd expect a smp_wmb
> > > is needed.
> > 
> > Hopefully addressed in the current version.
> > 
> > 
> > > > Index: monotonic-cleanup/kernel/time/timekeeping.c
> > > > ===================================================================
> > > > --- monotonic-cleanup.orig/kernel/time/timekeeping.c	2008-01-16 12:21:46.000000000 -0800
> > > > +++ monotonic-cleanup/kernel/time/timekeeping.c	2008-01-16 14:15:31.000000000 -0800
> > > > @@ -71,10 +71,12 @@
> > > >   */
> > > >  static inline s64 __get_nsec_offset(void)
> > > >  {
> > > > -	cycle_t cycle_delta;
> > > > +	cycle_t now, cycle_delta;
> > > >  	s64 ns_offset;
> > > >
> > > > -	cycle_delta = clocksource_get_cycles(clock, clocksource_read(clock));
> > > > +	now = clocksource_read(clock);
> > > > +	cycle_delta = (now - clock->cycle_last) & clock->mask;
> > > > +	cycle_delta += clock->cycle_accumulated;
> > > 
> > > Is the above just to decouple the two methods?
> > 
> > Yep. clocksource_get_cycles() ended up not being as useful as an helper
> > function (I was hoping the arch vsyscall implementations could use it,
> > but they've done too much optimization - although that may reflect a
> > need up the chain to the clocksource structure).
> > 
> 
> The problem with vsyscall is that we will have a hard time disabling
> preemption :( Therefore, insuring that the read of the data is done in a
> timely manner is hard to do.
> 

Sorry for self-reply, but I thought, in the past, of a way to make this
possible.

It would imply the creation of a new vsyscall : vgetschedperiod

It would read a counter that would increment each time the thread is
scheduled out (or in). It would be a per thread counter (not a per cpu
counter) so we can deal appropriately with a stopped thread that would
happen to come back running a loooong time afterward (if we do per-cpu
counters, we could get the same 32 bits counter value falsely if it is
shared with other thread activity).

Then, the clocksource read code would look like :

int period;

do {
  period = vgetschedperiod();

  perform the clocksource read..

} while (period != vgetschedperiod());

Therefore, we would be sure that we have not been scheduled out while
reading the value. I think this new vsyscall could be useful for others.
Actually, it would make implementation of RCU in user-space possible (as
long as the read-side can retry the read operation).

Mathieu

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

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

  reply	other threads:[~2008-01-17  3:00 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
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 [this message]
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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