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

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Message-ID: <20080117024000.GD2322@Krystal>
Date:	Wed, 16 Jan 2008 21:40:00 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	john stultz <johnstul@...ibm.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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>,
	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>,
	Paul Mackerras <paulus@...ba.org>,
	Daniel Walker <dwalker@...sta.com>
Subject: Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles

* john stultz (johnstul@...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.

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
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