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

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Message-Id: <1200536889.6127.52.camel@localhost.localdomain>
Date:	Wed, 16 Jan 2008 18:28:09 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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


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).

thanks
-john


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