linux-kernel - Re: [RFC PATCH 12/23 -v4] Use RCU algorithm for monotonic cycles.

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Date:	Mon, 21 Jan 2008 19:49:25 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	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>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	John Stultz <johnstul@...ibm.com>,
	John Stultz <johstul@...ibm.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH 12/23 -v4] Use RCU algorithm for monotonic cycles.



On Tue, 22 Jan 2008, Nick Piggin wrote:

> On Tuesday 22 January 2008 02:22, Steven Rostedt wrote:
> > From: john stultz <johnstul@...ibm.com>
>
> >  static inline cycle_t
> > -clocksource_get_cycles(struct clocksource *cs, cycle_t now)
> > +clocksource_get_basecycles(struct clocksource *cs)
> >  {
> > -	cycle_t offset = (now - cs->cycle_last) & cs->mask;
> > -	offset += cs->cycle_accumulated;
> > +	int num;
> > +	cycle_t now, offset;
> > +
> > +	preempt_disable();
> > +	num = cs->base_num;
> > +	smp_read_barrier_depends();
>
> All barriers need comments in the code. eg. with read barriers, the
> comment should contain a list of the loads being ordered, and a
> reference to the places where stores come from.
>
> I know it isn't too hard to follow _now_, but it makes the code more
> maintainable.

Yep, thanks for the reminder. This was a quick patch from John. I'll add
the necessary comment for the next release. Hmm, I'm surprised that
checkpatch didn't complain.

>
>
> > +	now = clocksource_read(cs);
> > +	offset = (now - cs->base[num].cycle_base_last);
> > +	offset &= cs->mask;
> > +	offset += cs->base[num].cycle_base;
> > +	preempt_enable();
> > +
> >  	return offset;
> >  }
> >
> > @@ -197,14 +215,26 @@ clocksource_get_cycles(struct clocksourc
> >   * @now:	current cycle value
> >   *
> >   * Used to avoids clocksource hardware overflow by periodically
> > - * accumulating the current cycle delta. Must hold xtime write lock!
> > + * accumulating the current cycle delta. Uses RCU-like update, but
> > + * ***still requires the xtime_lock is held for writing!***
> >   */
> >  static inline void clocksource_accumulate(struct clocksource *cs, cycle_t
> > now) {
> > -	cycle_t offset = (now - cs->cycle_last) & cs->mask;
> > +	/* First update the monotonic base portion.
> > +	 * The dual array update method allows for lock-free reading.
> > +	 */
> > +	int num = 1 - cs->base_num;
> > +	cycle_t offset = (now - cs->base[1-num].cycle_base_last);
> > +	offset &= cs->mask;
> > +	cs->base[num].cycle_base = cs->base[1-num].cycle_base + offset;
> > +	cs->base[num].cycle_base_last = now;
> > +	wmb();
> > +	cs->base_num = num;
>
> Ditto for the wmb. Also, I think the wmb() can probably just be
> smp_wmb().

Yep, this too. We suggested earlier to John to switch this to a
smp_wmb, but I took his patch before the update. This will be fixed in the
next round too.

Thanks,

-- Steve

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