linux-kernel - Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU

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Message-ID: <20100127115927.GQ6807@linux.vnet.ibm.com>
Date:	Wed, 27 Jan 2010 03:59:27 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com
Subject: Re: [PATCH RFC tip/core/rcu] accelerate grace period if last
	non-dynticked CPU

On Wed, Jan 27, 2010 at 10:39:22PM +1100, Nick Piggin wrote:
> On Wed, Jan 27, 2010 at 02:04:34AM -0800, Paul E. McKenney wrote:
> > I could indeed do that.  However, there is nothing stopping the
> > more-active CPU from going into dynticks-idle mode between the time
> > that I decide to push the callback to it and the time I actually do
> > the pushing.  :-(
> > 
> > I considered pushing the callbacks to the orphanage, but that is a
> > global lock that I would rather not acquire on each dyntick-idle
> > transition.
> 
> Well we already have to do atomic operations on the nohz mask, so
> maybe it would be acceptable to actually have a spinlock there to
> serialise operations on the nohz mask and also allow some subsystem
> specific things (synchronisation here should allow either one of
> those above approaches).
> 
> It's not going to be zero cost, but seeing as there is already the
> contended cacheline there, it's not going to introduce a
> fundamentally new bottleneck.

Good point, although a contended global lock is nastier than a contended
cache line.

							Thanx, Paul
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