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

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Message-ID: <20100127052133.GD6807@linux.vnet.ibm.com>
Date:	Tue, 26 Jan 2010 21:21:33 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, 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 Tue, Jan 26, 2010 at 06:55:16PM -0500, Mathieu Desnoyers wrote:
> * Andi Kleen (andi@...stfloor.org) wrote:
> > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> writes:
> > 
> > Kind of offtopic to the original patch, but I couldn't 
> > resist...
> > 
> > > +config RCU_FAST_NO_HZ
> > > +	bool "Accelerate last non-dyntick-idle CPU's grace periods"
> > > +	depends on TREE_RCU && NO_HZ && SMP
> > 
> > Having such a thing as a config option doesn't really make 
> > any sense to me. Who would want to recompile their kernel
> > to enable/disable this? If anything it should be runtime, or better
> > just unconditionally on.
> > 
> > In general RCU could probably reduce its number of "weird"
> > Kconfig options.
> > 
> > While I think I have a better understanding of RCU than a lot
> > of normal users I often have no clue what to set there when
> > building a kernel.
> 
> Maybe we could keep them under a CONFIG_DEBUG_RCU umbrella. Compiling
> out parts of the rcu options can be useful for debugging purposes, but
> I agree with you that end users should not be bothered with that much
> options when some of them are "obviously" wanted.
> 
> OTOH, I understand that Paul seems to want to introduce new RCU
> features gradually, without hitting all kernel users with bugs in newer
> features. That's a sane approach to keep things generally stable, but
> maybe it is time to set some of the stabilized RCU options to default Y
> and move their config to the debug menu.

That is indeed a big part of the motivation.  ;-)

							Thanx, Paul

> Let's see what Paul has to say about this...
> 
> Thanks,
> 
> Mathieu
> 
> > 
> > -Andi
> > 
> > -- 
> > ak@...ux.intel.com -- Speaking for myself only.
> 
> -- 
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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