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

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Message-ID: <20100126235516.GA15855@Krystal>
Date:	Tue, 26 Jan 2010 18:55:16 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	paulmck@...ux.vnet.ibm.com, 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

* 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.

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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