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

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Message-ID: <1264585850.4283.1992.camel@laptop>
Date:	Wed, 27 Jan 2010 10:50:50 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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, 2010-01-27 at 10:43 +0100, Andi Kleen wrote:
> 
> Can't you simply check that at runtime then?
> 
> if (num_possible_cpus() > 20) 
>         ...
> 
> BTW the new small is large. This years high end desktop PC will come with 
> upto 12 CPU threads. It would likely be challenging to find a good
> number for 20 that holds up with the future. 

If only scalability were that easy :/

These massive core/thread count things are causing more problems as
well, the cpus/node ratios are constantly growing, giving grief in the
page allocator as well as other places that used to scale per node.

As to the current problem, the call_rcu() interface doesn't make a hard
promise that the callback will be done on the same cpu, right? So why
not simply move the callback list over to a more active cpu?

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