linux-kernel - Re: [patch -mm] cpusets: add memory_slab

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Message-ID: <20090312010330.GK7086@linux.vnet.ibm.com>
Date:	Wed, 11 Mar 2009 18:03:30 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Matt Mackall <mpm@...enic.com>,
	Paul Menage <menage@...gle.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch -mm] cpusets: add memory_slab_hardwall flag

On Mon, Mar 09, 2009 at 02:50:06PM -0400, Christoph Lameter wrote:
> Again these are fastpath modifications.
> 
> Scanning the partial list for matching nodes is an expensive operation.
> 
> Adding RCU into the fast paths is also another big worry.

Hello, Christoph,

Adding synchronize_rcu() into a fast path would certainly be a problem,
but call_rcu() should be OK.  If the data structure is updated often
(old elements removed and new elements added), then the cache misses
from elements that were removed, went cache-cold, and then were added
again could potentially cause trouble, but read-mostly data structures
should be OK.

Or were you worried about some other aspect of RCU overhead?

							Thanx, Paul
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