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

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Date:	Mon, 9 Mar 2009 13:13:06 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
cc:	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, 9 Mar 2009, Christoph Lameter wrote:

> Again these are fastpath modifications.
> 

The nature of the change requires the logic to be placed in the fastpath 
to determine whether a cpu slab's node is allowed by the allocating task's 
cpuset.

You have previously stated that you would prefer that this feature be 
tunable from userspace.  This patch adds the `memory_slab_hardwall' cpuset 
flag which defaults to off.

> Scanning the partial list for matching nodes is an expensive operation.
> 

It depends on how long you scan for a matching node, but again: this 
should be assumed by the user if the option has been enabled.

> Adding RCU into the fast paths is also another big worry.
> 

This could be mitigated by adding a PF_SLAB_HARDWALL flag similiar to 
PF_SPREAD_PAGE and PF_SPREAD_SLAB.  I'd prefer not to add additional 
cpuset-specific task flags, but this would address your concern.
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