linux-kernel - Re: [PATCH 20/35] Use a pre-calculated value for num_online

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Date:	Thu, 19 Mar 2009 22:52:52 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 20/35] Use a pre-calculated value for num_online_nodes()

On Thu, Mar 19, 2009 at 06:42:39PM -0400, Christoph Lameter wrote:
> On Thu, 19 Mar 2009, Mel Gorman wrote:
> 
> > I posted an amalgamation. Sorry for the cross-over mails but I wanted to
> > get tests going before I fell asleep. They take a few hours to complete.
> >
> > > >  static inline void node_set_state(int node, enum node_states state)
> > > >  {
> > > >  	__node_set(node, &node_states[state]);
> > > > +	if (state == N_ONLINE)
> > > > +		nr_online_nodes = num_node_state(N_ONLINE);
> > > >  }
> > >
> > > That assumes uses of node_set_state N_ONLINE. Are there such users or are
> > > all using node_set_online()?
> > >
> >
> > node_set_online() calls node_set_state(node, N_ONLINE) so it should have
> > worked out.
> 
> But this adds a surprising side effect to all uses of node_set_state.
> Node_set_state is generating more code now.
> 

Fair point.

> > > if you want to check if the system could ever bring up a second node
> > > (which would make the current optimization not viable) whereas
> > > nr_online_nodes is the check for how many nodes are currently online.
> > >
> >
> > I redid your patch to drop the nr_possible_nodes() because I couldn't convince
> > myself it was correct in all cases and it isn't as important as avoiding
> > num_online_nodes() in fast paths.
> 
> I was more thinking about getting the infrastructure right so that we can
> avoid future hacks like the one in slab.
> 

Which is fair enough and you're right in that it's worth fixing.  One horribly
large patchset and associcate thread at a time though so I'll be putting
it on the wrong finger rather than adding this to the pile right now :).

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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