linux-kernel - Re: [Patch] numa:x86_64: Cacheline aliasing makes for_each_populated

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Message-ID: <20100820135822.GA3220@sgi.com>
Date:	Fri, 20 Aug 2010 08:58:22 -0500
From:	Robin Holt <holt@....com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Robin Holt <holt@....com>, Jack Steiner <steiner@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, x86@...nel.org,
	Yinghai Lu <yinghai@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Joerg Roedel <joerg.roedel@....com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Stable Maintainers <stable@...nel.org>
Subject: Re: [Patch] numa:x86_64: Cacheline aliasing makes
 for_each_populated_zone extremely expensive -V2.

On Thu, Aug 19, 2010 at 03:54:36PM -0700, H. Peter Anvin wrote:
> On 08/18/2010 11:30 AM, Robin Holt wrote:
> >  
> > -	node_data[nodeid] = early_node_mem(nodeid, start, end, pgdat_size,
> > +	/*
> > +	 * Allocate an extra cacheline per node to reduce cacheline
> > +	 * aliasing when scanning all node's node_data.
> > +	 */
> > +	cache_alias_offset = nodeid * SMP_CACHE_BYTES;
> > +	node_data[nodeid] = cache_alias_offset +
> > +			    early_node_mem(nodeid, start, end,
> > +					   pgdat_size + cache_alias_offset,
> >  					   SMP_CACHE_BYTES);
> > -	if (node_data[nodeid] == NULL)
> > +	if (node_data[nodeid] == (void *)cache_alias_offset)
> >  		return;
> >  	nodedata_phys = __pa(node_data[nodeid]);
> >  	reserve_early(nodedata_phys, nodedata_phys + pgdat_size, "NODE_DATA");
> 
> I'm concerned about this, because it really seems to rely on subtleties
> in the behavior of early_node_mem, as well as the direction of
> find_e820_area -- which is pretty much intended to change anyway.  It's
> the "action at a distance" effect.
> 
> What we really want, I think, is to push the offsetting into
> find_early_area().  Right now we have an alignment parameter, but what
> we need is an alignment and a color parameter (this is just a classic
> case of cache coloring, after all) which indicates the desirable offset
> from the alignment base.

That sounds reasonable.  Are there other examples you can think of which
I can build upon?

Robin
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