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

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Date:	Thu, 19 Aug 2010 15:54:36 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Robin Holt <holt@....com>
CC:	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 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.

	-hpa
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