linux-kernel - Re: [RFC PATCH V5] mm readahead: Fix readahead fail for no local memory and limit readahead pages

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Date:	Thu, 13 Feb 2014 14:41:04 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	David Cohen <david.a.cohen@...ux.intel.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Damien Ramonda <damien.ramonda@...el.com>,
	Jan Kara <jack@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V5] mm readahead: Fix readahead fail for no local
 memory and limit readahead pages

On Thu, 13 Feb 2014, Raghavendra K T wrote:

> Thanks David, unfortunately even after applying that patch, I do not see
> the improvement.
> 
> Interestingly numa_mem_id() seem to still return the value of a
> memoryless node.
> May be  per cpu _numa_mem_ values are not set properly. Need to dig out ....
> 

I believe ppc will be relying on __build_all_zonelists() to set 
numa_mem_id() to be the proper node, and that relies on the ordering of 
the zonelist built for the memoryless node.  It would be very strange if 
local_memory_node() is returning a memoryless node because it is the first 
zone for node_zonelist(GFP_KERNEL) (why would a memoryless node be on the 
zonelist at all?).

I think the real problem is that build_all_zonelists() is only called at 
init when the boot cpu is online so it's only setting numa_mem_id() 
properly for the boot cpu.  Does it return a node with memory if you 
toggle /proc/sys/vm/numa_zonelist_order?  Do

	echo node > /proc/sys/vm/numa_zonelist_order
	echo zone > /proc/sys/vm/numa_zonelist_order
	echo default > /proc/sys/vm/numa_zonelist_order

and check if it returns the proper value at either point.  This will force 
build_all_zonelists() and numa_mem_id() to point to the proper node since 
all cpus are now online.

So the prerequisite for CONFIG_HAVE_MEMORYLESS_NODES is that there is an 
arch-specific set_numa_mem() that makes this mapping correct like ia64 
does.  If that's the case, then it's (1) completely undocumented and (2) 
Nishanth's patch is incomplete because anything that adds 
CONFIG_HAVE_MEMORYLESS_NODES needs to do the proper set_numa_mem() for it 
to be any different than numa_node_id().
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