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

[Nishanth Aravamudan <nacc@linux.vnet.ibm.com>]

On 13.02.2014 [14:41:04 -0800], David Rientjes wrote:
> 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().

I'm working on this latter bit now. I tried to mirror ia64, but it looks
like they have CONFIG_USER_PERCPU_NUMA_NODE_ID, which powerpc doesn't.
It seems like CONFIG_USER_PERCPU_NUMA_NODE_ID and
CONFIG_HAVE_MEMORYLESS_NODES should be tied together in Kconfig?

I'll keep working, but would appreciate any further insight.

-Nish

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