linux-kernel - Re: [PATCH] online CPU before memory failed in pcpu_alloc

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Date:	Fri, 21 May 2010 09:21:47 -0400
From:	Lee Schermerhorn <Lee.Schermerhorn@...com>
To:	minskey guo <chaohong_guo@...ux.intel.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	prarit@...hat.com, andi.kleen@...el.com,
	linux-kernel@...r.kernel.org, minskey guo <chaohong.guo@...el.com>,
	Tejun Heo <tj@...nel.org>, stable@...nel.org
Subject: Re: [PATCH] online CPU before memory failed in pcpu_alloc_pages()

On Fri, 2010-05-21 at 17:12 +0800, minskey guo wrote:
> On 05/21/2010 04:39 PM, KAMEZAWA Hiroyuki wrote:
> > On Fri, 21 May 2010 16:22:19 +0800
> > minskey guo<chaohong_guo@...ux.intel.com>  wrote:
> >
> >> Yes.  I can use cpu_to_mem().  only some little difference during
> >> CPU online:  1st cpu within memoryless node gets memory from current
> >> node or the node to which the cpu0 belongs,
> >>
> >>
> >> But I have a question about the patch:
> >>
> >>      numa-slab-use-numa_mem_id-for-slab-local-memory-node.patch,
> >>
> >>
> >>
> >>
> >> @@ -2968,9 +2991,23 @@ static int __build_all_zonelists(void *d
> >> ...
> >>
> >> -	for_each_possible_cpu(cpu)
> >> +	for_each_possible_cpu(cpu) {
> >> 		setup_pageset(&per_cpu(boot_pageset, cpu), 0);
> >> ...
> >>
> >> +#ifdef CONFIG_HAVE_MEMORYLESS_NODES
> >> + 	if (cpu_online(cpu))
> >> +		cpu_to_mem(cpu) = local_memory_node(cpu_to_node(cpu));
> >> +#endif
> 
> Look at the above code,  int __build_all_zonelists(),  cpu_to_mem(cpu)
> is set only when cpu is onlined.  Suppose that a node with local memory,
> all memory segments are onlined first, and then,  cpus within that node
> are onlined one by one,  in this case,  where does the cpu_to_mem(cpu)
> for the last cpu get its value ?

Minskey:

As I mentioned to Kame-san, x86 does not define
CONFIG_HAVE_MEMORYLESS_NODES, so this code is not compiled for that
arch.  If x86 did support memoryless nodes--i.e., did not hide them and
reassign the cpus to other nodes, as is the case for ia64--then we could
have on-line cpus associated with memoryless nodes.  The code above is
in __build_all_zonelists() so that in the case where we add memory to a
previously memoryless node, we re-evaluate the "local memory node" for
all online cpus.

For cpu hotplug--again, if x86 supports memoryless nodes--we'll need to
add a similar chunk to the path where we set up the cpu_to_node map for
a hotplugged cpu.  See, for example, the call to set_numa_mem() in
smp_callin() in arch/ia64/kernel/smpboot.c.  But currently, I don't
think you can use the numa_mem_id()/cpu_to_mem() interfaces for your
purpose.  I suppose you could change page_alloc.c to compile
local_memory_node() #if defined(CONFIG_HAVE_MEMORYLESS_NODES) || defined
(CPU_HOTPLUG) and use that function to find the nearest memory.  It
should return a valid node after zonelists have been rebuilt.

Does that make sense?

Lee
> 
> 
> >
> > So, cpu_to_node(cpu) for possible cpus will have NUMA_NO_NODE(-1)
> > or the number of the nearest node.
> >
> > IIUC, if SRAT is not broken, all pxm has its own node_id.
> 
> Thank you very much for the info,  I have been thinking why node_id
> is (-1) in some cases.
> 
> 
> -minskey
> 
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