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Re: [Qemu-devel] [RFC PATCH v0] numa: API to lookup NUMA node by address


From: Eduardo Habkost
Subject: Re: [Qemu-devel] [RFC PATCH v0] numa: API to lookup NUMA node by address
Date: Mon, 8 Jun 2015 12:51:39 -0300
User-agent: Mutt/1.5.23 (2014-03-12)

On Mon, Jun 08, 2015 at 11:51:03AM +0200, Igor Mammedov wrote:
> On Mon, 8 Jun 2015 11:28:18 +0530
> Bharata B Rao <address@hidden> wrote:
> 
> > On Mon, May 25, 2015 at 02:42:40PM -0300, Eduardo Habkost wrote:
> > > On Mon, May 25, 2015 at 01:17:57PM +0530, Bharata B Rao wrote:
> > > > On Thu, May 14, 2015 at 11:39:06AM +0200, Paolo Bonzini wrote:
> > > > > On 13/05/2015 20:06, Eduardo Habkost wrote:
> > > > > > Also, this introduces a circular dependency between pc-dimm.c and
> > > > > > numa.c. Instead of that, pc-dimm could simply notify us when a new
> > > > > > device is realized (with just (addr, end, node) as arguments), so 
> > > > > > we can
> > > > > > save the list of memory ranges inside struct node_info.
> > > > > > 
> > > > > > I wonder if the memory API already provides something that would 
> > > > > > help
> > > > > > us. Paolo, do you see a way we could simply use a MemoryRegion as 
> > > > > > input
> > > > > > to lookup the NUMA node?
> > > > > 
> > > > > No, but I guess you could add a numa_get/set_memory_region_node_id API
> > > > > that uses a hash table.  That's a variant of the "pc-dimm could simply
> > > > > notify" numa.c that you propose above.
> > > > 
> > > > While you say we can't use MemoryRegion as input to lookup the NUMA 
> > > > node,
> > > > you suggest that we add numa_get/set_memory_region_node_id. Does this 
> > > > API
> > > > get/set NUMA node id for the given MemoryRegion ? 
> > > 
> > > I was going to suggest that, but it would require changing the
> > > non-memdev code path to create a MemoryRegion for each node, too. So
> > > having a numa_set_mem_node_id(start_addr, end_addr, node_id) API would
> > > be simpler.
> > 
> > In order to save the list of memory ranges inside node_info, I tried this
> > approach where I call
> > 
> > numa_set_mem_node_id(dimm.addr, dimm.size, dimm.node) from
> > 
> > pc_dimm_realize(), but
> > 
> > the value of dimm.addr is finalized only later in ->plug().
> > 
> > So we would have to call this API from arch code like pc_dimm_plug().
> > Is that acceptable ?

It looks acceptable to me, as pc.c already has all the rest of the
NUMA-specific code for PC. I believe it would be interesting to keep all
numa.o dependencies contained inside machine code.

> Could you query pc_dimms' numa property each time you need mapping
> instead of additionally storing that mapping elsewhere?

The original patch did that, but I suggested the
numa_set_mem_node_id() API for two reasons: 1) not requiring special
cases for hotplug inside numa_get_node(); 2) not introducing a circular
dependency between pc-dimm.c and numa.c.

Having a numa_set_memory_region_node_id(MemoryRegion *mr, int node) API
would probably be better, and make the discussion about pc_dimm.addr
moot. But it would require changing
memory_region_allocate_system_memory() to avoid
allocate_system_memory_nonnuma() even in the !have_memdevs case.

-- 
Eduardo



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