RE: [PATCH 22/37] xen/arm: use NR_MEM_BANKS to override default NR_NODE_MEMBLKS

[Stefano Stabellini <sstabellini@kernel.org>]

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On Sun, 26 Sep 2021, Wei Chen wrote:
> > -----Original Message-----
> > From: Stefano Stabellini <sstabellini@kernel.org>
> > Sent: 2021年9月24日 9:35
> > To: Wei Chen <Wei.Chen@arm.com>
> > Cc: xen-devel@lists.xenproject.org; sstabellini@kernel.org; julien@xen.org;
> > Bertrand Marquis <Bertrand.Marquis@arm.com>
> > Subject: Re: [PATCH 22/37] xen/arm: use NR_MEM_BANKS to override default
> > NR_NODE_MEMBLKS
> > 
> > On Thu, 23 Sep 2021, Wei Chen wrote:
> > > As a memory range described in device tree cannot be split across
> > > multiple nodes. So we define NR_NODE_MEMBLKS as NR_MEM_BANKS in
> > > arch header.
> > 
> > This statement is true but what is the goal of this patch? Is it to
> > reduce code size and memory consumption?
> > 
> 
> No, when Julien and I discussed this in last version[1], we hadn't thought
> so deeply. We just thought a memory range described in DT cannot be split
> across multiple nodes. So NR_MEM_BANKS should be equal to NR_MEM_BANKS.
> 
> https://lists.xenproject.org/archives/html/xen-devel/2021-08/msg00974.html 
> 
> > I am asking because NR_MEM_BANKS is 128 and
> > NR_NODE_MEMBLKS=2*MAX_NUMNODES which is 64 by default so again
> > NR_NODE_MEMBLKS is 128 before this patch.
> > 
> > In other words, this patch alone doesn't make any difference; at least
> > doesn't make any difference unless CONFIG_NR_NUMA_NODES is increased.
> > 
> > So, is the goal to reduce memory usage when CONFIG_NR_NUMA_NODES is
> > higher than 64?
> > 
> 
> I also thought about this problem when I was writing this patch.
> CONFIG_NR_NUMA_NODES is increasing, but NR_MEM_BANKS is a fixed
> value, then NR_MEM_BANKS can be smaller than CONFIG_NR_NUMA_NODES
> at one point.
> 
> But I agree with Julien's suggestion, NR_MEM_BANKS and NR_NODE_MEMBLKS
> must be aware of each other. I had thought to add some ASSERT check,
> but I don't know how to do it better. So I post this patch for more
> suggestion.

OK. In that case I'd say to get rid of the previous definition of
NR_NODE_MEMBLKS as it is probably not necessary, see below.



> > 
> > > And keep default NR_NODE_MEMBLKS in common header
> > > for those architectures NUMA is disabled.
> > 
> > This last sentence is not accurate: on x86 NUMA is enabled and
> > NR_NODE_MEMBLKS is still defined in xen/include/xen/numa.h (there is no
> > x86 definition of it)
> > 
> 
> Yes.
> 
> > 
> > > Signed-off-by: Wei Chen <wei.chen@arm.com>
> > > ---
> > >  xen/include/asm-arm/numa.h | 8 +++++++-
> > >  xen/include/xen/numa.h     | 2 ++
> > >  2 files changed, 9 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/xen/include/asm-arm/numa.h b/xen/include/asm-arm/numa.h
> > > index 8f1c67e3eb..21569e634b 100644
> > > --- a/xen/include/asm-arm/numa.h
> > > +++ b/xen/include/asm-arm/numa.h
> > > @@ -3,9 +3,15 @@
> > >
> > >  #include <xen/mm.h>
> > >
> > > +#include <asm/setup.h>
> > > +
> > >  typedef u8 nodeid_t;
> > >
> > > -#ifndef CONFIG_NUMA
> > > +#ifdef CONFIG_NUMA
> > > +
> > > +#define NR_NODE_MEMBLKS NR_MEM_BANKS
> > > +
> > > +#else
> > >
> > >  /* Fake one node for now. See also node_online_map. */
> > >  #define cpu_to_node(cpu) 0
> > > diff --git a/xen/include/xen/numa.h b/xen/include/xen/numa.h
> > > index 1978e2be1b..1731e1cc6b 100644
> > > --- a/xen/include/xen/numa.h
> > > +++ b/xen/include/xen/numa.h
> > > @@ -12,7 +12,9 @@
> > >  #define MAX_NUMNODES    1
> > >  #endif
> > >
> > > +#ifndef NR_NODE_MEMBLKS
> > >  #define NR_NODE_MEMBLKS (MAX_NUMNODES*2)
> > > +#endif

This one we can remove it completely right?