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

[Julien Grall <julien.grall.oss@gmail.com>]

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On Mon, 27 Sep 2021, 06:59 Stefano Stabellini, <sstabellini@kernel.org>
wrote:

> +x86 maintainers
>
> On Mon, 27 Sep 2021, Wei Chen wrote:
> > > -----Original Message-----
> > > From: Stefano Stabellini <sstabellini@kernel.org>
> > > Sent: 2021年9月27日 11:26
> > > To: Wei Chen <Wei.Chen@arm.com>
> > > Cc: Stefano Stabellini <sstabellini@kernel.org>; xen-
> > > devel@lists.xenproject.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 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?
> >
> > How about define NR_MEM_BANKS to:
> > #ifdef CONFIG_NR_NUMA_NODES
> > #define NR_MEM_BANKS (CONFIG_NR_NUMA_NODES * 2)
> > #else
> > #define NR_MEM_BANKS 128
> > #endif
> > for both x86 and Arm. For those architectures do not support or enable
> > NUMA, they can still use "NR_MEM_BANKS 128". And replace all
> NR_NODE_MEMBLKS
> > in NUMA code to NR_MEM_BANKS to remove NR_NODE_MEMBLKS completely.
> > In this case, NR_MEM_BANKS can be aware of the changes of
> CONFIG_NR_NUMA_NODES.
>
> x86 doesn't have NR_MEM_BANKS as far as I can tell. I guess you also
> meant to rename NR_NODE_MEMBLKS to NR_MEM_BANKS?
>
> But NR_MEM_BANKS is not directly related to CONFIG_NR_NUMA_NODES because
> there can be many memory banks for each numa node, certainly more than
> 2. The existing definition on x86:
>
> #define NR_NODE_MEMBLKS (MAX_NUMNODES*2)
>
> Doesn't make a lot of sense to me. Was it just an arbitrary limit for
> the lack of a better way to set a maximum?
>
>
> On the other hand, NR_MEM_BANKS and NR_NODE_MEMBLKS seem to be related.
> In fact, what's the difference?
>
> NR_MEM_BANKS is the max number of memory banks (with or without
> numa-node-id).
>
> NR_NODE_MEMBLKS is the max number of memory banks with NUMA support
> (with numa-node-id)?
>
> They are basically the same thing. On ARM I would just do:
>
> #define NR_NODE_MEMBLKS MAX(NR_MEM_BANKS, (CONFIG_NR_NUMA_NODES * 2))
>

As you wrote above, the second part of the MAX is totally arbitrary. In
fact, it is very likely than if you have more than 64 nodes, you may need a
lot more than 2 regions per node.

So, for Arm, I would just define NR_NODE_MEMBLKS as an alias to
NR_MEM_BANKS so it can be used by common code.

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