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

[Wei Chen <Wei.Chen@xxxxxxx>]

Hi Stefano,

> -----Original Message-----
> From: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Sent: 2021年9月27日 11:26
> To: Wei Chen <Wei.Chen@xxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxxxxxxxxx; julien@xxxxxxx; Bertrand Marquis
> <Bertrand.Marquis@xxxxxxx>
> 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@xxxxxxxxxx>
> > > Sent: 2021年9月24日 9:35
> > > To: Wei Chen <Wei.Chen@xxxxxxx>
> > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; sstabellini@xxxxxxxxxx;
> julien@xxxxxxx;
> > > Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
> > > 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@xxxxxxx>
> > > > ---
> > > >  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.