RE: [XEN RFC PATCH 24/40] xen/arm: introduce a helper to parse device tree NUMA distance map

[Wei Chen <Wei.Chen@xxxxxxx>]

Hi Stefano,

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of
> Stefano Stabellini
> Sent: 2021年9月1日 5:36
> To: Wei Chen <Wei.Chen@xxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxxxxxxxxx; julien@xxxxxxx; Bertrand Marquis
> <Bertrand.Marquis@xxxxxxx>
> Subject: RE: [XEN RFC PATCH 24/40] xen/arm: introduce a helper to parse
> device tree NUMA distance map
> 
> On Tue, 31 Aug 2021, Wei Chen wrote:
> > Hi Stefano,
> >
> > > -----Original Message-----
> > > From: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > Sent: 2021年8月31日 8:48
> > > To: Wei Chen <Wei.Chen@xxxxxxx>
> > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; sstabellini@xxxxxxxxxx;
> julien@xxxxxxx;
> > > jbeulich@xxxxxxxx; Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
> > > Subject: Re: [XEN RFC PATCH 24/40] xen/arm: introduce a helper to
> parse
> > > device tree NUMA distance map
> > >
> > > On Wed, 11 Aug 2021, Wei Chen wrote:
> > > > A NUMA aware device tree will provide a "distance-map" node to
> > > > describe distance between any two nodes. This patch introduce a
> > > > new helper to parse this distance map.
> > > >
> > > > Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
> > > > ---
> > > >  xen/arch/arm/numa_device_tree.c | 67
> +++++++++++++++++++++++++++++++++
> > > >  1 file changed, 67 insertions(+)
> > > >
> > > > diff --git a/xen/arch/arm/numa_device_tree.c
> > > b/xen/arch/arm/numa_device_tree.c
> > > > index bbe081dcd1..6e0d1d3d9f 100644
> > > > --- a/xen/arch/arm/numa_device_tree.c
> > > > +++ b/xen/arch/arm/numa_device_tree.c
> > > > @@ -200,3 +200,70 @@ device_tree_parse_numa_memory_node(const void
> *fdt,
> > > int node,
> > > >
> > > >      return 0;
> > > >  }
> > > > +
> > > > +/* Parse NUMA distance map v1 */
> > > > +int __init
> > > > +device_tree_parse_numa_distance_map_v1(const void *fdt, int node)
> > > > +{
> > > > +    const struct fdt_property *prop;
> > > > +    const __be32 *matrix;
> > > > +    int entry_count, len, i;
> > > > +
> > > > +    printk(XENLOG_INFO "NUMA: parsing numa-distance-map\n");
> > > > +
> > > > +    prop = fdt_get_property(fdt, node, "distance-matrix", &len);
> > > > +    if ( !prop )
> > > > +    {
> > > > +        printk(XENLOG_WARNING
> > > > +               "NUMA: No distance-matrix property in distance-
> map\n");
> > > > +
> > > > +        return -EINVAL;
> > > > +    }
> > > > +
> > > > +    if ( len % sizeof(uint32_t) != 0 )
> > > > +    {
> > > > +        printk(XENLOG_WARNING
> > > > +               "distance-matrix in node is not a multiple of
> u32\n");
> > > > +        return -EINVAL;
> > > > +    }
> > > > +
> > > > +    entry_count = len / sizeof(uint32_t);
> > > > +    if ( entry_count <= 0 )
> > > > +    {
> > > > +        printk(XENLOG_WARNING "NUMA: Invalid distance-matrix\n");
> > > > +
> > > > +        return -EINVAL;
> > > > +    }
> > > > +
> > > > +    matrix = (const __be32 *)prop->data;
> > > > +    for ( i = 0; i + 2 < entry_count; i += 3 )
> > > > +    {
> > > > +        uint32_t from, to, distance;
> > > > +
> > > > +        from = dt_read_number(matrix, 1);
> > > > +        matrix++;
> > > > +        to = dt_read_number(matrix, 1);
> > > > +        matrix++;
> > > > +        distance = dt_read_number(matrix, 1);
> > > > +        matrix++;
> > > > +
> > > > +        if ( (from == to && distance != NUMA_LOCAL_DISTANCE) ||
> > > > +            (from != to && distance <= NUMA_LOCAL_DISTANCE) )
> > > > +        {
> > > > +            printk(XENLOG_WARNING
> > > > +                   "Invalid nodes' distance from node#%d to node#%d
> > > = %d\n",
> > > > +                   from, to, distance);
> > > > +            return -EINVAL;
> > > > +        }
> > > > +
> > > > +        printk(XENLOG_INFO "NUMA: distance from node#%d to node#%d
> > > = %d\n",
> > > > +               from, to, distance);
> > > > +        numa_set_distance(from, to, distance);
> > > > +
> > > > +        /* Set default distance of node B->A same as A->B */
> > > > +        if (to > from)
> > > > +             numa_set_distance(to, from, distance);
> > >
> > > I am a bit unsure about this last 2 lines: why calling
> numa_set_distance
> > > in the opposite direction only when to > from? Wouldn't it be OK to
> > > always do both:
> > >
> > > numa_set_distance(from, to, distance);
> > > numa_set_distance(to, from, distance);
> > >
> > > ?
> > >
> > I borrowed this code from Linux, but here is my understanding:
> >
> > First, I read some notes in Documentation/devicetree/bindings/numa.txt
> > 1. Each entry represents distance from first node to second node.
> > The distances are equal in either direction.
> > 2. distance-matrix should have entries in lexicographical ascending
> > order of nodes.
> >
> > Here is an example of distance-map node in DTB:
> > Sample#1, full list:
> >             distance-map {
> >                      compatible = "numa-distance-map-v1";
> >                      distance-matrix = <0 0  10>,
> >                                        <0 1  20>,
> >                                        <0 2  40>,
> >                                        <0 3  20>,
> >                                        <1 0  20>,
> >                                        <1 1  10>,
> >                                        <1 2  20>,
> >                                        <1 3  40>,
> >                                        <2 0  40>,
> >                                        <2 1  20>,
> >                                        <2 2  10>,
> >                                        <2 3  20>,
> >                                        <3 0  20>,
> >                                        <3 1  40>,
> >                                        <3 2  20>,
> >                                        <3 3  10>;
> >             };
> >
> > Call numa_set_distance when "to > from" will prevent Xen to call
> > numa_set_distance(0, 1, 20) again when it's setting distance for <1 0
> 20>.
> > But, numa_set_distance(1, 0, 20) will be call twice.
> >
> > Normally, distance-map node will be optimized in following sample#2,
> > all redundant entries are removed:
> > Sample#2, partial list:
> >             distance-map {
> >                      compatible = "numa-distance-map-v1";
> >                      distance-matrix = <0 0  10>,
> >                                        <0 1  20>,
> >                                        <0 2  40>,
> >                                        <0 3  20>,
> >                                        <1 1  10>,
> >                                        <1 2  20>,
> >                                        <1 3  40>,
> >                                        <2 2  10>,
> >                                        <2 3  20>,
> >                                        <3 3  10>;
> >             };
> >
> > There is not any "from > to" entry in the map. But using this partial
> map
> > still can set all distances for all pairs. And numa_set_distance(1, 0,
> 20)
> > will be only once.
> 
> I see. I can't find in Documentation/devicetree/bindings/numa.txt where
> it says that "from > to" nodes can be omitted. If it is not written
> down, then somebody could easily optimize it the opposite way:
> 
>                        distance-matrix = <0 0  10>,
>                                          <1 0  20>,
>                                          <2 0  40>,
>                                          <3 0  20>,
>                                          <1 1  10>,
>                                          <2 1  20>,
>                                          <3 1  40>,
>                                          <2 2  10>,
>                                          <3 2  20>,
>                                          <3 3  10>;
> 

Yes, you're right. Spec doesn't say opposite way is unallowed.

> I think the code in Xen should be resilient and able to cope with a
> device tree like the one you wrote or the one I wrote. From a code
> perspective, it should be very easy to do. If nothing else it would make
> Xen more resilient against buggy firmware.
> 
> 

I don't disagree with that.

> > > But in any case, I have a different suggestion. The binding states
> that
> > > "distances are equal in either direction". Also it has an example
> where
> > > only one direction is expressed unfortunately (at the end of the
> > > document).
> > >
> >
> > Oh, I should see this comment first, then I will not post above
> > comment : )
> >
> > > So my suggestion is to parse it as follows:
> > >
> > > - call numa_set_distance just once from
> > >   device_tree_parse_numa_distance_map_v1
> > >
> > > - in numa_set_distance:
> > >     - set node_distance_map[from][to] = distance;
> > >     - check node_distance_map[to][from]
> > >           - if unset, node_distance_map[to][from] = distance;
> > >           - if already set to the same value, return success;
> > >           - if already set to a different value, return error;
> >
> > I don't really like this implementation. I want the behavior of
> > numa_set_distance just like the function name, do not include
> > implicit operations. Otherwise, except the user read this function
> > implementation before he use it, he probably doesn't know this
> > function has done so many things.
> 
> You can leave numa_set_distance as-is without any implicit operations.
> 
> In that case, just call numa_set_distance twice from numa_set_distance
> for both from/to and to/from. numa_set_distance could return error is

I am OK for the first sentence. But...

> the entry was already set to a different value or success otherwise
> (also in the case it was already set to the same value). This would

... I prefer not to check the previous value. Subsequent numa_set_distance
call will override previous calls. Keep numa_set_distance as simple as
it can. And when you pass new data to numa_set_distance, it doesn't
know whether the previous data was correct or the new data is correct.
Only caller may have known.  

> allow Xen to cope with both scenarios above.