On Tue, 31 Aug 2021, Wei Chen wrote: > Hi Stefano, > > > -----Original Message----- > > From: Stefano Stabellini > > Sent: 2021年8月31日 8:48 > > To: Wei Chen > > Cc: xen-devel@lists.xenproject.org; sstabellini@kernel.org; julien@xen.org; > > jbeulich@suse.com; Bertrand Marquis > > 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 > > > --- > > > 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>; 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. > > 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 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 allow Xen to cope with both scenarios above.