spapr_numa: consider user input when defining associativity

A new function called spapr_numa_define_associativity_domains()
is created to calculate the associativity domains and change
the associativity arrays considering user input. This is how
the associativity domain between two NUMA nodes A and B is
calculated:

- get the distance D between them

- get the correspondent NUMA level 'n_level' for D. This is done
via a helper called spapr_numa_get_numa_level()

- all associativity arrays were initialized with their own
numa_ids, and we're calculating the distance in node_id ascending
order, starting from node id 0 (the first node retrieved by
numa_state). This will have a cascade effect in the algorithm because
the associativity domains that node 0 defines will be carried over to
other nodes, and node 1 associativities will be carried over after
taking node 0 associativities into account, and so on. This
happens because we'll assign assoc_src as the associativity domain
of dst as well, for all NUMA levels beyond and including n_level.

The PPC kernel expects the associativity domains of the first node
(node id 0) to be always 0 [1], and this algorithm will grant that
by default.

Ultimately, all of this results in a best effort approximation for
the actual NUMA distances the user input in the command line. Given
the nature of how PAPR itself interprets NUMA distances versus the
expectations risen by how ACPI SLIT works, there might be better
algorithms but, in the end, it'll also result in another way to
approximate what the user really wanted.

To keep this commit message no longer than it already is, the next
patch will update the existing documentation in ppc-spapr-numa.rst
with more in depth details and design considerations/drawbacks.

[1] https://lore.kernel.org/linuxppc-dev/5e8fbea3-8faf-0951-172a-b41a2138fbcf@gmail.com/

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20201007172849.302240-5-danielhb413@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 109 insertions, 1 deletions

diff --git a/hw/ppc/spapr_numa.c b/hw/ppc/spapr_numa.c
index 16badb1f4b..b50796bbe3 100644
--- a/hw/ppc/spapr_numa.c
+++ b/hw/ppc/spapr_numa.c
@@ -37,12 +37,108 @@ static bool spapr_numa_is_symmetrical(MachineState *ms)
     return true;
 }
 
+/*
+ * This function will translate the user distances into
+ * what the kernel understand as possible values: 10
+ * (local distance), 20, 40, 80 and 160, and return the equivalent
+ * NUMA level for each. Current heuristic is:
+ *  - local distance (10) returns numa_level = 0x4, meaning there is
+ *    no rounding for local distance
+ *  - distances between 11 and 30 inclusive -> rounded to 20,
+ *    numa_level = 0x3
+ *  - distances between 31 and 60 inclusive -> rounded to 40,
+ *    numa_level = 0x2
+ *  - distances between 61 and 120 inclusive -> rounded to 80,
+ *    numa_level = 0x1
+ *  - everything above 120 returns numa_level = 0 to indicate that
+ *    there is no match. This will be calculated as disntace = 160
+ *    by the kernel (as of v5.9)
+ */
+static uint8_t spapr_numa_get_numa_level(uint8_t distance)
+{
+    if (distance == 10) {
+        return 0x4;
+    } else if (distance > 11 && distance <= 30) {
+        return 0x3;
+    } else if (distance > 31 && distance <= 60) {
+        return 0x2;
+    } else if (distance > 61 && distance <= 120) {
+        return 0x1;
+    }
+
+    return 0;
+}
+
+static void spapr_numa_define_associativity_domains(SpaprMachineState *spapr)
+{
+    MachineState *ms = MACHINE(spapr);
+    NodeInfo *numa_info = ms->numa_state->nodes;
+    int nb_numa_nodes = ms->numa_state->num_nodes;
+    int src, dst, i;
+
+    for (src = 0; src < nb_numa_nodes; src++) {
+        for (dst = src; dst < nb_numa_nodes; dst++) {
+            /*
+             * This is how the associativity domain between A and B
+             * is calculated:
+             *
+             * - get the distance D between them
+             * - get the correspondent NUMA level 'n_level' for D
+             * - all associativity arrays were initialized with their own
+             * numa_ids, and we're calculating the distance in node_id
+             * ascending order, starting from node id 0 (the first node
+             * retrieved by numa_state). This will have a cascade effect in
+             * the algorithm because the associativity domains that node 0
+             * defines will be carried over to other nodes, and node 1
+             * associativities will be carried over after taking node 0
+             * associativities into account, and so on. This happens because
+             * we'll assign assoc_src as the associativity domain of dst
+             * as well, for all NUMA levels beyond and including n_level.
+             *
+             * The PPC kernel expects the associativity domains of node 0 to
+             * be always 0, and this algorithm will grant that by default.
+             */
+            uint8_t distance = numa_info[src].distance[dst];
+            uint8_t n_level = spapr_numa_get_numa_level(distance);
+            uint32_t assoc_src;
+
+            /*
+             * n_level = 0 means that the distance is greater than our last
+             * rounded value (120). In this case there is no NUMA level match
+             * between src and dst and we can skip the remaining of the loop.
+             *
+             * The Linux kernel will assume that the distance between src and
+             * dst, in this case of no match, is 10 (local distance) doubled
+             * for each NUMA it didn't match. We have MAX_DISTANCE_REF_POINTS
+             * levels (4), so this gives us 10*2*2*2*2 = 160.
+             *
+             * This logic can be seen in the Linux kernel source code, as of
+             * v5.9, in arch/powerpc/mm/numa.c, function __node_distance().
+             */
+            if (n_level == 0) {
+                continue;
+            }
+
+            /*
+             * We must assign all assoc_src to dst, starting from n_level
+             * and going up to 0x1.
+             */
+            for (i = n_level; i > 0; i--) {
+                assoc_src = spapr->numa_assoc_array[src][i];
+                spapr->numa_assoc_array[dst][i] = assoc_src;
+            }
+        }
+    }
+
+}
+
 void spapr_numa_associativity_init(SpaprMachineState *spapr,
                                    MachineState *machine)
 {
     SpaprMachineClass *smc = SPAPR_MACHINE_GET_CLASS(spapr);
     int nb_numa_nodes = machine->numa_state->num_nodes;
     int i, j, max_nodes_with_gpus;
+    bool using_legacy_numa = spapr_machine_using_legacy_numa(spapr);
 
     /*
      * For all associativity arrays: first position is the size,
@@ -56,6 +152,17 @@ void spapr_numa_associativity_init(SpaprMachineState *spapr,
     for (i = 0; i < nb_numa_nodes; i++) {
         spapr->numa_assoc_array[i][0] = cpu_to_be32(MAX_DISTANCE_REF_POINTS);
         spapr->numa_assoc_array[i][MAX_DISTANCE_REF_POINTS] = cpu_to_be32(i);
+
+        /*
+         * Fill all associativity domains of non-zero NUMA nodes with
+         * node_id. This is required because the default value (0) is
+         * considered a match with associativity domains of node 0.
+         */
+        if (!using_legacy_numa && i != 0) {
+            for (j = 1; j < MAX_DISTANCE_REF_POINTS; j++) {
+                spapr->numa_assoc_array[i][j] = cpu_to_be32(i);
+            }
+        }
     }
 
     /*
@@ -85,7 +192,7 @@ void spapr_numa_associativity_init(SpaprMachineState *spapr,
      * 1 NUMA node) will not benefit from anything we're going to do
      * after this point.
      */
-    if (spapr_machine_using_legacy_numa(spapr)) {
+    if (using_legacy_numa) {
         return;
     }
 
@@ -95,6 +202,7 @@ void spapr_numa_associativity_init(SpaprMachineState *spapr,
         exit(EXIT_FAILURE);
     }
 
+    spapr_numa_define_associativity_domains(spapr);
 }
 
 void spapr_numa_write_associativity_dt(SpaprMachineState *spapr, void *fdt,