sched/topology: Introduce NUMA identity node sched domain

On AMD Family17h-based (EPYC) system, a logical NUMA node can contain
upto 8 cores (16 threads) with the following topology.

             ----------------------------
         C0  | T0 T1 |    ||    | T0 T1 | C4
             --------|    ||    |--------
         C1  | T0 T1 | L3 || L3 | T0 T1 | C5
             --------|    ||    |--------
         C2  | T0 T1 | #0 || #1 | T0 T1 | C6
             --------|    ||    |--------
         C3  | T0 T1 |    ||    | T0 T1 | C7
             ----------------------------

Here, there are 2 last-level (L3) caches per logical NUMA node.
A socket can contain upto 4 NUMA nodes, and a system can support
upto 2 sockets. With full system configuration, current scheduler
creates 4 sched domains:

  domain0 SMT       (span a core)
  domain1 MC        (span a last-level-cache)
  domain2 NUMA      (span a socket: 4 nodes)
  domain3 NUMA      (span a system: 8 nodes)

Note that there is no domain to represent cpus spaning a logical
NUMA node.  With this hierarchy of sched domains, the scheduler does
not balance properly in the following cases:

Case1:

 When running 8 tasks, a properly balanced system should
 schedule a task per logical NUMA node. This is not the case for
 the current scheduler.

Case2:

 In some cases, threads are scheduled on the same cpu, while other
 cpus are idle. This results in run-to-run inconsistency. For example:

  taskset -c 0-7 sysbench --num-threads=8 --test=cpu \
                          --cpu-max-prime=100000 run

Total execution time ranges from 25.1s to 33.5s depending on threads
placement, where 25.1s is when all 8 threads are balanced properly
on 8 cpus.

Introducing NUMA identity node sched domain, which is based on how
SRAT/SLIT table define a logical NUMA node. This results in the following
hierarchy of sched domains on the same system described above.

  domain0 SMT       (span a core)
  domain1 MC        (span a last-level-cache)
  domain2 NODE      (span a logical NUMA node)
  domain3 NUMA      (span a socket: 4 nodes)
  domain4 NUMA      (span a system: 8 nodes)

This fixes the improper load balancing cases mentioned above.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bp@suse.de
Link: http://lkml.kernel.org/r/1504768805-46716-1-git-send-email-suravee.suthikulpanit@amd.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 86e81f06d36b3cdd859ed549e4640504c285f2c4..f51d123f9fe1c4bc8b6dd96511625820c0cc085e 100644 (file)
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1332,6 +1332,10 @@ void sched_init_numa(void)
        if (!sched_domains_numa_distance)
                return;
 
+       /* Includes NUMA identity node at level 0. */
+       sched_domains_numa_distance[level++] = curr_distance;
+       sched_domains_numa_levels = level;
+
        /*
         * O(nr_nodes^2) deduplicating selection sort -- in order to find the
         * unique distances in the node_distance() table.
@@ -1379,8 +1383,7 @@ void sched_init_numa(void)
                return;
 
        /*
-        * 'level' contains the number of unique distances, excluding the
-        * identity distance node_distance(i,i).
+        * 'level' contains the number of unique distances
         *
         * The sched_domains_numa_distance[] array includes the actual distance
         * numbers.
@@ -1441,10 +1444,19 @@ void sched_init_numa(void)
        for (i = 0; sched_domain_topology[i].mask; i++)
                tl[i] = sched_domain_topology[i];
 
+       /*
+        * Add the NUMA identity distance, aka single NODE.
+        */
+       tl[i++] = (struct sched_domain_topology_level){
+               .mask = sd_numa_mask,
+               .numa_level = 0,
+               SD_INIT_NAME(NODE)
+       };
+
        /*
         * .. and append 'j' levels of NUMA goodness.
         */
-       for (j = 0; j < level; i++, j++) {
+       for (j = 1; j < level; i++, j++) {
                tl[i] = (struct sched_domain_topology_level){
                        .mask = sd_numa_mask,
                        .sd_flags = cpu_numa_flags,