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 <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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