[PATCH v4 1/6] x86/sched: Decrease further the priorities of SMT siblings

[Ricardo Neri]

When scheduling, it is better to prefer a separate physical core rather
than the SMT sibling of a high priority core. The existing formula to
compute priorities takes such fact in consideration. There may exist,
however, combinations of priorities (i.e., maximum frequencies) in which
the priority of high-numbered SMT siblings of high-priority cores collides
with the priority of low-numbered SMT siblings of low-priority cores.

Consider for instance an SMT2 system with CPUs [0, 1] with priority 60 and
[2, 3] with priority 30(CPUs in brackets are SMT siblings. In such a case,
the resulting priorities would be [120, 60], [60, 30]. Thus, to ensure
that CPU2 has higher priority than CPU1, divide the raw priority by the
squared SMT iterator. The resulting priorities are [120, 30]. [60, 15].

Cc: Aubrey Li <aubrey.li at intel.com>
Cc: Ben Segall <bsegall at google.com>
Cc: Daniel Bristot de Oliveira <bristot at redhat.com>
Cc: Dietmar Eggemann <dietmar.eggemann at arm.com>
Cc: Joel Fernandes (Google) <joel at joelfernandes.org>
Cc: Mel Gorman <mgorman at suse.de>
Cc: Quentin Perret <qperret at google.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki at intel.com>
Cc: Srinivas Pandruvada <srinivas.pandruvada at linux.intel.com>
Cc: Steven Rostedt <rostedt at goodmis.org>
Cc: Tim Chen <tim.c.chen at linux.intel.com>
Originally-by: Len Brown <len.brown at intel.com>
Signed-off-by: Len Brown <len.brown at intel.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon at linux.intel.com>
---
Changes since v3:
  * Introduced this patch

Changes since v2:
  * N/A

Changes since v1:
  * N/A
---
 arch/x86/kernel/itmt.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/itmt.c b/arch/x86/kernel/itmt.c
index 1afbdd1dd777..9ff480e94511 100644
--- a/arch/x86/kernel/itmt.c
+++ b/arch/x86/kernel/itmt.c
@@ -198,7 +198,7 @@ void sched_set_itmt_core_prio(int prio, int core_cpu)
 		 * of the priority chain and only used when
 		 * all other high priority cpus are out of capacity.
 		 */
-		smt_prio = prio * smp_num_siblings / i;
+		smt_prio = prio * smp_num_siblings / (i * i);
 		per_cpu(sched_core_priority, cpu) = smt_prio;
 		i++;
 	}
-- 
2.17.1