[tip: sched/core] x86/sched: Decrease further the priorities of SMT siblings

[tip-bot2 for Ricardo Neri]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     183b8ec38f1ec6c1f8419375303bf1d09a2b8369
Gitweb:        https://git.kernel.org/tip/183b8ec38f1ec6c1f8419375303bf1d09a2b8369
Author:        Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
AuthorDate:    Fri, 10 Sep 2021 18:18:14 -07:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 05 Oct 2021 15:51:59 +02:00

x86/sched: Decrease further the priorities of SMT siblings

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].

Originally-by: Len Brown <len.brown@xxxxxxxxx>
Signed-off-by: Len Brown <len.brown@xxxxxxxxx>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210911011819.12184-2-ricardo.neri-calderon@xxxxxxxxxxxxxxx
---
 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 1afbdd1..9ff480e 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++;
 	}