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x86/sched: Decrease further the priorities of SMT siblings

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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@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210911011819.12184-2-ricardo.neri-calderon@linux.intel.com
This commit is contained in:
Ricardo Neri 2021-09-10 18:18:14 -07:00 committed by Peter Zijlstra
parent 1a7243ca40
commit 183b8ec38f
1 changed files with 1 additions and 1 deletions
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arch/x86/kernel/itmt.c
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@ -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++;
}