[RFC PATCH] sched/fair: scale wake_wide() threshold by SMT width

[Zhang Qiao]

wake_wide() uses sd_llc_size as the spreading threshold to detect wide
waker/wakee relationships and to disable wake_affine() for those cases.

On SMT systems, sd_llc_size counts logical CPUs rather than physical
cores. This inflates the wake_wide() threshold, allowing wake_affine()
to pack more tasks into one LLC domain than the actual compute capacity
of its physical cores can sustain. The resulting SMT interference may
cost more than the cache-locality benefit wake_affine() intends to gain.

Scale the factor by the SMT width of the current CPU so that it
approximates the number of independent physical cores in the LLC domain,
making wake_wide() more likely to kick in before SMT interference
becomes significant. On non-SMT systems the SMT width is 1 and behaviour
is unchanged.

Signed-off-by: Zhang Qiao <zhangqiao22@xxxxxxxxxx>
---
 kernel/sched/fair.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f07df8987a5ef..4896582c6e904 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7334,6 +7334,11 @@ static int wake_wide(struct task_struct *p)
 	unsigned int slave = p->wakee_flips;
 	int factor = __this_cpu_read(sd_llc_size);
 
+	/* Scale factor to physical-core count to account for SMT interference. */
+	if (sched_smt_active())
+		factor = DIV_ROUND_UP(factor,
+				cpumask_weight(cpu_smt_mask(smp_processor_id())));
+
 	if (master < slave)
 		swap(master, slave);
 	if (slave < factor || master < slave * factor)
-- 
2.18.0