Messages in this thread Patch in this message |  | | From | Zhang Qiao <> | | Subject | [RFC PATCH] sched/fair: scale wake_wide() threshold by SMT width | | Date | Tue, 7 Apr 2026 14:39:15 +0800 |
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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@huawei.com> --- 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
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