Messages in this thread Patch in this message |  | | Date | Tue, 7 Apr 2026 16:38:21 +0200 | | Subject | Re: [RFC PATCH] sched/fair: scale wake_wide() threshold by SMT width | | From | Dietmar Eggemann <> |
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On 07.04.26 08:39, Zhang Qiao wrote: > 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)
I assume not a lot of people care since this needs:
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c index 5847b83d9d55..596c5d590532 100644 --- a/kernel/sched/topology.c +++ b/kernel/sched/topology.c @@ -1691,7 +1691,7 @@ sd_init(struct sched_domain_topology_level *tl, .flags = 1*SD_BALANCE_NEWIDLE | 1*SD_BALANCE_EXEC | 1*SD_BALANCE_FORK - | 0*SD_BALANCE_WAKE + | 1*SD_BALANCE_WAKE | 1*SD_WAKE_AFFINE | 0*SD_SHARE_CPUCAPACITY | 0*SD_SHARE_LLC And then it's a trade-off between one busy thread per core vs. wakeup cost.
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