Messages in this thread |  | | Date | Tue, 7 Apr 2026 23:46:57 +0530 | | Subject | Re: [RFC PATCH] sched/fair: scale wake_wide() threshold by SMT width | | From | Shrikanth Hegde <> |
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On 4/7/26 8:08 PM, Dietmar Eggemann wrote: > 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:
wake_affine machinery needs SD_WAKE_AFFINE. No?
> > 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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