Messages in this thread |  | | Date | Tue, 7 Apr 2026 23:28:48 +0530 | | Subject | Re: [RFC PATCH] sched/fair: scale wake_wide() threshold by SMT width | | From | Shrikanth Hegde <> |
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Hi.
On 4/7/26 12:09 PM, 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. >
Isn't load balance to move it out? What does the workload do?
> 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. >
There are systems where LLC_SIZE == SMT_SIZE. i.e one core in the LLC. This would effectively disable wake_affine feature in such systems.
Power10 being a major example.
> 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)
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