Re: [PATCH 0/4] sched/fair: SMT-aware asymmetric CPU capacity

[Andrea Righi <arighi@nvidia.com>]

On Fri, Apr 03, 2026 at 04:46:03PM +0200, Andrea Righi wrote:
> On Fri, Apr 03, 2026 at 01:47:17PM +0200, Dietmar Eggemann wrote:
...
> > > Looking at the data:
> > >  - SIS_UTIL doesn't seem relevant in this case (differences are within
> > >    error range),
> > >  - ASYM_CPU_CAPACITY seems to provide a small throughput gain, but it seems
> > >    more beneficial for tail latency reduction,
> > >  - the ILB SMT patch seems to slightly improve throughput, but the biggest
> > >    benefit is still coming from ASYM_CPU_CAPACITY.
> > 
> > > Overall, also in this case it seems beneficial to use ASYM_CPU_CAPACITY
> > > rather than equalizing the capacities.
> > > 
> > > That said, I'm still not sure why ASYM is helping. The frequency asymmetry
> > 
> > OK, I still would be more comfortable with this when I would now why
> > this is :-)
> 
> Working on this. :)

Alright, I think I found something. I tried to make sis() behave more like sic()
by adding the same SMT "full idle core" check in the fast path and removing the
extra select_idle_smt(prev) hop from the LLC idle path.

Essentially this:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7bebceb5ed9df..19fffa2df2d36 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7651,29 +7651,6 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
 	return -1;
 }
 
-/*
- * Scan the local SMT mask for idle CPUs.
- */
-static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
-{
-	int cpu;
-
-	for_each_cpu_and(cpu, cpu_smt_mask(target), p->cpus_ptr) {
-		if (cpu == target)
-			continue;
-		/*
-		 * Check if the CPU is in the LLC scheduling domain of @target.
-		 * Due to isolcpus, there is no guarantee that all the siblings are in the domain.
-		 */
-		if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
-			continue;
-		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
-			return cpu;
-	}
-
-	return -1;
-}
-
 #else /* !CONFIG_SCHED_SMT: */
 
 static inline void set_idle_cores(int cpu, int val)
@@ -7690,11 +7667,6 @@ static inline int select_idle_core(struct task_struct *p, int core, struct cpuma
 	return __select_idle_cpu(core, p);
 }
 
-static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
-{
-	return -1;
-}
-
 #endif /* !CONFIG_SCHED_SMT */
 
 /*
@@ -7859,7 +7831,7 @@ static inline bool asym_fits_cpu(unsigned long util,
 		       (util_fits_cpu(util, util_min, util_max, cpu) > 0);
 	}
 
-	return true;
+	return !sched_smt_active() || is_core_idle(cpu);
 }
 
 /*
@@ -7964,16 +7936,9 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	if (!sd)
 		return target;
 
-	if (sched_smt_active()) {
+	if (sched_smt_active())
 		has_idle_core = test_idle_cores(target);
 
-		if (!has_idle_core && cpus_share_cache(prev, target)) {
-			i = select_idle_smt(p, sd, prev);
-			if ((unsigned int)i < nr_cpumask_bits)
-				return i;
-		}
-	}
-
 	i = select_idle_cpu(p, sd, has_idle_core, target);
 	if ((unsigned)i < nr_cpumask_bits)
 		return i;

---

With this applied, I see identical performance between NO_ASYM and ASYM+SMT.

I'm not suggesting to apply this, but that seems to be the reason why ASYM+SMT
performs better in my case.

-Andrea