Re: [PATCH 2/2] sched/fair: Honor asymmetric SMT priority in idle selection
From: Shrikanth Hegde
Date: Mon Sep 07 2026 - 12:57:29 EST
On 9/7/26 9:27 AM, K Prateek Nayak wrote:
Hello Andrea,
On 9/4/2026 2:48 PM, Andrea Righi wrote:
+/*
+ * Return true when @cpu has a higher asymmetric-packing priority than
+ * @other in their shared SMT scheduling domain.
+ */
+static bool sched_smt_asym_prefer(int cpu, int other)
+{
+ struct sched_domain *sd = rcu_dereference_all(cpu_rq(cpu)->sd);
+
+ if (!sd)
+ return false;
+
+ if (!(sd->flags & SD_SHARE_CPUCAPACITY) ||
+ !(sd->flags & SD_ASYM_PACKING))
+ return false;
+
+ if (!cpumask_test_cpu(other, sched_domain_span(sd)))
+ return false;
+
+ return sched_asym_prefer(cpu, other);
+}
+
+/*
+ * Return the highest-priority available CPU in @cpu's SMT core that is also in @cpus.
+ */
+static int __select_idle_smt_cpu(struct task_struct *p, int cpu, const struct cpumask *cpus)
+{
+ int best = cpu;
+ int sibling;
+
+ for_each_cpu_and(sibling, cpu_smt_mask(cpu), cpus) {
+ if (sibling == best || !choose_idle_cpu(sibling, p))
+ continue;
+
+ if (sched_smt_asym_prefer(sibling, best))
+ best = sibling;
nit. Since sched_smt_asym_prefer() is only used here, and we know rq->sd
is the one that can have SD_SHARE_CPUCAPACITY | SD_ASYM_PACKING, perhaps
you can inline the check here do a:
sd = rcu_dereference_all(cpu_rq(cpu)->sd);
if (!sd)
return cpu;
if (!(sd->flags & SD_SHARE_CPUCAPACITY) || !(sd->flags & SD_ASYM_PACKING))
return cpu;
for_each_cpu_and (sibling, sched_domain_span(sd), cpus) {
...
}
...
That way, you don't need to dereference cpu_rq(cpu)->sd every time in
sched_smt_asym_prefer() and check cpumask_test_cpu(). Both, domain
span and task affinity will be covered at once.
Thoughts?
+ }
+
+ return best;
+}
+
+static inline int
+select_idle_smt_cpu(struct task_struct *p, int cpu, const struct cpumask *cpus)
+{
+ if (!sched_smt_asym_active())
+ return cpu;
+
+ return __select_idle_smt_cpu(p, cpu, cpus);
+}
+
+/*
+ * Redirect an available SMT CPU to a higher-priority available sibling allowed by task affinity.
+ */
+static inline int select_idle_smt_priority(struct task_struct *p, int cpu)
+{
+ return select_idle_smt_cpu(p, cpu, p->cpus_ptr);
+}
+
/*
* Scans the local SMT mask to see if the entire core is idle, and records this
* information in sd_balance_shared->has_idle_cores.
@@ -8645,7 +8702,7 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
}
if (idle)
- return core;
+ return select_idle_smt_cpu(p, core, cpus);
cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
return -1;
@@ -8668,7 +8725,7 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
continue;
if (choose_idle_cpu(cpu, p))
- return cpu;
+ return select_idle_smt_priority(p, cpu);
}
return -1;
@@ -8720,7 +8777,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
return -1;
idle_cpu = __select_idle_cpu(cpu, p);
if ((unsigned int)idle_cpu < nr_cpumask_bits)
- return idle_cpu;
+ return select_idle_smt_priority(p, idle_cpu);
Question for Shrikanth: On larger SMT (SMT-4, SMT-8), does the ranking
make that big of a difference if the core is already busy?
Only on Power7 we had AYSM PACKING.
There IPC of CPU0 > CPU1 > CPU2 > CPU4 for the four siblings IIRC irrespective of busy
or idle.
PS: I haven't seen the patches in detail yet.
Does the overehead of additional search get offset by the benefit of
being placed on a better ranked thread? If not, maybe the paths for
!has_idle_core can stay as is?
}
}
cpumask_andnot(cpus, cpus, sched_group_span(sg));
@@ -8745,7 +8802,8 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
if (has_idle_core)
set_idle_cores(target, false);
- return idle_cpu;
+ return (unsigned int)idle_cpu < nr_cpumask_bits ?
+ select_idle_smt_priority(p, idle_cpu) : idle_cpu;
Since every path does a select_idle_smt_priority() - be it coming from
select_idle_core(), the early-return from the cluster scan, or just an
idle CPU from the LLc scan, can't we simply just do it once in
select_idle_sibling()?
Something like:
(Only build tested)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f79fcba4afec..7c97585141dd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8964,7 +8964,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
if (choose_idle_cpu(target, p) &&
asym_fits_cpu(task_util, util_min, util_max, target))
- return target;
+ goto out;
/*
* If the previous CPU is cache affine and idle, don't be stupid:
@@ -8974,8 +8974,10 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
asym_fits_cpu(task_util, util_min, util_max, prev)) {
if (!static_branch_unlikely(&sched_cluster_active) ||
- cpus_share_resources(prev, target))
- return prev;
+ cpus_share_resources(prev, target)) {
+ target = prev;
+ goto out;
+ }
prev_aff = prev;
}
@@ -8993,7 +8995,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
prev == smp_processor_id() &&
this_rq()->nr_running <= 1 &&
asym_fits_cpu(task_util, util_min, util_max, prev)) {
- return prev;
+ target = prev;
+ goto out;
}
/* Check a recently used CPU as a potential idle candidate: */
@@ -9007,8 +9010,10 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
asym_fits_cpu(task_util, util_min, util_max, recent_used_cpu)) {
if (!static_branch_unlikely(&sched_cluster_active) ||
- cpus_share_resources(recent_used_cpu, target))
- return recent_used_cpu;
+ cpus_share_resources(recent_used_cpu, target)) {
+ target = recent_used_cpu;
+ goto out;
+ }
} else {
recent_used_cpu = -1;
@@ -9030,7 +9035,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
*/
if (sd) {
i = select_idle_capacity(p, sd, target);
- return ((unsigned)i < nr_cpumask_bits) ? i : target;
+ target = ((unsigned)i < nr_cpumask_bits) ? i : target;
+ goto out;
}
}
@@ -9043,27 +9049,31 @@ static int select_idle_sibling(struct task_struct *p, int prev, int 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;
+ if ((unsigned int)i < nr_cpumask_bits) {
+ target = i;
+ goto out;
+ }
}
}
i = select_idle_cpu(p, sd, has_idle_core, target);
if ((unsigned)i < nr_cpumask_bits)
- return i;
-
+ target = i;
/*
* For cluster machines which have lower sharing cache like L2 or
* LLC Tag, we tend to find an idle CPU in the target's cluster
* first. But prev_cpu or recent_used_cpu may also be a good candidate,
* use them if possible when no idle CPU found in select_idle_cpu().
*/
- if ((unsigned int)prev_aff < nr_cpumask_bits)
- return prev_aff;
- if ((unsigned int)recent_used_cpu < nr_cpumask_bits)
- return recent_used_cpu;
+ else if ((unsigned int)prev_aff < nr_cpumask_bits)
+ target = prev_aff;
+ else if ((unsigned int)recent_used_cpu < nr_cpumask_bits)
+ target = recent_used_cpu;
+out:
+ if (!sched_smt_asym_active())
+ return target;
- return target;
+ return select_idle_smt_priority(p, target);
}
/**
---
That way, it lives in a single place, and we don't have to pepper
select_idle_smt_priority() everywhere. Thoughts?