Re: [PATCH 2/2] sched/fair: Honor asymmetric SMT priority in idle selection
From: K Prateek Nayak <kprateek.nayak@amd.com>
Date: 2026-09-08 19:41:19
Also in:
lkml
Subsystem:
scheduler, the rest · Maintainers:
Ingo Molnar, Peter Zijlstra, Juri Lelli, Vincent Guittot, Linus Torvalds
Hello Andrea, On 9/8/2026 1:53 PM, Andrea Righi wrote:
quoted hunk ↗ jump to hunk
@@ -9747,8 +9796,10 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags) } /* Slow path */ - if (unlikely(sd)) - return sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag); + if (unlikely(sd)) { + new_cpu = sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag); + return select_idle_smt_cpu(p, new_cpu);
nit. I personally feel this can be better integrated into the sched_balance_find_dst_cpu(). Something like the following: (Only build tested)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b8bd308c2d5b..1012dfb33f08 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c@@ -12353,6 +12353,17 @@ static inline void update_sg_wakeup_stats(struct sched_domain *sd, } + /* + * If we are on a SD_SHARE_CPUCAPACITY | SD_ASYM_PACKING + * domain, use the group_asym_packing classification to + * decide placement based on rankings of idle siblings. + */ + if (unlikely(sched_smt_asym_active() && + (sd->flags & SD_SHARE_CPUCAPACITY) && + (sd->flags & SD_ASYM_PACKING) && + sgs->idle_cpus)) + sgs->group_asym_packing = 1; + sgs->group_capacity = group->sgc->capacity; sgs->group_weight = group->group_weight;
@@ -12393,9 +12404,15 @@ static bool update_pick_idlest(struct sched_group *idlest, return false; break; + case group_asym_packing: + /* + * Only possible for sched_smt_asym_active(). + * Select the idle SMT that is more preferred. + */ + return sched_asym_prefer(idlest->asym_prefer_cpu, + group->asym_prefer_cpu); case group_llc_balance: case group_imbalanced: - case group_asym_packing: case group_smt_balance: /* Those types are not used in the slow wakeup path */ return false; ---
It leads to slightly more branches but they are super predictable when iterating at a particular sched_domain level so the overhead should be negligible. I don't have any strong feelings either ways. Thoughts? -- Thanks and Regards, Prateek