From 9e005ed21152d4a4bb0ceea71045ff8a642a6feb Mon Sep 17 00:00:00 2001 From: K Prateek Nayak Date: Tue, 19 May 2026 05:14:23 +0000 Subject: sched/topology: Allow multiple domains to claim sched_domain_shared Recent optimizations of sd->shared assignment moved to allocating a single instance of per-CPU sched_domain_shared objects per s_data. Recent optimizations to select_idle_capacity() moved the sd->shared assignments to "sd_asym" domain when ASYM_CPUCAPACITY is detected but cache-aware scheduling mandates the presence of "sd_llc_shared" to compute and cache per-LLC statistics. Use an "alloc_flags" union in sched_domain_shared to claim a sched_domain_shared object per sched_domain. Allocation starts searching for an available / matching sched_domain_shared instance from the first CPU of sched_domain_span(sd) (sd can be sd_llc, or sd_asym). If the shared object is claimed by another domain, the instance corresponding to next CPU in the domain span is explored until a matching / available instance is found. In case of a single CPU in sched_domain_span(), the domain will be degenerated and a temporary overlap of ->shared objects across different domains is acceptable. "alloc_flags" forms a union with "nr_idle_scan" and the stale flags are left as is when the sd->shared is published. The expectation is for the first load balancing instance to correct the value just like the current behavior, except the initial value is no longer 0. Originally-by: Peter Zijlstra Signed-off-by: K Prateek Nayak Signed-off-by: Peter Zijlstra (Intel) Tested-by: Andrea Righi --- include/linux/sched/topology.h | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) (limited to 'include') diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h index fe09d3268bc9..b5d9d7c2b8ad 100644 --- a/include/linux/sched/topology.h +++ b/include/linux/sched/topology.h @@ -67,7 +67,21 @@ struct sched_domain_shared { atomic_t ref; atomic_t nr_busy_cpus; int has_idle_cores; - int nr_idle_scan; + union { + int nr_idle_scan; + /* + * Used during allocation to claim the sched_domain_shared + * object at multiple levels. + * + * Note: between build and the first periodic LB tick, which + * rewrites the union via update_idle_cpu_scan(), readers of + * nr_idle_scan may observe the transient SD_* flag value as + * the scan bound. The flag bits are small positive integers, + * so the effect is just a slightly relaxed scan bound for one + * window and self-heals on the first tick. + */ + int alloc_flags; + }; #ifdef CONFIG_SCHED_CACHE unsigned long util_avg; unsigned long capacity; -- cgit v1.2.3